Article

Genesis of Vanadium Ores in the Otavi Mountainland, Namibia

May 2007
Economic Geology 102(3):441-469

May 2007
102(3):441-469

DOI:10.2113/gsecongeo.102.3.441

Authors:

Maria Boni

University of Naples Federico II

Rosario Terracciano

Noreen Evans

Curtin University

Carsten Laukamp

The Commonwealth Scientific and Industrial Research Organisation

Show all 6 authorsHide

Zu-Cu-Pb vanadate ores (descloizite, mottramite, and vanadinite) in the Otavi Mountainland, Namibia, once were considered the greatest vanadium deposits in the world, with resources estimated at several million tons of vanadium ore. The deposits, now mostly exhausted, occurred in collapse breccias and solution cavities related to a karstic network associated with the post-Gondwana land surfaces in carbonate rocks of the Neoproterozoic Otavi Group, which already contained primary sulfide orebodies. Although a supergene, post-Damaran origin has been generally accepted, the timing and genesis of this mineralization style remains controversial. Mottramite and Cu descloizite are particularly abundant around Cu sulfide deposits (Tsumeb type), whereas descloizite occurs in the areas surrounding primary sphalerite-willemite orebodies (Berg Aukas type). The V deposits the Otavi Mountainland represent a special low-temperature, weathering-related, nonsulfide ore type, also fairly widespread in other areas of southern Africa (e.g., Zambia, Angola). Here we describe the geologic setting of the main V deposits and their ore mineralogy, present some comprehensive major and trace element data for the vanadate ore minerals and gangue carbonates, as well as fluid inclusion microthermometry, O, C, Sr, and Pb isotope analyses, and direct (U-Th)/He descloizite thermochronometry. The vanadates are considered to have formed during a late phase in the depositional history of the post-Damaran supergene ores. Calcite and dolomite gangue are paragenetically coeval with the vanadates. occurring with the vein- and breccia-related ores and as cement in the deeper parts of the recent karst infills. The acidic conditions of sulfide alteration, coupled with a low Eh caused by the influence of organic matter, could have been favorable for vanadate precipitation. Geochemical results and isotope data indicate that Most vanadate deposits were formed at temperatures of similar to 40 degrees to 50 degrees C by meteoric waters interacting with Neoproterozoic shales, carbonates, and related primary sulfide ores, which were subjected to multiple weathering events in a karst environment. The age of most deposits appears to he generally confined to the Tertiary, with a distinct period of descloizite formation dated at 24 to 33 Ma. The V ores are genetically related to post-Gondwanan erosional episodes, controlled by tectonomorphological evolution following rifting phases in the Atlantic realm and spanning the period from the end of the Cretaceous to Pleistocene.

Vanadium Ore Resources of the African continent: State of the Art

Article

Full-text available

Apr 2023
ORE GEOL REV

As part of the critical metals group, vanadium is an essential commodity for the low- and zero-CO2 energy generation, storage and transport. This contribution aims to carry out a review of the most significant vanadium ore sources and mineralizations located in Africa, which are highly diversified in their geological and mineralogical characteristics, and can be classified in: 1. Vanadiferous (titano)magnetite deposits; 2. Sandstone-hosted (U)-vanadium deposits; 3. Calcrete-hosted (U)-vanadium deposits; 4. Vanadate deposits; 5. Vanadium deposits associated with crude oil, coal, and shale-hosted deposits; 6. Graphite-associated vanadium deposits; 7. Vanadium occurrences associated with laterite, bauxite, and phosphate ores. The economically most significant vanadium sources in Africa are associated with titanomagnetite layers in mafic-ultramafic magmatic intrusions (e.g., the Bushveld Complex in South Africa and the Great Dyke in Zimbabwe). Vanadium has been historically mined also in vanadate deposits deriving from the supergene alteration of Pb-Zn-Cu sulfide ores in Namibia and Zambia. Several areas in these countries, where potentially re-processable old tailings and slags have been accumulated, still have economic potential. Vanadium mineralizations are associated with graphite bodies in the Mozambique Metamorphic Belt. Vanadium is also enriched in uranium ores occurring in the Upper Paleozoic-Mesozoic Karoo continental sediments: typical examples are found in Botswana, South Africa, and Zimbabwe. Significant uranium-vanadium concentrations (where carnotite prevails) occur in relatively recent (Tertiary-Quaternary) calcrete duricrusts in paleo-fluviatile beds, which are widespread throughout the African continent. These derive from the weathering of U-(V)-fertile source rocks, which under favorable paleoclimatic conditions resulted in the vanadium precipitation in the critical zone. Variable vanadium amounts have been also recorded in iron ore deposits, phosphorites, and laterites, even though the phosphate deposits seem to have the most favorable characteristics for potentially economic vanadium concentrations. On the whole, South Africa holds the highest-grade vanadium ore resources globally. However, also many other African countries, where this metal could be profitably extracted as a by-product from other economic ores, will probably be at the forefront of vanadium production in the near future.

Characterization and genesis of supergene karstic vanadium ores in the Djebba Pb–Zn district (Triassic Diapirs zone, North Eastern Tunisia)

Article

Aug 2022
J AFR EARTH SCI

The Djebba district hosts hypogene Pb–Zn sulfides and two successive stages of supergene Pb–Zn–Fe–V mineralization. This study mainly focuses on the supergene karstic vanadium mineralization of Jebel Goraa deposit. The hypogene Pb–Zn mineralization is considered to have been formed during Late Miocene (Alpine Orogeny) by Mississippi-Valley-Type (MVT) brines, similar to that proposed for many other Pb–Zn ore deposits of the Diapirs zone in Northern Africa. During the Late Neogene-Pleistocene, several episodes of uplift and erosion enabled the exhumation and karstification of the Early Eocene host rocks, and spatially related hypogene Pb–Zn-(Fe) sulfides, their subsequent weathering, and the formation of supergene ores. Vanadium mineralization is developed in karst cavities filled with oxidised ores (goethite, smithsonite and hemimorphite) and argillaceous materials (smectite). The paragenesis of supergene ores reveals two successive stages of deposition: 1) early supergene weathering with formation of Fe-bearing carbonates in turn oxidized to goethite, smithsonite, hemimorphite, and cerussite; 2) late supergene weathering with deposition of Pb–Zn vanadate minerals (vanadinite, descloizite, and vanadinite-pyromorphite intermediate compositions), minor mimetite and late calcite.Vanadium, phosphorous and probably chlorine of the vanadate ores are inferred to have originated from the immature organic matter of Early Eocene country rocks (Tmax <435 °C and TOC average 3.4 wt%) by descending low-temperature aqueous solutions. Pb, Zn and minor As were derived from primary Pb–Zn sulfides undergoing oxidation. Genesis of supergene vanadium ores is genetically related to post-Miocene uplifting-related weathering episodes, controlled by tectono-morphological and climatic evolution following the Alpine orogeny. The formation of the vanadium mineralization spans through the Late Miocene to Holocene. However, with paleoclimatic considerations it could be generally confined to the Middle Pliocene. Genetically, the supergene vanadium ores of Djebba Pb–Zn district belong to the vanadate ores type, which have been reported in other areas of southern and northern Africa (e.g., Namibia and Morocco). The association of vanadium, particularly in highly fractured and faulted zones with high levels of immature organic matter and spatially close to surface or near-surface MVT Pb–Zn ore deposits, can be used as an exploration tool for new vanadium-bearing deposits.

Vanadium as a critical material: economic geology with emphasis on market and the main deposit types

Article

Full-text available

Aug 2022
B Appl Earth Sci

Vanadium is currently considered a critical material in the European Union, the U.S.A., and other jurisdictions. The vanadium mine production for 2021 is estimated at more than 120 000 tonnes; however, the market base is expected to grow rapidly due to the increase in the use of vanadium for redox flow batteries. Currently, worldwide , many projects are in the advanced stages of exploration and development. In the longer term, should vanadium cease to be a critical material and the law of supply and demand applies, the marginal mines will be decommissioned, and the best deposits will remain economic. Depending on the prevailing regulations in specific jurisdictions, geological settings, and the most up-to-date metallurgical research results, the main vanadium deposit types that could be considered as potential exploration and development targets are the vanadiferous titanomagnetite deposits, sandstone-hosted uranium-vanadium deposits (Salt Wash category), shale-hosted vanadium deposits, and base metal-related vanadate deposits. However, placer deposits, surficial uranium-vanadium type mineralisation, and the Minas Ragra type patrónite deposits should also be considered. ARTICLE HISTORY

Mineralogical and geochemical studies of copper mineralization in the Paleozoic sedimentary section in southwestern Sinai, Egypt

Article

Jun 2022
ORE GEOL REV

Adel Mady

The present study provides new insights into the distribution and characteristics of copper-bearing Paleozoic clastic-carbonate units, Araba and Um Bogma formations in southwestern Sinai, Egypt. Malachite, azurite, atacamite, chrysocolla, and cuprite occur as disseminations or stratiform, layer-parallel streaks, and lenticular bodies that are intercalated with vanadiferous sandstones in the middle unit of the Araba Formation. The vanadiferous sandstones are rich in mottramite, mimetite, duftite, beudantite, descloizite and vanadinite. The Um Bogma Formation is characterized by uncommon chalcanthite, antlerite, malachite, chrysocolla, covellite, and chalcocite in addition to traces of pyrite and barite. These ore minerals occur as acicular, fibrous, fracturefilling, disseminated, and pore-filling patches and are associated with Mn-minerals. Copper minerals form also thin laminae in the manganiferous and black organic-rich shales of the Um Bogma Formation. Copper and vanadium minerals are disseminated in the arkose, subarkose, sublitharenite, graywacke, shale, siltstone and dolostone members of the host formations. Elevated copper concentrations (up to 5.04 wt%) and vanadium (≤362.3 ppm) are associated with high CaO and MgO contents in the intergranular patches and discontinuous laminae. In the Um Bogma Formation, the mineralized layers show high contents of MnO (up to 84.27 wt%) and Fe2O3 (up to 72.1 wt%), Ba (0.03–2.86 wt%), S (up to 0.452 wt%) and Ni (0.012–0.041 wt%). Mineralogical and textural characteristics of the studied rocks indicate that copper and vanadium minerals in the Araba Formation are related to supergene processes and telodiagenesis after oxidation of sulfides and dissolution of feldspar grains and carbonate cement in the clastic rocks most probably during the Upper Devonian-Lower Carboniferous Hercynian event. In the Um Bogma Formation, copper mineralization is a supergene alteration of hydrothermal Fe-Mn minerals and associated primary Cu-Zn sulfides. Hydrothermal fluids likely mobilized several metals, i.e., Fe, Mn, and Cu, within dissolved and karstified carbonates in the Um Bogma Formation.

Minerals of Berg Aukas, Otavi Mountainland, Namibia

Article

Mar 2021

Bruce Cairncross

Mineralogy and Genesis of the Kihabe Zn-Pb-V Prospect, Aha Hills, Northwest Botswana

Article

Full-text available

Jul 2020

The Kihabe Zn-Pb-V > (Cu-Ag-Ge) prospect is located at the boundary between Namibia and Botswana (Aha Hills, Ngamiland District) in a strongly deformed Proterozoic fold belt, corresponding to the NE extension of the Namibian Damara Orogen. The Kihabe prospect contains Zn-Pb resources of 14.4 million tonnes at 2.84% zinc equivalent, Ag resources of 3.3 million ounces, and notable V-Ge amounts, still not evaluated at a resource level. The ores are represented by a mixed sulfide-nonsulfide mineralization. Sulfide minerals consist mainly of sphalerite, galena and pyrite in a metamorphic quartzwacke. Among the nonsulfide assemblage, two styles of mineralization occur in the investigated samples: A first one, characterized by hydrothermal willemite and baileychlore, and a second one consisting of supergene smithsonite, cerussite, hemimorphite, Pb-phosphates, arsenates and vanadates. Willemite is present in two generations, which postdate sulfide emplacement and may also form at their expenses. These characteristics are similar to those observed in the willemite occurrences of the nearby Otavi Mountainland, which formed through hydrothermal processes, during the final stages of the Damara Orogeny. The formation of the Kihabe willemite is likely coeval. Baileychlore is characterized by textures indicating direct precipitation from solutions and dissolution-crystallization mechanisms. Both processes are typical of hydrothermal systems, thus suggesting a hydrothermal genesis for the Kihabe Zn-chlorite as well. Baileychlore could represent an alteration halo possibly associated either with the sulfide or with willemite mineralization. The other nonsulfide minerals, smithsonite, cerussite, various Pb-phosphates and vanadates, are clearly genetically associated with late phases of supergene alteration, which overprinted both the sulfide and the willemite-and baileychlore-bearing mineralizations. Supergene alteration probably occurred in this part of Botswana from the Late Cretaceous to the Miocene.

Mineral Processing and Metallurgical Treatment of Lead Vanadate Ores

Article

Full-text available

Feb 2020

Vanadium has been strongly moving into focus in the last decade. Due to its chemical properties, vanadium is vital for applications in the upcoming renewable energy revolution as well as usage in special alloys. The uprising demand forces the industry to consider the exploration of less attractive sources besides vanadiferous titanomagnetite deposits, such as lead vanadate deposits. Mineral processing and metallurgical treatment of lead vanadate deposits stopped in the 1980s, although the deposits contain a noteworthy amount of the desired resource vanadium. There has been a wide variety of research activities in the first half of the last century, including density sorting and flotation to recover concentrates as well as pyro- and hydrometallurgical treatment to produce vanadium oxide. There have been ecological issues and technical restrictions in the past that made these deposits uninteresting. Meanwhile, regarding the development of mineral processing and metallurgy, there are methods and strategies to reconsider lead vanadates as a highly-potential vanadium resource. This review does not merely provide an overview of lead vanadate sources and the challenges in previous mechanical and metallurgical processing activities, but shows opportunities to ensure vanadium production out of primary sources in the future.

New insights into the genesis of willemite (Zn2SiO4) from zinc nonsulfide deposits, through trace elements and oxygen isotope geochemistry

Article

Mar 2020
ORE GEOL REV

Download the full-text here: https://authors.elsevier.com/a/1abgmcTGy28A4

Genesis of the mottramite-bearing Jbel Haouanit Cu–Pb– Zn–V supergene ore deposit (Eastern High Atlas, Morocco)

Conference Paper

Full-text available

Aug 2017

The Jbel Haouanit ore deposit, hosted in dolomitized Jurassic limestones, is located in the Oriental High Atlas (Morocco). Pb-Zn mineralizations have been exploited in an ancient underground mine, while Cu-Pb-V occurrences are currently mined in handmade (" artisanal ") excavations. The primary sulfides of Jbel Haouanit, galena, sphalerite, pyrite, and chalcopyrite, underwent weathering processes leading to the precipitation of typical supergene minerals, mainly carbonates. Their formation was allowed by the buffering of the fluids acidity associated with the oxidation of sulphides, through the dissolution of the dolomitic host rocks. The paragenetic sequence is rather simple: galena has been successively replaced by anglesite and cerussite, sphalerite has been weathered in smithsonite, which was later replaced by hydrozincite, and pyrite has been pseudomorphosed in hematite and goethite. The weathering of chalcopyrite successively lead to the precipitation of chalcocite, covellite, malachite, and goethite. Late iron oxyhydroxides are mainly found at the top of the deposit. Occurrences of mottramite indicate that V-rich fluids have circulated in the deposits. The presence of secondary sulfides (galena, covellite) and mottramite is related to local fluctuations of the hydrostatic level.

Implications of the occurrence of mottramite (Pb Cu[VO 4 ][OH]) in the Jbel Haouanit Cu–Pb–Zn–V supergene Ore deposit (Eastern High Atlas, Morocco)

Poster

Full-text available

Apr 2017

The Jbel Haouanit deposit, hosted in dolomitized Jurassic limestones, is located at the Northern edge of the Oriental High Atlas (Morocco). The deposit is known for the Pb-Zn main mine that was exploited during the first half of the 20th century (until 1954), providing 150.000 tons of ore. Handcrafted excavations are currently exploited 2.5 km away from the principal mine, for Cu and Cu-Pb-V ores. The primary sulfides of Jbel Haouanit, galena, sphalerite, pyrite, and chalcopyrite, underwent weathering processes leading to the precipitation of typical supergene minerals. Carbonates are the predominant secondary phases at Jbel Haouanit, leading to neoformation of calamine (typical lead and zinc weathering minerals). The carbonates result from the buffering of the fluids acidity associated to the oxidation of sulfides, through the dissolution of the dolomitic host rocks. Galena is successively replaced by anglesite and cerussite, sphalerite is weathered into smithsonite, which is subsequently replaced by hydrozincite, and pyrite is pseudomorphosed in hematite and goethite. The weathering of chalcopyrite successively leads to the precipitation of chalcocite, covellite, malachite, and goethite. Small grains of barite, copper-lead sulfosalts and phospho-arsenates are disseminated in the ore. Mottramite (PbCu[VO4][OH]) is observed in veins in the cementation zone, as euhedral crystals surrounded by (ferriferous) calcite and dolomite. Malachite also contains numerous small crystals of mottramite. The occurrence of copper minerals and mottramite at Jbel Haouanit indicates that Cu, Pb, V bearing fluids also circulated through the Jurassic host rocks. Mottramite is typically found in the southern part of Africa (Namibia, Angola, Zambia), in oxidized zones of vanadium-bearing supergene deposits, but it may also be abundant around Cu-sulfides ores (Boni, 2007, and references herein). This vanadate forms at low temperatures (40–50°C), in arid, acidic and slightly reduced environments, such as the boundary between the phreatic and vadose zones (Takahashi, 1960; Boni et al., 2007). Jbel Haouanit mottramite shows a positive Ce anomaly suggesting its neoformation in relation with Eh variations (Leybourne et al., 2000). The slight enrichment in LREE (Light Rare Earth Elements), compared to HREE (Heavy Rare Earth Elements), also indicates that mottramite precipitated from slightly more acidic fluids than the other supergene minerals, where LREE are further soluble than HREE (De Putter et al., 1999). Vanadium is initially restricted in minor quantities in the primary sulfides (e.g. galena) and in the host dolostone of Jbel Haouanit, at the range of 5–10 ppm. When the host rock is weathered, V is released with other ions. It is probably transported as a calcium metavanadate (Ca[VO3]2) far away from its source before precipitation by recombination with metal cations in favourable environments (Van der Westhuizen et al., 1988). Following Boni et al. (2007), the released Ca may then react with bicarbonate ions to form dolomite and calcite. All the Jbel Haouanit samples are V-rich, and “bulk” calamine rocks are also enriched in this metal compared to galena, suggesting that V-bearing fluids have percolated in the whole deposit, and that mottramite may result from the same weathering step as the The First West African Craton and Margins International Workshop “WACMA1” Dakhla, Morocco, 24 to 29th April 2017 other secondary minerals. Once formed, mottramite does not seem to undergo further weathering. Note that descloizite, the Zn analogue of mottramite, was not found in this deposit. The High Atlas was built in several steps, the two most dominant being the Mesozoic rifting and the Cenozoic tectonic inversion and uplift. Three major episodes of Cenozoic uplift are defined by Leprêtre et al. (2015) in the eastern High Atlas: late Eocene, early to middle Miocene, and late Pliocene to Quaternary. Neoformation of mottramite, as the other supergene minerals, should be associated to these geodynamic events (as proposed by Choulet et al., 2014). Keywords. Moroccan High Atlas, Jbel Haouanit, weathering, supergene, mottramite, vanadium. Acknowledgements Michèle Verhaert thanks the Belgian Fund for Scientific Research (FNRS-FRIA), for providing a FRIA PhD grant. The authors are grateful to the administration of Bou Arfa for its role in providing field access. References: 1. Boni M, Terracciano R, Evans NJ, Laukamp C, Schneider J, Bechstädt T (2007) Genesis of vanadium ores in the Otavi Mountainland, Namibia. Econ. Geol. 102:441–469. 2. Choulet F, Charles N, Barbanson L, Branquet Y, Sizaret S, Ennaciri A, Badra L, Chena Y (2014) Non-sulfide zinc deposits of the Moroccan High Atlas: multi-scale characterization and origin. Ore Geol. Rev. 56:115–140. 3. De Putter T, Charlet JM, Quinif Y (1999) REE, Y and U concentration at the fluid–iron oxide interface in late Cenozoic cryptodolines from Southern Belgium. Chem. Geol. 153:139–150. 4. Leprêtre R, Missenard Y, Saint-Bezar B, Barbarand J, Delpech G, Yans J, Dekoninck A, Saddiqi O (2015) The three main steps of the Marrakech High Atlas building in Morocco: structural evidences from the southern foreland, Imini area. J. Afr. Earth Sci. 109:177–194. 5. Leybourne MI, Goodfellow WD, Boyle DR, Hall GM (2000) Rapid development of negative Ce anomalies in surface waters and contrasting Rare Earth Elements patterns in groundwaters associated with Zn-Pb massive sulphide deposits. Appl. Geochem. 15:695–723. 6. Van der Westhuizen WA, Tordiffe EA, de Bruiyn H, Beukes GJ (1988) The composition of descloizite-mottramite in relation to the trace element distribution of Pb, Zn, Cu and V in the Otavi Mountain Land, South West Africa/Namibia. J. Geochem. Explor. 34:21–29.

First vanadium isotope analyses of V-rich minerals by femtosecond laser ablation and solution-nebulization MC-ICP-MS

Article

Oct 2016
ORE GEOL REV

Vanadium is a redox-sensitive trace metal that occurs in nature as VII, VIII, VIV, and VV, e.g. in a variety of oreforming minerals like vanadates (hosting VV), sulfides (e.g., patrónite: VIV?S4), silicates (e.g., roscoelite, hosting VIII; cavansite, hosting VIV), and phosphates (e.g., sincosite, hosting VIV). Similar to other redox-sensitive metals (e.g., Fe, Cu, U), the V isotope compositions of V minerals may provide valuable information for source fingerprinting and redox-controlled processes during ore formation. The first in situ V isotope analyses of several natural V minerals (cavansite, descloizite, patrónite, sincosite, vanadinite) were performed in this study employing femtosecond-laser ablation-high resolution-MC-ICP-MS. The δ51V values are determined relative to a V metal (Alfa Aesar, 99.5% V) via standard-sample-standard bracketing, and are recast to the Oxford Alfa Aesar solution value. Mass biaswas monitored by addition of an Fe standard to the sample aerosol during analyses. For comparison, aliquots of all mineral specimenswere analyzed after chromatographic separation by solution-nebulization MC-ICP-MS. Our laser ablation and solution typically data agree well with each other (within 0.1‰ units), and indicate a significant variation of δ51V values between the analyzed minerals, ranging from −0.5 to +1.3‰ (2 s.e. per analysis typically between 0.04 and 0.06‰). This spread is significantly larger than reported for various reference rocks, and also exceeds the difference for δ51V between the bulk silicate Earth and the chondrite average. The extended range of δ51V values demonstrates that stable V isotope analyses may provide a new proxy for redox-processes in high- and low-temperature studies, including the fingerprinting of redox-induced V mobilization and enrichment processes during the formation of V-rich ores.

Vanadium: Global (bio)geochemistry

Article

Sep 2015
CHEM GEOL

Redox-sensitive transition group elements are involved in almost all fundamental geochemical processes. Of these elements, vanadium (V) contributes a particularly powerful tool to decipher the Earth's history and its link to extraterrestrial bodies. A comprehensive view of V includes the formation and interaction between the Earth's interior layers, the evolution of the Earth's surface to a habitable zone, biogeochemical cycling, and anthropogenic impacts on the environment. Tracing the geochemical behavior of V through the Earth's compartments reveals critical connections between almost all disciplines of Earth sciences. Vanadium has a history of application as a redox tracer to address the early accretion history of the Earth, to identify connections between the mantle and crust by subduction and melting, and to interpret past surface environments. The geochemical cycling of V from the deep Earth to the surface occurs through magmatism, weathering and digenesis, reflecting variations of fO2 and V species in different Earth compartments. Minerals form a link between deep Earth reservoirs of vanadium and surface environments, and the study of V in minerals has increased the understanding of V cycling. Finally, the exploitation of V has been increasing since the Industrial Revolution, and significant amounts of V have been released as a consequence into natural systems. Environmental concerns are promoting new areas of research to focus on V cycling between water, air, soil and sediment compartments. An increased understanding of V in all compartments, and knowledge of the processes that connect the compartments, is vital to tracing the fate of this intriguing element in natural systems.

The age of supergene manganese deposits in Katanga and its implications for the Neogene evolution of the African Great Lakes Region

Article

Jun 2015
ORE GEOL REV

The “Calamines” and the “Others”: The great family of supergene nonsulfide zinc ores

Article

Jun 2015
ORE GEOL REV

“Nonsulfides” is a term, which comprises a series of oxidized Zn(Pb)-ore minerals. It has also been used to define a special deposit type, mainly considered as derived from the weathering of Zn(Pb) sulfide concentrations. However, nonsulfide zinc deposits have been distinguished between supergene and hypogene, according to their mineralogy, geological characteristics and genetic setting. The supergene deposits formed by weathering and oxidation at ambient temperatures, whereas the hypogene ones are considered hydrothermal, or associated with metamorphic processes on primary sulfide ores. In this review paper, a comparison between a number of several nonsulfide deposits has been carried out: typical “Calamines”, peculiar “Calamines” and “Others”. The whole group comprises deposits of typical supergene origin, mixed supergene–hypogene mineralizations, and oxidized concentrations characterized by different metals only locally associated with zinc. The Zn–Pb nonsulfide concentrations hosted in carbonate rocks, which are mainly attributed to “wall-rock replacement” and “direct-replacement” supergene processes, are the typical “Calamines” (Liège district, Belgium; Iglesias district, Italy; Silesia–Cracow district, Poland). Peculiar “Calamine” deposits are those mineralizations that have been generally considered as supergene, but which are instead genetically related, at least partly, to hypogene processes (e.g. Angouran, Iran; Jabali, Yemen), though mineralogically and texturally similar to supergene nonsulfide deposits. The “Others” are prevailingly supergene nonsulfide zinc deposits not hosted in carbonate rocks (Skorpion, Namibia; Yanque, Peru), or characterized by other metals as main commodities, like lead (Magellan, Australia), silver (Sierra Mojada, Mexico; Wonawinta, Australia) or vanadium (Otavi Mountainland, Namibia). Minerals of current economic importance in most “Calamine” deposits are smithsonite, hydrozincite, and cerussite. This mineralogical association is generally simple but, when the “Calamines” are dolomite-hosted, one of the consequences of the “wall-rock replacement” process is the generation of a series of economically useless Zn- and Mg-bearing mixed carbonate phases. Secondary deposits hosted in silicatic (sedimentary or volcanic) rocks mainly contain hemimorphite and/or sauconite. Lead-, Ag- and V-rich nonsulfide ores are characterized by a more complex mineralogical association: mixed Pb-carbonates, Pb-sulfates, Pb-phosphates, Pb-arsenates, various Ag-sulfosalts, and Zn–Pb–Cu-vanadates. Carbon and oxygen stable isotope studies allow distinguishing between supergene and hypogene nonsulfide deposits, evaluating the effects of oxidative heating and even gaining indirect paleoclimatic information. The oxygen-isotope variation of the individual carbonate minerals within a deposit is relatively small, indicating constant formation temperatures and a single, meteoric fluid source. Carbon-isotope values are highly variable, thus suggesting several isotopically distinct carbon sources. Periods of paleoclimatic switch-overs from seasonally humid/arid to hyperarid have been considered as the most favorable conditions for the formation and preservation of supergene nonsulfide deposits. However, while several recent nonsulfide deposits throughout the world are positioned between 15° and 45° N latitude, thus pointing to a warm and humid weathering climate, others have been deposited in sub-Arctic regions. The economic value of the nonsulfide Zn(Pb–Ag–V) ores is highly variable, because more than in the case of metallic sulfide deposits, it resides not only on the geological setting, but also on their mineralogy that can directly influence processing and metallurgy.

Influence of Genetic Processes on Geochemistry of Fe-oxy-Hydroxides in SupergeneZnNon-SulfideDeposits

Article

Full-text available

Jan 2020

Influence of Genetic Processes on Geochemistry of Fe-oxy-hydroxides in Supergene Zn Non-Sulfide Deposits

Article

Full-text available

Jan 2020

In supergene Zn non-sulfide deposits, the Fe-oxy-hydroxides (FeO/OH) are mainly concentrated in the residual zones (gossan) on top of the oxidized ore bodies, although they can also be found throughout the whole weathering profile coexisting with the primary and secondary ore assemblages. Fe-oxy-hydroxides are rarely pure as they form in systems where a wide range of metals, most of them of economic importance (e.g., Zn, Pb, Co, REE, Sc, Ga, Ge, V, etc.), freely circulate and can be "captured" under specific conditions. Although their occurrence can be widespread, and they have a potential to scavenge and accumulate critical metals, FeO/OH are considered gangue phases during the existing processing routes of Zn non-sulfide ores. Moreover, very little is known about the role of the deposit type on the geochemistry of FeO/OH formed in a specific association. Therefore, this paper provides a comprehensive assessment of the trace element footprint of FeO/OH from a number of Zn non-sulfide deposits, in order to define parameters controlling the metals' enrichment process in the mineral phase. To achieve this, we selected FeO/OH-bearing mineralized samples from four supergene Zn non-sulfide ores in diverse settings, namely Hakkari (Turkey), Jabali (Yemen), Cristal (Peru) and Kabwe (Zambia). The petrography of FeO/OH was investigated by means of scanning electron microscope energy dispersive analysis (SEM-EDS), while the trace element composition was assessed using laser ablation-ICP-MS (LA-ICP-MS). Statistical analyses performed on LA-ICP-MS data defined several interelement associations, which can be ascribed to the different nature of the studied deposits, the dominant ore-formation process and subsequent evolution of the deposits and the environmental conditions under which FeO/OH phases were formed. Based on our results, the main new inferences are: (A) Zinc, Si, Pb, Ga and Ge enrichment in FeO/OH is favored in ores where the direct replacement of sulfides is the dominant process and/or where the pyrite is abundant (e.g., Cristal and Hakkari). (B) When the dissolution of the host-rock is a key process during the supergene ore formation (i.e., Jabali), the buffering toward basic pH of the solutions favors the uptake in FeO/OH of elements leached from the host carbonate rock (i.e., Mn), whilst restricting the uptake of elements derived from the dissolution of sulfides (i.e., Zn, Pb, Ga and Ge), as well as silica. (C) The input of exotic phases can produce significant enrichment in "unconventional" metals in FeO/OH (i.e., Cr and Co at Kabwe; Y at Cristal) depending on whether the optimal pH-Eh conditions are attained. (D) In the Kabwe deposit, FeO/OH records heterogeneous geochemical conditions within the system: where locally basic conditions prevailed during the alteration process, the V and U concentration in FeO/OH is favored; yet conversely, more acidic weathering produced Zn-and Si-bearing FeO/OH.

Mineralogical and geochemical characterization of supergene Cu–Pb–Zn–V ores in the Oriental High Atlas, Morocco

Article

Full-text available

Oct 2017
MINER DEPOSITA

In the Moroccan High Atlas, two sulfide deposits hosted by Jurassic dolostones underwent significant weathering. In the Cu deposit of Jbel Klakh, several stages of supergene mineralization are distinguished: (1) the replacement of hypogene sulfides in the protolith (chalcopyrite) by secondary sulfides in the cementation zone (bornite, digenite, chalcocite, covellite), (2) the formation of oxidized minerals in the saprolite (malachite, azurite, brochantite) where the environment becomes more oxidizing and neutral, and (3) the precipitation of late carbonates (calcite) and iron (hydr-)oxides in the laterite. The precipitation of carbonates is related to the dissolution of dolomitic host rocks, which buffers the fluid acidity due to the oxidation of sulfides. In the Jbel Haouanit Pb–Zn deposit, the mineral assemblage is dominated by typical calamine minerals, Cu minerals (chalcocite, covellite, malachite), and a Cu–Pb–Zn vanadate (mottramite). Galena is successively weathered in anglesite and cerussite. Sphalerite is weathered in smithsonite, which is rapidly replaced by hydrozincite. Late iron (hydr-)oxides are mainly found at the top of both deposits (laterite). Both deposits are thus characterized by specific mineral zoning, from laterite to protolith, related to variations in the mineralogy and ore grades and probably caused by varying Eh-pH conditions.

New C–O isotopic data on supergene minerals from the Skorpion and Rosh Pinah ore deposits (Namibia): Genetic and paleoclimatic constraints

Article

Feb 2017
J AFR EARTH SCI

Geochemistry and mineralogy of vanadium in mine tailings at Berg Aukas, northeastern Namibia

Article

Aug 2014
J AFR EARTH SCI

Critical elements in non-sulfide Zn deposits: a reanalysis of the Kabwe Zn-Pb ores (central Zambia)

Article

Sep 2021

The Kabwe Zn-Pb deposit (central Zambia) consists of a cluster of mixed sulfide and non-sulfide orebodies. The sulfide ores comprise sphalerite, galena, pyrite, chalcopyrite and accessory Ge-sulfides (±Ga and In). The non-sulfide ores comprise: (1) willemite-dominated zones encasing massive sulfide orebodies and (2) oxide-dominated alteration bands, overlying both the sulfide and Zn-silicate orebodies. This study focuses on the Ge, In and Ga distribution in the non-sulfide mineralization, and was carried out on a suite of Kabwe specimens, housed in the Natural History Museum Ore Collection (London). Petrography confirmed that the original sulfides were overprinted by at least two contrasting oxidation stages dominated by the formation of willemite (W1 and W2), and a further event characterized by weathering-related processes. Oxygen isotopic analyses have shown that W1 and W2 are unrelated genetically and furthermore not related to supergene Zn-Pb-carbonates in the oxide-dominated assemblage. The δ 18 O composition of 13.9-15.7‰ V-SMOW strongly supports a hydrothermal origin for W1. The δ 18 O composition of W2 (−3.0‰ to 0‰ V-SMOW) indicates that it precipitated from groundwaters of meteoric origin in either a supergene or a low-T hydrothermal environment. Gallium and Ge show a diversity of distribution among the range of Zn-bearing minerals. Gallium has been detected at the ppm level in W1, sphalerite, goethite and hematite. Germanium occurs at ppm levels in W1 and W2, and in scarcely detectable amounts in hemimorphite, goethite and hematite. Indium has low concentrations in goethite and hematite. These different deportments among the various phases are probably due to the different initial Ga, In and Ge abundances in the mineralization, to the different solubilities of the three elements at different temperatures and pH values, and finally to their variable affinities with the various minerals formed.

Erosion rates and weathering timescales in the eastern Great Escarpment, South Africa

Article

Jun 2021
CHEM GEOL

The eastern escarpment in South Africa has a combination of geology and climate that is unique in the entire Great Escarpment. Yet, no studies have been undertaken before to quantify the landscape changes at this locality. In this study, we assess the denudation history of the eastern escarpment using cosmogenic beryllium-10 (¹⁰Be) on quartz from rock outcrops and river sediments, and carry out uranium‑thorium‑helium ((U,Th)-He) and argon‑argon (⁴⁰Ar/³⁹Ar) dating of Fe-oxides and Mn-oxides, respectively, in the soils. The erosion rates obtained on the eastern escarpment vary from 1.8 m/Ma to 24 m/Ma and are similar in range to values from the entire Great Escarpment. We found that the catchment-averaged erosion rates of the gentle catchments above the eastern escarpment are lower, whereas those from steep catchments draining the escarpment edge are higher. We also determined that the catchment-average erosion rates of the eastern escarpment are similar to those of the western escarpment in Namibia, lower than those of the Drakensberg Escarpment and lower than those of the Lowveld adjacent to it. The ⁴⁰Ar/³⁹Ar dating of cryptomelane-bearing Mn nodules yield Palaeoproterozoic ages unrelated to the pedogenic processes, and might indicate that these nodules are not pedogenic, but inherited from weathered bedrock. The (U,Th)-He ages of goethite concretions range from 0.85 Ma to 1.05 Ma and they date the last period of intensive chemical weathering on the eastern escarpment, which coincides with the Mid-Pleistocene Transition from humid to more arid conditions on the southern African subcontinent. Given the low erosion rates above the escarpment, an area that experiences intensive chemical weathering, and the high erosion rates down the escarpment we conclude that at the eastern escarpment, pediplanation is a more important driver of denudation than peneplanation.

Vanadium in slags from smelting of African Pb-Zn vanadate ores: Mineralogy, extractability and potential recovery

Article

Nov 2020
J GEOCHEM EXPLOR

Slags originating from the metallurgical processing of Pb-Zn vanadate ores at Berg Aukas (Namibia) and Kabwe (Zambia) contain interestingly high concentrations of V (3750–6140 mg/kg) and can be considered potential sources of this metal. We focused on the mineralogical determination of the major V-bearing phases in these slags using a combination of mineralogical methods (XRD, SEM/EDS, EPMA) and found that V is mainly bound in clinopyroxene (Berg Aukas) and glass/vanadates (Kabwe). The abiotic extraction tests, simulating a hydrometallurgical recovery via agitation leaching, were carried out in 0.5 mol/l sulfuric acid (25 °C and 70 °C, pulp density of 1%) to determine the release of V (and other metals: Pb, Zn) from these slags. The leaching of the metals attained a steady state after 24 h and was systematically higher for the high-temperature trials. There was an incomplete dissolution of V-bearing clinopyroxene from the Berg Aukas slags, as opposed to the much higher extractabilities of V (>80%) observed for the Kabwe slags. Compared to the relatively high Zn extractabilities (>72%), lower yields were observed for Pb due to massive formation of secondary Pb sulfate (anglesite) as confirmed by the PHREEQC-3 calculations and the mineralogical investigation of the slag residues. Due to the high expenses related to the mechanical sample treatment and chemical extraction and low prices of V and Zn, the recovery of these metals from the studied slags is currently non-economical. However, the Kabwe slags have a much higher potential for secondary metal recovery in the future, because they are relatively fine-grained and the extraction could potentially be carried out on the as-is materials without expensive comminution processes (crushing/milling).

PREPUBLICATION: Critical elements in nonsulphide Zn deposits: A reanalysis of the Kabwe Zn-Pb ores

Article

Full-text available

Jan 2017

The Kabwe Zn-Pb deposit (central Zambia) consists of a cluster of mixed sulfide and non-sulfide orebodies. The sulfide ores comprise sphalerite, galena, pyrite, chalcopyrite and accessory Ge-sulfides (±Ga and In). The non-sulfide ores comprise: (1) willemite-dominated zones encasing massive sulfide orebodies and (2) oxide-dominated alteration bands, overlying both the sulfide and Zn-silicate orebodies. This study focuses on the Ge, In and Ga distribution in the non-sulfide mineralization, and was carried out on a suite of Kabwe specimens, housed in the Natural History Museum Ore Collection (London). Petrography confirmed that the original sulfides were overprinted by at least two contrasting oxidation stages dominated by the formation of willemite (W1 and W2), and a further event characterized by weathering-related processes. Oxygen isotopic analyses have shown that W1 and W2 are unrelated genetically and furthermore not related to supergene Zn-Pb-carbonates in the oxide-dominated assemblage. The δ ¹⁸ O composition of 13.9–15.7‰ V-SMOW strongly supports a hydrothermal origin for W1. The δ ¹⁸ O composition of W2 (−3.5‰ to 0‰ V-SMOW) indicates that it precipitated from groundwaters of meteoric origin in either a supergene or a low- T hydrothermal environment. Gallium and Ge show a diversity of distribution among the range of Zn-bearing minerals. Gallium has been detected at the ppm level in W1, sphalerite, goethite and hematite. Germanium occurs at ppm levels in W1 and W2, and in scarcely detectable amounts in hemimorphite, goethite and hematite. Indium has low concentrations in goethite and hematite. These different deportments among the various phases are probably due to the different initial Ga, In and Ge abundances in the mineralization, to the different solubilities of the three elements at different temperatures and pH values, and finally to their variable affinities with the various minerals formed.

Characterization and genesis of Cu-Pb-Zn-V-Fe-Mn supergene ore deposits in the area of Bou Arfa (Oriental High Atlas, Morocco)

Article

Full-text available

Dec 2016

Formation of basic lead phases during fire-setting and other natural and man-made processes

Article

Full-text available

Jul 2017

Basic lead phases are relatively rare compounds occurring in various natural and anthropogenically influenced environments, most importantly those related to fire-setting (FS). The medieval FS mining method and subsequent alteration processes lead to a complex set of basic lead phases including caledonite, hydrocerussite, leadhillite, and lanarkite. Although basic lead phases have been known for over 100 years, their mode of formation and stability relations are only insufficiently known. In this study, the formation of this interesting phase assemblage is described in detail including textures, genesis, and conditions of formation. Samples include ones collected in a medieval mining district in SW-Germany and ones which underwent short-term (50 days) experiments mimicking FS and subsequent mine dump processes. The mode of occurrence and the stability relation of basic lead phases formed during FS is discussed using thermodynamic models which are adapted to also explain their occurrence in other anthropogenic and in natural environments. Textures indicate a three step development of the FS assemblage starting with formation of cerussite during supergene weathering of primary galena prior to FS. This is followed by the decarbonisation of the supergene cerussite during FS leading to the formation of lead oxides. Finally, the newly formed lead oxides were hydrated by rain and soil water in the mine dumps producing basic lead phases. Chemical composition of partially produced melt indicates that FS temperatures of up to 950°C were reached in rare cases, whereas the lack of melt phase and predominance of litharge and lead oxycarbonates in most other samples implies that temperatures in most cases do not exceed 540°C. Calculated stability diagrams reveal that most basic lead phases are stable at moderate to high pH and low PCO2. Thermodynamic models quantitatively explain their formation in the medieval mine dumps by the reaction of the lead oxides with a weathering fluid which increases pH and consumes CO2 which favors the precipitation of basic lead phases. This also explains the occurrence of basic lead phases in other anthropogenic environments like slag dumps, lead contaminated soils or in contact to concrete, where the reaction of a fluid with portlandite produces high pH and low PCO2-environments. One possible explanation for the rare formation of basic lead minerals in natural oxidation zones in the absence of lead oxides is the alteration of primary galena under elevated temperatures, since the stability fields of the basic lead phases hydrocerussite and lanarkite are enlarged under elevated temperatures. The short-term experiments show that the precipitation of basic lead phases is almost independent of the external fluid from which they precipitate. Hence, their stability is controlled by microenvironments formed at the mineral-water interface. Consequently, no closed systems in terms of CO2 or external high pH-fluids are needed to stabilize basic lead phases in contact with lead oxides. Analyses of the experimental fluid phase show that the solubility of lead in environments, where lead oxides predominate, is mainly controlled by the basic lead phase hydrocerussite. The present study can be used to quantify the formation of basic lead phases at lead contaminated sites or in natural environments. The observations on the natural samples and the experiments show that in specific rock types, like the medieval FS ones, basic lead phases control the availability of the toxic element lead better than anglesite or cerussite over a wide pH-range. In addition, the described FS phase assemblage can help mining archeologists to understand the details of the FS method even without mining traces and provide constraints on temperatures reached during this process.

Basic lead minerals in medieval firesetting and geogenic environments:Mineralogy and stability relations

Conference Paper

Full-text available

Apr 2016

Maximilian Keim

Zinc-rich clays in supergene non-sulfide zinc deposits

Article

Full-text available

Apr 2016
MINER DEPOSITA

The nature and the origin of zinc clays are poorly understood. With the example of the Bou Arhous Zn-Pb ore deposit in the Moroccan High Atlas, this study presents new data for the mineralogical and chemical characterization of barren and zinc clays associated with non-sulfide zinc ores. In the field, white to ocher granular clays are associated with willemite (Zn2SiO4), while red clays fill karst-related cavities cutting across the non-sulfide ore bodies. Red clays (kaolinite, chlorite, illite, and smectite) present evidence of stratification that reflects internal sedimentation processes during the karst evolution. White clays contain 7-Å clay mineral/smectite irregular interstratified minerals with less than 20 % of smectite layers. Willemite is partially dissolved and is surrounded by authigenic zinc clay minerals. Together with XRD results, WDS analyses on newly formed clay aggregates suggest that this interstratified mineral is composed of fraipontite and sauconite. CEC measurements support that zinc is only located within the octahedral sheets. These new results support the following process: (i) dissolution of willemite, leading to release of Si and Zn, (ii) interaction between Zn-Si-rich solutions and residual-detrital clays, and (iii) dissolution of kaolinite and formation of interstratified zinc clay minerals that grew over detrital micas.

Characterization and origin of clays in the Bou Arhous (Morocco) non-sulfide Zn ore deposit

Article

Full-text available

Sep 2014

A Hydrothermal Origin for the Jaspilite-Hosted, Giant Serra Norte Iron Ore Deposits in the Carajás Mineral Province, Pará State, Brazil

Chapter

Full-text available

Jan 2008

Assessment of agricultural soil contamination by potentially toxic metals dispersed from improperly disposed tailings, Kombat mine, Namibia

Article

Sep 2014
J GEOCHEM EXPLOR

Late Cryogenian (Neoproterozoic) glacial and post-glacial successions at the southern margin of the Congo Craton, northern Namibia: Facies, palaeogeography and hydrocarbon perspective

Article

Full-text available

Oct 2009

The geological paradox of at least two Neoproterozoic glacial intervals at tropical latitudes intercalated within carbonates remains an unsolved puzzle. Several conceptual models have been proposed to explain these apparent rapid swings between climatic extremes and the associated isotopic changes in sea-water chemistry. In Oman, post-glacial transgressive sedimentary successions represent important hydrocarbon source rocks. Source rock characteristics of Neoproterozoic post-glacial successions in other parts of the world (even if not directly correlatable) are, therefore, of special economic interest. This paper concentrates on the Ghaub Formation diamictite interval in northern Namibia and the major environmental change in the aftermath of the assumed glaciation. The relationship of the post-glacial sediments with the underlying different types of cap carbonate and diamictite successions is discussed, and a model of the succession of events is presented. The palaeotopography, caused mostly by ongoing tectonic activity including uplift on the scale of thousands of metres, strongly influenced the petroleum system created and played an important role for the hydrocarbon prospectivity of this post-glacial succession. Tectonic activity on the shelf of the southern margin of the Congo Craton was repeated, and different sub-basins were created before, during and after the Ghaub glaciation. The newly formed relief was flooded, and the different sub-basins were affected by restricted circulation for quite some time. This general scenario bears many similarities to the late Ordovician–early Silurian petroleum system, also formed during post-glacial sea-level rise.

The Nonsulfide Zinc Deposit at Accha (Southern Peru): Geological and Mineralogical Characterization

Article

Apr 2009

The Accha-Yanque zinc belt is located in the southern Altiplano of Peru, a major zinc-rich metallogenic province hosting a number of economic mineral deposits (porphyry copper and skarn ores). Several nonsulfide-type occurrences, showings, and mineral deposits are situated in a belt, peripheral to the northern, northeastern, and northwestern edge of the Oligocene-(MioceneP) Yauri-Apurimac batholith. Mineralization is hosted in breccias of both sedimentary and tectonic origin in the limestones of the Middle to Upper Cretaceous Fer-robamba Formation. Primary ores belong to the carbonate replacement deposit type and are at least in part structurally controlled. Currently, the Zn mineralization is almost fully oxidized: the Accha deposit can be assigned to both direct replacement and wall-rock replacement types. The mineralized zone (indicated resources 5.1 Mt @ 8.2% Zn and 0.9% Pb) occupies the hinge of an anticlinal dome that has been exposed by erosion. The southern limb of the structure dips about 55° to the south-southwest, whereas its northern limb is truncated by faults. The nonsulfide concentrations, consisting of a mineralized zone 5 to 20 m thick, are continuous along strike to the west for at least 700 m. The mineralogy of the Accha deposit shares many characteristics with that of the typical carbonate-hosted calamine-type nonsulfide Zn ores. The nonsulfide mineral association consists mainly of smithsonite and hemi-morphite replacing both primary ore minerals and carbonate host rocks. Hydrozincite has been detected only in samples near the surface. Smithsonite occurs in zoned concretions with goethite, Mn (hydr)oxides and Zn clays, as well as replacive cement in the limestone intervals. One of the most peculiar nonsulfide Zn minerals at Accha is a sauconite-like, zincian smectite, variably concentrated throughout the deposit. Locally sauconite occurs as replacement of detrital feldspars and/or detrital fragments occurring in marly sediments or in infills of karst cavities. It also replaces both hemimorphite and smithsonite deposited during earlier stages. The age of the supergene products in the whole belt is poorly constrained, although there is geomorphologic evidence that the formation of supergene minerals postdates by more than 10 m.y. the last large-scale secondary enrichment event that terminated with central Andean climatic desiccation at -15 Ma. The age of the Accha deposit may be consistent witb a Pliocene K-Ar date of 3.3 ± 0.2 Ma obtained for supergene alunite from the top part of the leached cap in the nearby Cotabambas Cu deposit.

Hydrothermal Fluid Processes and Evolution of the Giant Serra Norte Jaspilite-Hosted Iron Ore Deposits, Carajás Mineral Province, Brazil

Article

Sep 2012

The Serra Norte Carajás banded iron-formation (BIF)-hosted iron ore deposits are located in the Carajás mineral province. The deposits are hosted in the ca. 2.7 Ga Grão Pará Group, a metamorphosed volcanic-sedimentary sequence where jaspilites are under- and overlain by basalts, both at greenschist facies conditions. They represent one of the largest high-grade (>60 wt % Fe) BIF iron ore deposits and resources in the world, with hypogene iron mineralization considered to be Paleoproterozoic. Four main open pits have, to date, produced about 1.2 billion metric tons (Bt) of high-grade iron ore with additional resources of 10 Bt. Ore types at the Serra Norte deposits include soft and hard ore; the latter consists of banded, massive and/or brecciated ores and is mainly localized along the contact with the surrounding hydrothermally altered basalts. Distinct hydrothermal alteration zones consist of veins and breccias that surround the hard ores, including: (1) an early alteration zone (distal portion of orebodies), characterized by recrystallization of jasper, formation of magnetite (± martite), and the local introduction of quartz and carbonate-sulfide (±quartz) veins; (2) intermediate alteration, synchronous with the main iron ore-forming event, which is accompanied by widespread development of martite, quartz-hematite and hematite-quartz veins, and dissolution of carbonate; and (3) proximal alteration zone having various types of hard and hard-porous hematite ores containing microplaty, anhedral, euhedral, and tabular hematite species. Locally, high-grade breccia ores contain dolomite and kutnahorite matrices indicating carbonate introduction. High-grade ore zones contain quartz ± carbonatehematite veins and breccias. Combined microthermometry, iron chromatography, and in situ laser ablation ICP-MS analyses on fluid inclusion assemblages from five vein types reveal that (1) early alteration vein-breccia quartz-carbonate contains high-salinity (up to 30 equiv wt % NaCl) fluid inclusions, with Ca, besides Na, K, and Mg, which were trapped at temperatures of 220° to 320°C. The quartz-hosted fluid inclusions have a wide range of Cl/Br ratios, presence of Li, base metals Cu-Pb-Zn, and Fe; (2) intermediate alteration vein quartz contains both low-salinity (Na-Fe-Mg-rich) and high-salinity (Ca-Mg-Fe-rich) fluid inclusions, with trapping temperatures of 210° to 290°C; (3) advanced alteration vein and breccia quartz-carbonate has low- to high-salinity fluid inclusions and trapping temperatures between 240° to 310°C, with the low-salinity inclusions being much more abundant in quartz. There is a gradual dilution of the metals signature in fluid inclusions from early to late- and/or advanced-stage veins and breccias. The large amount of Ca in the fluid inclusions is compatible with extensive exchange of the hydrothermal fluids with the surrounding chloritized-hematitized metabasaltic wall rock. Oxygen isotope analyses on different oxide species reveal that the heaviest δ18OSMOW values, up to 15.2‰, are recorded for jaspilites, followed by magnetite, between -0.4 to +4.3‰, and then by different hematite species such as microplaty, anhedral and tabular, which fall in the range of -9.5 to -2.4‰. These results show a progressive depletion in δ18O values from the earliest introduced hydrothermal oxide magnetite toward the latest tabular hematite. The advanced alteration stage in high-grade ore displays the most depleted 18Ov alues and represents the highest fluid/rock ratio during hydrothermal alteration. This depletion is interpreted to result from the progressive mixture of descending, heated meteoric water with ascending modified magmatic fluids. Sulfides from the distal zone of metabasaltic rocks have δ34S values close to 0‰, consistent with a magmatic origin for the sulfur. Heavier δ34S values, of up to 10.8‰, in vein sulfides hosted in jaspilite, may reflect interaction with meteoric waters or, alternatively, variations in fO2 and pH conditions during evolution of the hydrothermal fluid. Calcite-kutnahorite δ13C and δ18O values from the distal alteration zones show a large δ13C range of -5.5 to -2.4‰ and a relatively narrow δ18O range of 9.3 to 11.7‰. However, dolomite matrix breccias from the advanced hydrothermal zone, i.e., ore, exhibit a wider δ18O range from 15.1 to 21.8‰ and a more restricted δ13C range from -5.0 to -3.9‰. This latter range points to a single carbon source, of possible magmatic nature, whereas the larger δ18O range suggests multiple carbon and oxygen sources. The 87Sr/86Sr ratios for carbonates from the distal and advanced hydrothermal zones range between 0.7116 to 0.7460, suggesting incorporation of strontium from multiple crustal sources, including magmatic-hydrothermal fluids. A dual magmatic-meteoric hydrothermal fluid-flow model is proposed for the hematite ores in which an early, low Cl/Br ratio, saline, ascending modified magmatic fluid, caused widespread oxidation of magnetite to hematite. Progressive influx of light δ18O meteoric water, mixing with the ascending magmatic fluids, is interpreted to have been initiated during the intermediate stage of alteration. The advanced and final hydrothermal stage was dominated by a massive influx of low-salinity meteoric water, which maintained intermediate temperatures of 240° to 310°C, and concomitant formation of the paragenetically latest tabular hematite. The giant Carajás iron deposits are unique in their setting within an Archean granite-greenstone belt and their modified magmatic-meteoric hydrothermal system, compared to the other two end-member BIF iron deposit types, namely the basin-related Hamersley type and the metamorphosed metasedimentary- basin-related Iron-Quadrangle-type. The distinct hydrothermal alteration signature present in both wall-rock basalts and jaspilites, in combination with distinct fluid chemistry signatures, particularly the low δ18O values of paragenetically late oxides indicative of massive influx of meteoric water into the high-grade orebodies, provide distinctive parameters for defining the Carajás end-member type BIF deposit class.

Geodynamic and climate controls in the formation of Mio–Pliocene world-class oxidized cobalt and manganese ores in the Katanga province, DR Congo

Article

Full-text available

Oct 2010

The Katanga province, Democratic Republic of Congo, hosts world-class cobalt deposits accounting for ~50% of the world reserves. They originated from sediment-hosted stratiform copper and cobalt sulfide deposits within Neoproterozoic metasedimentary rocks. Heterogenite, the main oxidized cobalt mineral, is concentrated as “cobalt caps” along the top of silicified dolomite inselbergs. The supergene cobalt enrichment process is part of a regional process of residual ore formation that also forms world-class “manganese cap” deposits in western Katanga, i.e., the “black earths” that are exploited by both industrial and artisanal mining. Here, we provide constraints on the genesis and the timing of these deposits. Ar–Ar analyses of oxidized Mn ore and in situ U–Pb SIMS measurements of heterogenite yield Mio–Pliocene ages. The Ar–Ar ages suggest a multi-phase process, starting in the Late Miocene (10–5Ma), when the metal-rich substratum was exposed to the action of meteoric fluids, due to major regional uplift. Further oxidation took place in the Pliocene (3.7–2.3Ma) and formed most of the observed deposits under humid conditions: Co- and Mn-caps on metal-rich substrata, and coeval Fe laterites on barren areas. These deposits formed prior to the regional shift toward more arid conditions in Central Africa. Arid conditions still prevailed during the Quaternary and resulted in erosion and valley incision, which dismantled the metal-bearing caps and led to ore accumulation in valleys and along foot slopes. KeywordsHeterogenite-Katanga Copperbelt-Lateritic ore-Supergene-Congo

Mannardite as the main vanadium-hosting mineral in black shale-hosted vanadium deposits, South China

Article

Full-text available

Apr 2023

Black shale-hosted vanadium (V) deposits account for about 80% vanadium resources in the world, but only <2% vanadium in the black shale can be extracted mainly due to insufficient recognition on the occurrence mode of vanadium. It is commonly agreed that most vanadium in the black shale is hosted in clay minerals and organic matters, but it is not clear how the other parts of vanadium exist and whether there exists a vanadium mineral, which has limited our understanding of metallogenic mechanism of black shale-hosted vanadium deposits. The Jiujiang Basin at the Lower Yangtze Block is a significant black shale-hosted vanadium metallogenic district. In this work, we conducted systematic studies of mineralogy, lithology and geochemistry on the occurrence of vanadium hosted in the black shales. Electron probe microanalysis (EPMA), Raman spectroscopy, and X-ray diffraction (XRD) show that the main vanadium-hosting mineral in the black shale is mannardite, with a structural formula of [Ba0.96∙H2O](Ti5.87V1.873+V0.114+Si0.07Cr0.07Fe0.023+)O16.00, space group I41/a, unit-cell parameters a = b = 14.346(7) Å, c = 5.899(1) Å, α = β = γ = 90°, Z = 4. Data from EPMA, TESCAN integrated mineral analyzer (TIMA), and whole-rock geochemistry indicate that 12.32–44.06% (average 24.95%) vanadium exists in mannardite. Most vanadium atoms in mannardite occupy its structural sites as trivalent vanadium (V3+), forming chemical bonds with O atoms as VO2−, whereas a minor amount of vanadium atoms replace titanite atoms (Ti4+) as quadrivalent vanadium (V4+) by isomorphism. Mannardite precipitates under a strong reductive condition with sufficient trivalent vanadium species, titanium and biogenic barium (bio-barite). Our first identification of mannardite in black shale-hosted vanadium deposits thus sheds light on the occurrence mode of vanadium and the metallogenic mechanism of black shale-hosted vanadium deposits.

The Water Leaching Behavior of Vanadium from a Salt-roasted VTM Concentrate and the Preparation of High-concentration Vanadium Solution

Article

Apr 2022

Deposit classification scheme for the Critical Minerals Mapping Initiative Global Geochemical Database: U.S. Geological Survey Open-File Report 2021–1049, 60 p.

Technical Report

Jun 2021

A challenge for the global economy is to meet the growing demand for commodities used in today’s advanced technologies. Critical minerals are commodities (for example, elements, compounds, minerals) deemed vital to the economic and national security of individual countries that are vulnerable to supply disruption. The national geological agencies of Australia, Canada, and the United States recently joined forces to advance understanding and foster development of critical mineral resources in their respective countries through the Critical Minerals Mapping Initiative (CMMI). An initial goal of the CMMI is to fill the knowledge gap on the abundance of critical minerals in ores. To do this, the CMMI compiled modern multielement geochemical data generated by each agency on ore samples collected from historical and active mines and prospects from around the world. To identify relationships between critical minerals, deposit types, deposit environments, and mineral systems, a unified deposit classification scheme was needed. This report describes the scheme developed by the CMMI to classify the initial release of geochemical data. In 2021, the resulting database—along with basic query, statistical analysis, and display tools—will be served to the public through a web-based portal managed by Geoscience Australia. The database will enable users to trace critical minerals through mineral systems and identify individual deposits or deposit types that are potential sources of critical minerals.

A Review on the Deposit Geology and Mineralization Mechanism of Tsumeb Polymetallic Deposit, Namibia

Article

Full-text available

Jan 2021

The Tsumeb polymetallic deposit of Otavi Mountain Land (OML), Namibia, is a prominent deposit of remarkable and complex mineral species with the accreditation of about 337 valid minerals. A total of 72 species of these minerals are of Tsumeb as the type of locality. The deposit was first prospected in 1893 by the South West Africa Company and it was mined from the year 1897 to 1996, yielding a total of about 30 Mt of ore with the grade of 10% Pb, 4.3% Cu, and 3.5% Zn along with ore minerals of As, Sb, Ag, Cd, and Au. The orebody also typifies the largest renowned single sulfidic accumulation of germanium (Ge). However, like many other world-class deposits, the Tsumeb copper deposit has been exhausted. Hence, re-assessment of ore reserve and exploration is a crucial practice in the discovery of new mineral resources and occurrences. This practice requires extensive understanding of the geological characteristics and metallogenic mechanisms of the parent/exhausted ore deposit as a reference model. In this paper, we presented a summary description of the Tsumeb deposit of OML, with the main focus on the regional geological background, deposit geology and ore mineralization mechanism.

LA PARAGÉNESIS HIDROTERMAL PORTADORA DE Ag-Pb-Zn-V-Mo-Cu-P EN LA PEGMATITA EL CRIOLLO, CERRO BLANCO (CÓRDOBA)

Conference Paper

Jan 2010

An unusual mineral association was found in cavities and cracks in quartz adjacent to massive hematite in a pegmatite at Cerro Blanco, Córdoba province (31°21'22.9"S - 64°39'6.1"W). The primary assemblage consists of primary chalcocite with scarce inclusions (possibly exsolutions) of an unidentified Ag-bearing Cu sulfide; secondary minerals include covellite (as inwards replacement of chalcocite), pseudomalachite, wulfenite, calcian P- and Asbearing mottramite, malachite, azurite, chrysocolla, genthelvite, and clay minerals (one of them being dickite). With the exception of mottramite (and possibly genthelvite, which was not analyzed), EDS spectra of oxidized phases show only the expected elements. Pseudomalachite was stabilized instead of libethenite by a high a(Cu2+)/a(H2PO4)- ratio at a pH 6. However, a(H+) was not low enough to precipitate cornetite. The presence of V, Pb, Mo, As, Zn, Cu, Ag, Cr, Co and Ni (these last three found as trace constituents in blue genthelvite) in the hydrothermal minerals suggests an input from a source unrelated to the pegmatitic magma.

Chemical composition and source apportionment of atmospheric aerosols on the Namibian coast

Article

Full-text available

Dec 2020
ATMOS CHEM PHYS

The chemical composition of aerosols is of particular importance to assess their interactions with radiation, clouds and trace gases in the atmosphere and consequently their effects on air quality and the regional climate. In this study, we present the results of the first long-term dataset of the aerosol chemical composition at an observatory on the coast of Namibia, facing the south-eastern Atlantic Ocean. Aerosol samples in the mass fraction of particles smaller than 10 µm in aerodynamic diameter (PM10) were collected during 26 weeks between 2016 and 2017 at the ground-based Henties Bay Aerosol Observatory (HBAO; 22∘6′ S, 14∘30′ E; 30 m above mean sea level). The resulting 385 filter samples were analysed by X-ray fluorescence and ion chromatography for 24 inorganic elements and 15 water-soluble ions. Statistical analysis by positive matrix factorisation (PMF) identified five major components, sea salt (mass concentration: 74.7±1.9 %), mineral dust (15.7±1.4 %,), ammonium neutralised (6.1±0.7 %), fugitive dust (2.6±0.2 %) and industry (0.9±0.7 %). While the contribution of sea salt aerosol was persistent, as the dominant wind direction was south-westerly and westerly from the open ocean, the occurrence of mineral dust was episodic and coincided with high wind speeds from the south-south-east and the north-north-west, along the coastline. Concentrations of heavy metals measured at HBAO were higher than reported in the literature from measurements over the open ocean. V, Cd, Pb and Nd were attributed to fugitive dust emitted from bare surfaces or mining activities. As, Zn, Cu, Ni and Sr were attributed to the combustion of heavy oils in commercial ship traffic across the Cape of Good Hope sea route, power generation, smelting and other industrial activities in the greater region. Fluoride concentrations up to 25 µg m−3 were measured, as in heavily polluted areas in China. This is surprising and a worrisome result that has profound health implications and deserves further investigation. Although no clear signature for biomass burning could be determined, the PMF ammonium-neutralised component was described by a mixture of aerosols typically emitted by biomass burning, but also by other biogenic activities. Episodic contributions with moderate correlations between NO3-, nss-SO42- (higher than 2 µg m−3) and nss-K+ were observed, further indicative of the potential for an episodic source of biomass burning. Sea salt accounted for up to 57 % of the measured mass concentrations of SO42-, and the non-sea salt fraction was contributed mainly by the ammonium-neutralised component and small contributions from the mineral dust component. The marine biogenic contribution to the ammonium-neutralised component is attributed to efficient oxidation in the moist marine atmosphere of sulfur-containing gas phase emitted by marine phytoplankton in the fertile waters offshore in the Benguela Upwelling System. The data presented in this paper provide the first ever information on the temporal variability of aerosol concentrations in the Namibian marine boundary layer. This data also provide context for intensive observations in the area.

Genesis of As-Pb-Rich Supergene Mineralization: The Tazalaght and Agoujgal Cu Deposits (Moroccan Anti-Atlas Copperbelt)

Article

Full-text available

Dec 2020

In the Moroccan Anti-Atlas, sulfide deposits hosted by Neoproterozoic to Cambrian formations underwent significant weathering, leading to the formation of supergene profiles. In the Tazalaght Cu-As deposit, three mineralogical steps are distinguished: (1) the replacement of hypogene sulfides (chalcopyrite, pyrite, tennantite) by supergene sulfides (bornite, chalcocite) in the large cementation zone; (2) the formation of oxidized minerals (malachite, azurite, olivenite, and chenevixite, mainly) in a more oxidizing and neutral environment; and (3) the precipitation of goethite, hematite, and quartz in the gossan. In the Cu-As-Pb-V deposit of Agoujgal, the mineralogical units are spatially less confined than at Tazalaght. The narrow cementation zone hosts chalcocite, resulting from the weathering of hypogene chalcopyrite, pyrite, tennantite and galena, while the much more extended and diversified oxidized zone is rich in Cu and Pb carbonates, arsenates, sulfates, phosphates, vanadates, and oxides. Goethite, hematite, mottramite, and late calcite occur in the gossan. Both deposits are characterized by As-rich secondary ores that were formed through similar processes, despite some mineralogical and chemical variations highlighting the influence of the host rocks on weathering. The restricted oxidized mineralization at Tazalaght and the Agoujgal cementation zone most likely arise from the contrasting omnipresence of quartzite at Tazalaght that could not enable a fast and effective neutralization of the fluid’s acidity, and the large amounts of dolomitic host rocks that could be dissolved at Agoujgal. At both sites, the weathering of tennantite through a boxwork texture records the transition from the cementation zone (chalcocite), the oxidized zone (arsenates), and the gossan, and reflects the fluids evolution with time.

Economic Geology Principles and Practice: Metals, Minerals, Coal and Hydrocarbons - Introduction to Formation and Sustainable Exploitation of Mineral Deposits (1st ed.)

Book

May 2011

Walter Leopold Pohl

Wisely used, mineral resources create wealth, employment, a vital social and natural environment, and peace. If the reverse of these conditions occurs only too often, illustrating the so-called “resource curse”, this should be attributed to the true perpetrators, namely irresponsible, weak or selfish leaders. This book, however, does not intend to provide rules for good governance. I wrote it as a broad overview on geoscientific aspects of mineral deposits, including their origin, geological characteristics, the principles of the search for ores and minerals, and the investigation of newly found deposits. In addition, practical and environmental aspects are adressed that arise during the life cycle of a mine and after its closure. I am convinced that in our time, economic geology cannot be tought, studied, or practiced without an understanding of environmental issues. The scientific core of the book is the attempt to present the extraordinary genetic variability of mineral deposits in the frame of fundamental geological process systems. The comprehensive approach of this book – covering materials from metal ores to minerals and hydrocarbons – is both an advantage and a loss. The second concerns the sacrifice of much detail, but I promote the first for its benefit of a panoramic view over the whole field of economic geology. Being aware that the specialist level of subjects presented in this book fills whole libraries, I do hope that even experienced practitioners, academic teachers and advanced students of particular subjects will find the synopsis useful. Over more than 50 years, several editions of this book were published in German language. Since the first edition (Wilhelm & Walther E. Petrascheck 1950), the book was intended to provide a concise introduction to the geological setting of mineral deposits, including its application to exploration and mining. The target audience has changed, however. Originally, it was written for students of mining engineering. Today, it is mainly directed to aspiring and practicing geologists. Each of the seven chapters of the book was developed with my own students as a university course, and should be useful to fellow academic teachers. After initially working in industry I never lost contact with the applications of economic geology which is my motive for the constant interweaving of practical aspects in the text and for dedicating one of the chapters to the practice of economic geology. For professional reference purposes, practitioners in geology and mining should appreciate this melange of science and application. Frequent explanations and references to environmental and health aspects of extraction and processing of ores and minerals should assist users involved in environmental work. To those with no background in geology, I recommend they acquire an introductory geoscience text for looking up terms that are employed but not explained in the book. Compared with the last German-language edition (Pohl 2005), this book has been rewritten for an international public. Although it retains a moderate European penchant by referring to examples from this region, important deposits worldwide are preferentially used to explain genetic types and practical aspects. I trust that this will be useful to both scholars and practitioners, wherever they work. Generally, it was my ambition to represent the state of the art in economic geology, by referring to and citing recent publications as well as earlier fundamental concepts. This should assist and motivate students to pursue topics to greater depth. Many people have supported me in my life-long pursuit of theory and practice of economic geology, and helped with this book, especially by donating photographs. I cannot name them all but in captions, donors are acknowledged. Here, just let me say thank you.

Hydrothermal Processes and Mineral System

Chapter

Jan 2009

Franco Pirajno

The study of asteroid impacts has gained considerable momentum in the last 30 years or so, following the pioneering works of Eugene (Gene) Shoemaker (1928–1997) who, with his wife Carolyn and Edward Chao, discovered shock features diagnostic of very high pressures that can only be attained by hypervelocity impacts. They were instrumental in the recognition by the geological community of asteroidal impacts as a major planetary process. Other pioneers included Robert Dietz (1914–1995) and Richard Grieve, who realised the impact origin of Sudbury, Bevan French, Michael Dence and Dieter Stoffer.

Hydrothermal Processes and Mineral Systems

Chapter

Jan 2009

Franco Pirajno

In this chapter I discuss and describe a range of hydrothermal mineral systems that are generated at passive margins, in basin structures of intracontinental or back-arc rifts, with or without obvious connection to igneous activity, seafloor metalliferous sediments and iron and manganese oxide accumulations. Passive margins and rift-related ore deposits include those known as Mississipi Valley-type (MVT), sedimentary exhalative polymetallic massive sulphides (SEDEX) and the Cu-rich stratabound and stratiform disseminated sulphides, all of which can be broadly grouped under the wider family of sedimentary-hydrothermal ore systems.

Internal structures and dating of non-sulphide Zn deposits using rock magnetism: Insights from the Moroccan High Atlas

Article

Full-text available

Jun 2015
MINER DEPOSITA

The renewal of interest in Zn-Pb non-sulphide ores has been induced by mineral processing improvement and leads to new exploration and mining projects in the world. Although the mineralogy is often precisely known, and despite several studies linking ore deposition to regional tectonics, absolute dating of non-sulphide stages is rare and structure of ore bodies was largely disregarded. Geochronological data from non-sulphide ores are essential to timely constrain alteration episodes and to insert supergene ore genesis in the climate and tectonic evolution of the metallogenic province. The access to internal organization of ore could reveal postmineralization episodes related to supergene evolution. Thus, a rock magnetism study combining anisotropy of magnetic susceptibility (AMS) and palaeomagnetism was performed on four non-sulphide deposits from the Moroccan High Atlas. AMS generally shows similar horizontal magnetic fabrics for ores and the clayey and carbonaceous internal sediments filling karstic cavities. The palaeomagnetic directions of ores and internal sediments are compatible, and the calculated poles are consistent with the last 30 Ma of the Africa apparent polar wander path, with an upper age at 0.78 Ma. The proposed three-step scenario is placed within the evolution of the Moroccan High Atlas belt. Deposition of primary sulphides is contemporaneous with opening of the Tethyan and Atlantic oceans. During the Tertiary, intracontinental deformation gave rise to the High Atlas fold-and-thrust belt and to regional uplift. Finally, Zn-Pb sulphides hosted in carbonates experienced oxidation under an arid climate to form karst related Zn-Pb non-sulphide ores. These promising results pave the way for an efficient method to constrain the internal fabrics and age of Zn supergene deposits.

Non-sulfide zinc deposits of the Moroccan High Atlas: Multi-scale characterization and origin

Article

Jan 2014
ORE GEOL REV

To understand the processes and timing of their formation, six non-sulfide Zn-Pb ore deposits were investigated in the Moroccan High Atlas. Sulfide and non-sulfide ores are hosted in Lower Jurassic reefal to para-reefal limestone. Zn (Pb) carbonates, Zn silicates and associated hydrated phases directly replace the stratabound primary ore bodies or fill cavities along fractures related to the Atlasic compression. Field observation has been complemented by a multidisciplinary approach (e.g. XRD, Raman, SEM, EPMA) for the mineralogical characterization. All six ore deposits present similar paragenesis revealing three successive stages for ore deposition: 1) formation of the protore sulfides, 2) early supergene weathering with formation of Zn-Pb-bearing carbonates and iron oxi-hydroxides and 3) late supergene weathering with deposition of Zn-carbonates, Zn-silicates and hydrated phases. Direct replacement of primary sulfides is accompanied by precipitation of zinc non-sulfide minerals in cavities or internal sediments filling. The proposed three-step scenario can be placed within the tectonic evolution of the Moroccan High Atlas belt. Deposition of primary sulfides is contemporaneous with opening of the Tethyan and Atlantic oceans. During the Tertiary, intracontinental deformation has given rise to the High Atlas fold-and-thrust belt and to regional uplift. As a result, Zn-Pb sulfides, hosted in carbonates experienced oxidation under an arid climate to form karst-related Zn-Pb non-sulfide ore bodies.

Apatite thermochronology in modern geology

Article

Full-text available

Sep 2009

Fission-track and (U-Th-Sm)/He thermochronology on apatites are radiometric dating methods that refer to thermal histories of rocks within the temperature range of 40°-125 °C. Their introduction into geological research contributed to the development of new concepts to interpreting time-temperature constraints and substantially improved the understanding of cooling processes within the uppermost crust. Present geological applications of apatite thermochronological methods include absolute dating of rocks and tectonic processes, investigation of denudation histories and long-term landscape evolution of various geological settings, and basin analysis.

Chapter 5.7 Mineral Deposits

Article

Dec 2009

The different stages of the geodynamic evolution of Neoproterozoic basins in southwestern Africa were accompanied by a variety of mineralisation processes. These range from synsedimentary-exhalative base metal deposits in continental rift basins, Besshi-type cupriferous pyrite deposits in oceanic crustal rocks, syn-orogenic polymetallic Tsumeb-type mineralisation in foreland carbonate platforms, orogenic gold and magmatic U and Sn deposits, to REE-rich pegmatites and post-orogenic enrichment in the supergene zone. Of currently main economic significance amongst these are the Pb and Zn deposits around Rosh Pinah in the external Gariep Belt, the Tsumeb-type deposits at Kombat and the alaskite-hosted U deposits in the western part of the Central Zone in the Damara Belt.

The Otavi Mountain Land Cu-Pb-Zn-V deposits, Namibia

Article

Full-text available

Jan 1997

Bruce Cairncross

Post Gondwana African land surfaces and pedogenetic ferromanganese deposits on the Witwatersrand at the West Wits Gold Mine, South Africa

Article

Full-text available

Mar 1999

Several supergene ferromanganese wad deposits, mined on small scale for industrial applications, are developed on dolomite of the Neoarchaean Malmani Subgroup of the Transvaal Supergroup in South Africa. At the West Wits Gold Mine on the plateau of the Witwatersrand escarpment near Johannesburg, the ferromanganese wad is developed in manganiferous dolomite of the Oaktree Formation at the base of the Malmani Subgroup. The wad represents an ancient saprolite developed in the dolomite below a major unconformity with incised valleys filled by ferruginous silty mudstone and gravels containing abundant reworked ferromanganese soil nodules. The saprolite is up to 80 m thick. The incised valley deposits are cut by a second erosion surface below which another palaeosol is developed with characteristics of a ferric podzol. Large root marks extend from the top of this palaeosol through the ferruginous silty mudstone channel-fill deposits, into the underlying ferromanganese wad, up to depths of several tens of metres. The root marks are filled with yellow Kalahari sand. The second erosion surface is flat and draped by a poorly sorted pediment stone lag. Stones on the pediment were derived from the underlying channel-fill succession. The pediment is in turn overlain by a dark yellowish brown sandy transported soil (the Hutton soil). Finally the Hutton soil is incised by modern stream erosion and overlain by a thin modern soil covered by grass. The ferromanganese wad in the saprolite at the base of the incised valley succession is mainly composed of amorphous manganese- and iron-oxyhydroxides containing some crystalline birnessite, lithiophorite, and haematite, as well as subordinate quartz, muscovite, and kaolinite. Original sedimentary bedding in the dolomite is partly preserved in the saprolite, although mass balance calculations suggest that the saprolite has undergone 60 to 70% compaction in the transformation of dolomite to wad. Carbonaceous chloritic shale beds interbedded with the Oaktree dolomite are altered to kaolinitic clays in the saprolite. Ferromanganese soil nodules present in the incised valley-fill are composed of a nucleus of older abraded or broken nodules concentrically overgrown by several coatings of cryptomelane ± goethite. The saprolite and overlying erosion surfaces, sediments, and soil profiles provide new information on the Post-Gondwana tectonic and climatic history of the Witwatersrand plateau. This plateau forms the watershed between rivers draining into the Indian and Atlantic Oceans from the Highveld of South Africa. The ferromanganese wad apparently formed after the break-up of Gondwanaland, perhaps during the late Cretaceous to early Tertiary African cycle of erosion. Conditions must have been humid and warm to have allowed deep chemical weathering with wad formation, kaolinitization, and lateritization. The incised valley succession is correlated with the post-African I cycle of erosion, which took place between 30 and 2.5 Ma. During this period the climate became more arid with development of savannah-type vegetation and trees with very deep tap roots. The arid conditions climaxed with the formation of the pediment stone lag on top of the incised valley-fill succession and influx of Kalahari sand in the middle Pliocene. It is thought that the pediment developed at the start of the post-African II cycle of erosion in the Pliocene. After that the climate apparently became more humid, resulting in reworking of Kalahari sediment into the Hutton soil. This soil is at present being incised by stream erosion and covered by grassland below which a thin modern humic soil is developed.

Geochronological aspects of palaeohydrological and palaeoclimatological results from Namibia

Article

Full-text available

Jan 1995

Mebus A Geyh

During the last three decades the Hannover 14C laboratory has performed 14C age determinations of mainly carbonate samples collected in Namibia and South Africa in cooperation with several German Geographical Institutes in order to contribute to the efforts for the establishment of a reliable chronology of the palaeoclimate. Many methodical problems hampered the geochronological interpretation of the dates which may be interconnected with the arid climate of this region. The palaeoclimatological and palaeohydrological information of the accessible dates from Namibia is not yet definite due to methodological reasons and the suitability of the dated material. The experience gathered is presented and recommendations are made for improved sample selection and analysis in order to construct a reliable and precise palaeoclimatological and paleohydrological time scale in future. -from English summary

The Otavi Mountain Land in Namibia: Tsumeb, Germanium and Snowball Earth

Article

Full-text available

Jan 2003

Frank Melcher

Latest Cretaceous to Cenozoic Climate and Oceanographic Developments in the Weddell Sea, Antarctica: an Ocean-Drilling Perspective

Article

Full-text available

Jul 1990

This is a summary of principal findings made by ODP Leg 113 investigators concerning the latest Cretaceous-Cenozoic climatic, cryospheric, and oceanographic history, and biogeographic developments of the Weddell Sea region, Antarctica. During Leg 113, 22 holes were drilled at 9 sites that sampled 4 contrasting environments: open-ocean pelagic sedimentation on Maud Rise (Sites 689 and 690), hemipelagic and terrigenous sediments on the East Antarctic Continental Margin (Sites 691-693), a turbiditic sequence in the deep Weddell Basin (Site 694), and hemipelagic and biogenic sediments on the South Orkney microcontinent (Sites 695-697). A wide range of sedimentologic, biotic, and isotopic evidence obtained in Leg 113 material indicates that sequential cooling and cryospheric development of Antarctica and the surrounding oceans during the Cenozoic profoundly affected the ocean/atmosphere circulation, sediments, and biota. Important cooling steps occurred during the latest Cretaceous, the Middle Eocene, near the Eocene/Oligocene boundary, in the Middle Oligocene, the Middle Miocene, the early late Miocene, the latest Miocene, and the late Pliocene. Distinct but temporary warming trends occurred during the late Paleocene and the latest Oligocene to early Miocene. -from Authors

An 40Ar/39Ar age of supergene K-Mn oxyhydroxides in a post-Gondwana soil profile on the Highveld of South Africa

Article

Full-text available

Oct 1999

Supergene ferromanganese deposits occur widely on the dolomites of the Transvaal Supergroup in the area between Krugersdorp and Lichtenburg in the western Gauteng and Northwest provinces. Most of the deposits occur along a Cretaceous to early Tertiary African land surface of deep weathering and erosion that developed on the subcontinent after the breakup of Gondwanaland some 180 million years ago. Other deposits occur immediately above the post-African I surface of erosion that developed in middle Tertiary times. The absolute age of these land surfaces is unknown. We have now directly dated the timing of formation of a pedogenic manganese crust composed of potassium-bearing cryptomelane from immediately below the post-African I surface of erosion at 12-15 million years using the 40Ar/39Ar technique. Similar crusts are also present in soil nodules immediately above this surface. The results imply a minimum age of 15 million years for the post-African I surface of erosion with abundant formation of manganese nodules and crusts in the soils under lateritic conditions in the period 12-15 million years ago.

Geochemical Patterns of Meteoric Diagenetic Systems and Their Application to Studies of Paleokarst

Chapter

Full-text available

Jan 1988

Kyger C. Lohmann

The isotopic and cation chemistry of meteoric waters changes in response to the effects of rock—water interaction, uptake of organically derived CO2, and primary mineralogic differences among carbonate terranes. Moreover, variations in the dominance of these factors produce diverse chemical conditions within the meteoric systems which allow the sub- environments of vadose-phreatic, mixed-water, and spelean diagenesis to be distinguished. Therefore, geochemical patterns within the meteoric water system are examined to provide criteria for recognition of these subenvironments of meteoric diagenesis in ancient carbonate sequences.

Supergene Ferromanganese Wad Deposits Derived from Permian Karoo Strata along the Late Cretaceous-Mid-Tertiary African Land Surface, Ryedale, South Africa

Article

Full-text available

Jan 2000

A previously unknown type of ferromanganese wad deposit is described at the Ryedale mine situated 110 km west of Johannesburg in the northwestern province of South Africa. The wad was derived from supergene alteration of Glossopteris-bearing Permian strata of the Karoo Supergroup that fill shallow karstic depressions in Neoarchean Malmani dolomite of the Transvaal Supergroup. The depressions contain up to several million tons of friable and highly porous ferromanganese wad with an average Mn/Fe ratio of about 0.3 and high grades of between 77 and 91 wt percent Fe2O3 + MnO. A detailed mineralogical, petrographical, and geochemical study of the orebody suggests that the wad is a saprolitic residue of manganiferous blackband iron ores known to be associated with Permian coal measures of the Karoo Supergroup. The protore was deposited in shallow lakes in preexisting karstic depressions in the Malmani dolomite as finely laminated mud composed of biogenic detritus and Mn-Fe oxyhydroxide precipitates. Siliciclastic detritus is conspicuously absent, suggesting that the lakes were exclusively fed by reducing and acidic ground water that leached manganese and iron from the underlying Malmani dolomite. Anaerobic early diagenesis led to the transformation of oxide precipitates into Mn-Fe carbonates. Much later, during the African cycles of erosion and weathering, the blackband ores were exhumed, partly eroded, and altered by deep lateritic weathering to form the present ferromanganese wad.

Late Neogene passive margin denudation history - Cosmogenic isotope measurements from the central Namib desert

Article

Full-text available

Oct 2001
GLOBAL PLANET CHANGE

In this paper, we review ideas on the geomorphological history of the southwest African passive margin, focusing on the central Namib sector and presenting new evidence on the late Neogene landscape evolution of this region. The hyperarid central Namib Desert occupies the 100–150-km-wide pediment at the foot of the Great Escarpment and forms part of the southwest African passive margin, which formed after breakup in the South Atlantic at around 118 Ma. Previous apatite fission track (AFT) and cosmogenic isotope studies and numerical models of coupled tectonic-surface processes in the same area suggest that long-term denudation rates of this passive margin (after a period of significant post-rift denudation) have been very low, ∼5 m/my.

Post-Gondwana drainage and the development of diamond placers in western South Africa

Article

Full-text available

Aug 1999

MichielC.J. de Wit

The post-Gondwana history of the major rivers in the western part of South Africa is important because these rivers were instrumental in the development of diamond placers along the west coast of southern Africa. The evolution of the drainage systems that developed after breakup of west Gondwana can be viewed in three timeslots: the middle to Late Cretaceous, the early to middle Cenozoic, and the late Cenozoic periods. During the middle to Late Cretaceous there were two main river systems draining the interior. The one in the south, also referred to as the Karoo River, had its source in the present upper Orange/Vaal drainage basin and its outlet was at the present Olifants River mouth. The second and more northerly system, also known as the Kalahari River, drained southern Botswana and Namibia and entered the Atlantic Ocean via the lower Orange River. Erosion dominated the period immediately after breakup of west Gondwana and most of the diamonds released during erosion of Cretaceous kimberlites in central South Africa were transported by the Karoo River to the coast. By early Cenozoic times, the lower Kalahari River had captured the upper part of the Karoo River and the broad configuration of the present Orange River network was established. This capture and northerly shift of the Orange River, on the newly exhumed pre-Karoo surface, was the result of an accelerated uplift of the southern and eastern subcontinental margins ca. 100 to 80 Ma. During the early and middle Cenozoic, the climate was arid to semiarid. This resulted in a substantial reduction in erosion rates and hence few diamonds were released from the primary bodies during that time. Late Cenozoic fluvial gravels, however, dated as either middle Miocene or Plio-Pleistocene, contain diamonds that were reworked out of older Tertiary fluvial deposits. Sediments at the base of the Koa Valley and in the upper terraces in the Sak Valley formed the Koa River, a major tributary of the Orange River during the Miocene, and drained most of the area previously occupied by the lower Karoo River. The Koa River thus reworked diamonds trapped in the Cretaceous Karoo River deposits or terraces. Younger sediments of the Carnarvon Leegte were never part of the Koa system. In fact, the Sak River captured the upper Koa River by late Pliocene times and the Plio-Pleistocene lower terraces in the Sak Valley and the paleo-Carnarvon Leegte joined as the paleo-Hartbees River-another major tributary of the Orange in the Plio-Pleistocene. Although climatic changes were the major controls that initiated the alluvial pulses during the Cenozoic, asymmetric uplift of the subcontinent was ultimately responsible for the northwesterly shift of the Orange River.

Denudation and uplift at passive margins: The record on the Atlantic Margin of southern Africa

Article

Full-text available

Apr 1999
PHILOS T R SOC A

The onshore region of a passive margin forms an integral part of the geological evolution from continental break–up to late sedimentation in the offshore basins. The dominant surface process in the onshore region is denudation, which acts to remov any direct evidence of surface uplift. However, denudation can be constrained on geological time–scales through low temperatur thermochronological data, such as that obtained through apatite fission track analysis. Here, we present a suite of such dat from the Atlantic Margin of southern Africa. The data have been modelled in terms of their temperature histories since th Jurassic. These temperature histories have been combined with heat–flow data to estimate the equivalent depth of denudatio over these time–scales. Average denudation rates are of the order of a few tens of metres per million years, but show considerabl variations both temporally and spatially. These results demonstrate that passive margins experience complex patterns of denudation. Three landscape evolution models are considered. Our results imply the downwarping model is inappropriate. The other two models scarp retreat and pinned drainage divide, predict trends similar to those observed but the complexities inherent in the dat and the evolution of passive margin topography do not allow us to resolve one from the other. In practice, both models probabl operate to some extent as a margin evolves. Estimates of palaeotopography have been made, assuming simple isostatic respons models to denudational unloading. Flexural models with effective elastic thickness (EET) of 25 km predict elevations 2 k and more above the present day values, while models with EET of 0 km predict elevations up to 750 m higher than the present–day. These models ignore any post–break–up tectonic uplift and need independent constraints on surface elevation to assess thei validity.

Variation in Metamorphic Style along the Northern Margin of the Damara Orogen, Namibia

Article

Full-text available

Jun 2004
J PETROL

The northern margin of the Inland Branch of the Pan-African Damara Orogen in Namibia shows dramatic along-strike variation in metamorphic character during convergence between the Congo and Kalahari Cratons (M3 metamorphic cycle). Low-P contact metamorphism with anticlockwise P-T paths dominates in the western domains (Ugab Zone and western Northern Zone), and high-P Barrovian metamorphism with a clockwise P-T path is documented from the easternmost domain (eastern Northern Zone). The sequence of M3 mineral growth in contact aureoles shows early growth of cordierite porphyroblasts that were pseudomorphed to biotite-chlorite-muscovite at the same time as an andalusite-biolite-muscovite transposed foliation was developed in the matrix. The peak-T metamorphic assemblages and fabrics were overprinted by crenulations and retrograde chlorite-muscovite. The KFMASH P-T pseudosection for metopelites in the Ugab Zone and western Northern Zone contact aureoles indicates tight anticlockwise P-T loops through peak metamorphic conditions of 540-570°C and 2·5-3·2 kbar. These semi-quantitative P-T loops are consistent with average P-T calculations using THERMOCALC, which give pooled mean of 556 ± 26°C and 3·2 ± 0·6 kbar, indicating a high average thermal gradient of 50° C/km. In contrast, the eastern Northern Zone experienced deep burial, high-P/moderate-T Barrovian M3 metamorphism with an average thermal gradient of 21°C/km and peak metamorphic conditions of c. 635°C and 8·7 kbar. The calculated P-T pseudosection and garnet compositional isopleths in KFMASH, appropriate for the metopelite sample from this region, document a clockwise P-T path. Early plagioclase-kyanite-biotite parageneses evolved by plagioclase consumption and the growth of garnet to increasing XFe, XMg and XCa and decreasing XMn compositions, indicating steep burial with heating. The developed kyanite-garnet-biotite peak metamorphic parageneses were followed by the resorption of garnet and formation of plagioclase moats, indicating decompression, which was followed by retrogressive cooling and chlorite-muscovite growth. The clockwise P-T loop is consistent with the foreland vergent fold-thrust belt geometry in this part of the northern margin. Earlier formed (580-570 Ma) pervasive matrix foliations (M2) were overprinted by contact metamorphic parageneses (M3) in the aureoles of 530 ± 3 Ma granites in the Ugab Zone and 553-514 Ma granites in the western Northern Zone. Available geochronological data suggest that convergence between the Congo and Kalahari Cratons was essentially coeval in all parts of the northern margin, with similar ages of 535-530 Ma for the main phase of deformation in the eastern Northern Zone and Northern Platform and 538-505 Ma high-grade metamorphism of the Central Zone immediately to the south. Consequently, NNE-SSW-directed convergent deformation and associated M3 metamorphism of contrasting styles are interpreted to be broadly contemporaneous along the length of the northern margin of the Inland Branch. In the west heat transfer was dominated by conduction and externally driven by granites, whereas in the east heat transfer was dominated by advection and internally driven radiogenic heat production. The ultimate cause was along-orogen variation in crustal architecture, including thickness of the passive margin lithosphere and thickness of the overlying sedimentary succession.

First 40 Ar/ 39 Ar geochronology of lateritic manganiferous pisolites: Implications for the Palaeogene history of a West African landscape

Article

Full-text available

Sep 2005
EARTH PLANET SC LETT

A pisolitic superficial formation deriving from the geochemical degradation of a previous massive manganiferous crust has been investigated in West Africa using 40 Ar/ 39 Ar laser probe analysis on different generations of cryptomelane from Fe–Mn pisolites and their embedding lateritic matrices. Prior to the 40 Ar– 39 Ar geochronological analysis, a detailed petrographical study has allowed the identification of successive assemblages from Mn-pisolites to (Fe,Mn)-pisolites embedded in Mn-rich and Fe-rich matrices, respectively. The age spectra obtained for pisolites, including cores, cortices and matrices enables the definition of three age clusters around 56–59, 44–47, and 24–27 Ma. The first two age clusters with the petrographical and geochemical results obtained on the different assemblages indicate that the development of the pisolitic crust first induced Mn-leaching and Fe-Al accumulations due to warmer and wetter climatic conditions in the age interval 56–47 Ma that characterizes the Tertiary greenhouse effect period propitious to bauxite formation in West Africa. The absence of 40 Ar/ 39 Ar ages between 44 and 27 Ma only means that geochemical conditions for cryptomelane crystallisation were not fulfilled, but could still be favourable to Fe-and/or Al-oxy-hydroxides formation, before drier climatic conditions became rather effective at the Oligocene period to sustain mechanical erosion rather than chemical weathering processes. The age cluster 24–27 Ma indicates however a reactivation of the manganiferous lateritic weathering late Oligocene. The 40 Ar/ 39 Ar dating results are discussed in terms of climatic condition changes during the Palaeogene that were favourable to the genesis of either Al-and Fe-or Mn-oxy-hydroxides in the course of development and evolution of the pisolitic formation. The 40 Ar/ 39 Ar dating also provides a new highlight to large-scale geomorphological patterns of West Africa. D 2005 Elsevier B.V. All rights reserved.

A Neoproterozoic Snowball Earth

Article

Full-text available

Sep 1998

Negative carbon isotope anomalies in carbonate rocks bracketing Neoproterozoic glacial deposits in Namibia, combined with estimates of thermal subsidence history, suggest that biological productivity in the surface ocean collapsed for millions of years. This collapse can be explained by a global glaciation (that is, a snowball Earth), which ended abruptly when subaerial volcanic outgassing raised atmospheric carbon dioxide to about 350 times the modern level. The rapid termination would have resulted in a warming of the snowball Earth to extreme greenhouse conditions. The transfer of atmospheric carbon dioxide to the ocean would result in the rapid precipitation of calcium carbonate in warm surface waters, producing the cap carbonate rocks observed globally.

The great West African Tertiary coastal uplift: Fact or fiction? A perspective from the Angolan divergent margin

Article

Full-text available

Dec 2005

1] We explore exhumation in the coastal Kwanza Basin by combining analyses of Tertiary hiatuses and apatite fission tracks. Planktonic biozones show five major hiatuses in the Oligo-Miocene and Plio-Pleistocene. Between gaps, Oligo-Miocene strata accumulated under marine conditions. A marine setting refutes the idea of a massively raised coastal plateau in the mid-Tertiary. Marine conditions continued until $5 Ma. Fission track data suggest three thermal events: $150 Ma, during rifting and volcanism; $100 –70 Ma, during shortening and volcanism; and $20–10 Ma, during exhumation. Tertiary uplift was spatially highly variable. For the Kwanza Basin, we infer that Tertiary uplift on the West African margin is indeed a fact but that estimates of uplift timing and size are unreliable when extrapolated to adjoining areas. Massive uplift (2000–4000 m) of the Precambrian craton had little structural effect in the outer basin. Instead, minor uplifts on the shelf drove late Tertiary deformation on the slope. Citation: (2005), The great West African Tertiary coastal uplift: Fact or fiction? A perspective from the Angolan divergent margin, Tectonics, 24, TC6014, doi:10.1029/2005TC001836.

A major perturbation of the carbon cycle before the Ghaub glaciation (Neoproterozoic) in Namibia: Prelude to snowball Earth?

Article

Full-text available

Jun 2002
GEOCHEM GEOPHY GEOSY

1] A large (11–15%) negative shift in d 13 C is observed in shallow water carbonates directly beneath Neoproterozoic glacial deposits (or correlative disconformity) in northwest Namibia ascribed to a snowball Earth. Reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin, and field relations show it predates the fall in sea level associated with the Ghaub glaciation. We crudely estimate the duration of the isotopic shift as $0.6 Â 10 6 years from a simple thermal subsidence model. Similar or larger d 13 C anomalies are found directly beneath Neoproterozoic glacial units in Australia, Canada, China, Scotland, and Svalbard. After considering conventional interpretations for negative d 13 C anomalies, we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale, magnitude, and geological context of the anomaly in Namibia. Counterintuitively, an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation. Components: 11,873 words, 10 figures. major perturbation of the carbon cycle before the Ghaub glaciation (Neoproterozoic) in Namibia: Prelude to snowball Earth?, Geochem. Geophys. Geosyst., 3(6), 10.1029/ 2001GC000244, 2002.

Lead isotopes of the carbonate-hosted Kabwe, Tsumeb, and Kipushi Pb-Zn-CU sulphide deposits in relation to Pan African orogenesis in the Damaran-Lufilian Fold Belt of Central Africa

Article

Full-text available

Mar 1999

Lead isotope ratios of galena from the carbonate-hosted massive sulphide deposits of Kabwe (Pb-Zn) and Tsumeb (Pb-Zn-Cu) in Zambia and Namibia, respectively, have been measured and found to be homogeneous and characteristic of upper crustal source rocks. Kabwe galena has average isotope ratios of 206/204Pb = 17.997 ± 0.007, 207/204Pb = 15.713 ± 0.010 and 208/204Pb = 38.410 ± 0.033. Tsumeb galena has slightly higher 206/204Pb (18.112 ± 0.035) and slightly lower 207/204Pb (15.674 ± 0.016) and 208/204Pb (38.276 ± 0.073) ratios than Kabwe galena. The isotopic differences are attributed to local differences in the age and composition of the respective source rocks for Kabwe and Tsumeb. The homogeneity of the ore lead in the two epigenetic deposits suggests lead sources of uniform isotopic composition or, alternatively, thorough mixing of lead derived from sources with relatively similar isotopic compositions. Both deposits have relatively high 238U/204Pb ratios of 10.31 and 10.09 for Kabwe and Tsumeb galenas, respectively. These isotope ratios are considered to be typical of the upper continental crust in the Damaran-Lufilian orogenic belt, as also indicated by basement rocks and Cu-Co sulphides in stratiform Katangan metasediments which have a mean μ-value of 10.25 ± 0.12 in the Copperbelt region of Zambia and the Democratic Republic of Congo (formerly Zaire). The 232Th/204Pb isotope ratios of 43.08 and 40.42 for Kabwe and Tsumeb suggest Th-enriched source regions with 232Th/235U (κ-values) of 4.18 and 4.01, respectively. Model isotopic ages determined for the Kabwe (680 Ma) and Tsumeb (530 Ma) deposits indicate that the timing of the mineralisation was probably related to phases of orogenic activity associated with the Pan-African Lufilian and Damaran orogenies, respectively. Galena from the carbonate-hosted Kipushi Cu-Pb-Zn massive sulphide deposit in the Congo also has homogeneous lead isotope ratios, but its isotopic composition is comparable to that of the average global lead evolution curve for conformable massive sulphide deposits. The μ (9.84) and κ (3.69) values indicate a significant mantle component, and the isotopic age of the Kipushi deposit (456 Ma) suggests that the emplacement of the mineralisation was related to a post-tectonic phase of igneous activity in the Lufilian belt. The isotope ratios (206/204Pb, 207/204Pb, 208/204Pb) of the three deposits are markedly different from the heterogeneous lead ratios of the Katangan Cu-Co stratiform mineralisation of the Copperbelt as well as those of the volcanogenic Nampundwe massive pyrite deposit in the Zambezi belt which typically define radiogenic linear trends on lead-lead plots. The host-rock dolomite of the Kabwe deposit also has homogeneous lead isotope ratios identical to the ore galena. This observation indicates contamination of the Kabwe Dolomite Formation with ore lead during mineralisation.

XVIII. Mineralogische und chemische Untersuchung von Descloizit und Mottramit aus dem Otavibergland (Südwestafrika)

Article

Sep 2015

Albert. Diefenbach

Thesis (doctoral)--Albert-Ludwigs-Universität Freiburg im Breisgau, 1930.

Géochronologie Africaine 1956

Article

Jan 1957

Implications of a new 40Ar/39Ar age for a basalt flow interbedded with the Etjo Formation, northeast Namibia

Article

Dec 2003

A reliable 40Ar/39Ar plateau age of 180 ± 1.2 Ma (1σ) has been obtained for fresh basalt lava interbedded with aeolian Etjo Formation sandstones south-southwest of Grootfontein in northeast Namibia. This indicates that the Early Jurassic Karoo flood basalt sequence extended from Botswana into northeast Namibia at least as far as 18 east and that this may mark the eastern extent of the Early Cretaceous Etendeka Igneous Province. If so it would confirm the marked asymmetry of the Paraná-Etendeka Flood Basalt Province relative to the Atlantic Rift as noted by others. This age also supports the correlation of the Etjo Formation of the Waterberg Plateau, Namibia with the Clarens Formation of the Karoo Sequence in South Africa, and the use of the name 'Twyfelfontein Formation' for the aeolianites in the Etendeka region and its incorporation into the Cretaceous Etendeka Group.

Discovery of 12 new late Cainozoic fossiliferous sites in palaeokarsts of the Otavi Mountains, Namibia

Article

Jan 1992

During a field reconnaissance in May-June 1991, several Neogene and Plio-Pleistocene sites, including primate-bearing breccias, were discovered in fissure fillings in Precambrian dolomites in the Otavi Mountains, northern Namibia. -Authors

The isotopic and trace element geochemistry of dolomite: the state of the art.

Article

Jan 1980

LYNTON S. LAND

Mathematical modeling of the diagenetic behavior of the trace element Sr in conjunction with oxygen isotopic changes during the diagenesis of limestone substantiates recent suggestions that experimentally determined distribution coefficients for calcite do not apply under actual diagenetic conditions. An analogous situation probably exists with respect to dolomite. Most ancient dolomite is trace-element-depleted relative to Holocene analogs. Present, quantitative interpretation of absolute isotopic or trace element values of dolomite is tenuous. Because recrystallization is neither completely open nor completely closed-system, chemical information from early events is incompletely erased by later ones. -from Author

The mineralogy and genesis of the lead-zinc-vanadium deposit of Abenab West in the Otavi Mountains, South West Africa

Article

Jan 1957

W.J. Verwoerd

Shortening the odds in exploration

Article

Jan 1981

Of diamonds, dinosaurs and diastrophism: 150 million years of landscape evolution in southern Africa

Article

Sep 1998

T.C. Partridge

The state of knowledge on the tectonic and geomorphological evolution of southern Africa during the post-Gondwana period is reviewed in the context of Alex du Toit's fundamental contributions to these fields of geology. Basic to an understanding of post-rifting events are, firstly, the high elevation which much of Africa possessed prior to rifting; secondly, the erosion of one to three kilometres from its surface during the Cretaceous; and thirdly, the role of Neogene uplift in re-establishing high elevations, particularly within the eastern half of the subcontinent. This history is traced through the massive denudation of the early Cretaceous, which was followed by the establishment of a dense, integrated drainage net on a well-planed land surface from the Santonian onwards. The configuration of the Upper Cretaceous river system is fundamental to a comprehension of the present distribution of alluvial diamonds and of gems transported into the sea via these conduits. Equally significant for an appreciation of the present macro-geomorphology of southern Africa is the continent-wide planation surface - known as the African Surface - generated by the multi-phase cycle of Cretaceous erosion. This surface forms a readily identifiable datum across the high plains because of the widespread preservation of deep weathering and massive cappings of laterite and silcrete on remnants which have survived later dissection. The African silcretes reflect a world-wide shift to greater aridity at the beginning of the Palaeocene. The evidence for large-scale Neogene uplift, particularly within the eastern half of the subcontinent, is now beyond question and argues for the late development of at least the southern part of the African Superswell. The largest movements post-date the Miocene and have contributed both to the anomalous elevations of the eastern hinterland and to the strong east-west climatic gradient across southern Africa. Controversies surrounding the mechanisms underlying these recent movements appear to have been resolved in favour of buoyancy forces originating from a massive low-density anomaly in the Earth's mantle below East and southern Africa.

On the Formation and Occurrence of Saddle Dolomite

Article

Jan 1980

Saddle dolomite is a variety of dolomite that has a warped crystal lattice; it is characterized by curved crystal faces and cleavage, and sweeping extinction. Saddle dolomite occurs as both a void-filling cement and a replacement mineral and is commonly associated with hydrocarbons, epigenetic base-metal mineralization, and sulfate-rich carbonates. These associations imply late diagenetic formation by sulfate reduction processes. Saddle dolomite is slightly enriched in Ca and has significant variations in composition within individual growth laminae. It has potential as a geothermometer, being indicative of elevated temperatures (60-150 oC). -from Authors

The application of stable isotopes to studies of the origin of dolomite and to problems of diagenesis of clastic sediments

Article

Jan 1983

L.S. Land

Demonstrates the application of stable isotope geochemistry to the origin of sedimentary dolomite and diagenesis in clastic systems - clay mineral authigenesis, quartz, carbonate, and zeolite cementation. Case studies on diagenesis illustrate the mode of application of stable isotopic analyses. -from Editor

Precise U-Pb zircon ages for early Damaran magmatism in the Summas Mountains and Welwitschia Inlier, northern Damara Belt, Namibia

Article

Jan 1996

Geomorphological Studies in Southern Africa

Article

Jul 1989

Quantitative Analysis of Homogeneous or Stratified Microvolumes Applying the Model “PAP”

Chapter

Jan 1991

For about 20 years, quantitative analysis of homogeneous microvolumes has been performed with the aid of correction models which transform into mass concentrations C A the ratio k A between the emerging intensities from the specimen and a standard obtained for a characteristic line of element A:

South African Scenery

Article

Dec 1963

Global Geomorphology

Book

Jan 1991

Michael A. Summerfield

The plate tectonics revolution in the earth sciences has provided a valuable new framework for understanding long-term landform development. This innovative text provides a comprehensive introduction to the subject of global geomorphology, with the emphasis placed on large-scale processes and phenomena. Integrating global tectonics into the study of landforms and incorporating planetary geomorphology as a major component the author discusses the impact of climatic change and the role of catastrophic events on landform genesis and includes a comprehensive study of surface geomorphic processes.

Implications of a new 40Ar/ 39Ar age for a basalt flow interbedded with the Etjo Formation, Northeast Namibia

Article

Dec 2003

J. S. Marsh

A reliable 40Ar/39Ar plateau age of 180 ± 1.2 Ma (1σ) has been obtained for fresh basalt lava interbedded with aeolian Etjo Formation sandstones south-southwest of Grootfontein in northeast Namibia. This indicates that the Early Jurassic Karoo flood basalt sequence extended from Botswana into northeast Namibia at least as far as 18 east and that this may mark the eastern extent of the Early Cretaceous Etendeka Igneous Province. If so it would confirm the marked asymmetry of the Paraná-Etendeka Flood Basalt Province relative to the Atlantic Rift as noted by others. This age also supports the correlation of the Etjo Formation of the Waterberg Plateau, Namibia with the Clarens Formation of the Karoo Sequence in South Africa, and the use of the name ‘Twyfelfontein Formation’ for the aeolianites in the Etendeka region and its incorporation into the Cretaceous Etendeka Group.

The Descloizite-Mottramite Series of Vanadates from the Otavi Mountain Land, South West Africa: An X-Ray Study

Article

Mar 1986

W.A. van der Westhuizen

Descloizite, cuprian descloizite, and mottramite from most of the known prospects and mines in the Otavi Mountain Land were sampled and subsequently analysed chemically and by X-ray diffraction. Unit cell parameters were refined with the aid of a computer program. It was found that, contrary to previous opinion, chemical parameters of these vanadates can be deduced from their X-ray powder data according to the equations (correlation coefficients in parenthesis): ZnO = 75.369 c −0.119 b −31.177 a −456.512(0.9634) Zn ²⁺ = 369.568 c + 236.583 b − 146.42 a − 3726.344 (0.9642) CuO = − 74.991 c + 0.086 b + 29.419 a + 486.204 (0.9649) Cu ²⁺ = − 369.568 c − 236.583 b + 146.420 c + 3826.344 (0.9642).

The Identity of Mottramite and Psittacinite with Cupriferous Descloizite

Article

Jan 1933

F. A. Bannister

Concepts and models of dolomitization: A critical reappraisal

Article

Jan 2004

Hans Machel

Despite intensive research over more than 200 years, the origin of dolomite, the mineral and the rock, remains subject to considerable controversy. This is partly because some of the chemical and/or hydrological conditions of dolomite formation are poorly understood, and because petrographic and geochemical data commonly permit more than one genetic interpretation. This paper is a summary and critical appraisal of the state of the art in dolomite research, highlighting its major advances and controversies, especially over the last 20–25 years. The thermodynamic conditions of dolomite formation have been known quite well since the 1970s, and the latest experimental studies essentially confirm earlier results. The kinetics of dolomite formation are still relatively poorly understood, however. The role of sulphate as an inhibitor to dolomite formation has been overrated. Sulphate appears to be an inhibitor only in relatively low-sulphate aqueous solutions, and probably only indirectly. In sulphate-rich solutions it may actually promote dolomite formation. Mass-balance calculations show that large water/rock ratios are required for extensive dolomitization and the formation of massive dolostones. This constraint necessitates advection, which is why all models for the genesis of massive dolostones are essentially hydrological models. The exceptions are environments where carbonate muds or limestones can be dolomitized via diffusion of magnesium from seawater rather than by advection. Replacement of shallow-water limestones, the most common form of dolomitization, results in a series of distinctive textures that form in a sequential manner with progressive degrees of dolomitization, i.e. matrix-selective replacement, overdolomitization, formation of vugs and moulds, emplacement of up to 20 vol% calcium sulphate in the case of seawater dolomitization, formation of two dolomite populations, and — in the case of advanced burial — formation of saddle dolomite. In addition, dolomite dissolution, including karstification, is to be expected in cases of influx of formation waters that are dilute, acidic, or both. Many dolostones, especially at greater depths, have higher porosities than limestones, and this may be the result of several processes, i.e. mole-per-mole replacement, dissolution of unreplaced calcite as part of the dolomitization process, dissolution of dolomite due to acidification of the pore waters, fluid mixing (mischungskorrosion), and thermochemical sulphate reduction. There also are several processes that destroy porosity, most commonly dolomite and calcium sulphate cementation. These processes vary in importance from place to place. For this reason, generalizations about the porosity and permeability development of dolostones are difficult, and these parameters have to be investigated on a case-by-case basis. A wide range of geochemical methods may be used to characterize dolomites and dolostones, and to decipher their origin. The most widely used methods are the analysis and interpretation of stable isotopes (O, C), Sr isotopes, trace elements, and fluid inclusions. Under favourable circumstances some of these parameters can be used to determine the direction of fluid flow during dolomitization. The extent of recrystallization in dolomites and dolostones is much disputed, yet extremely important for geochemical interpretations. Dolomites that originally form very close to the surface and from evaporitic brines tend to recrystallize with time and during burial. Those dolomites that originally form at several hundred to a few thousand metres depth commonly show little or no evidence of recrystallization. Traditionally, dolomitization models in near-surface and shallow diagenetic settings are defined and/or based on water chemistry, but on hydrology in burial diagenetic settings. In this paper, however, the various dolomite models are placed into appropriate diagenetic settings. Penecontemporaneous dolomites form almost syndepositionally as a normal consequence of the geochemical conditions prevailing in the environment of deposition. There are many such settings, and most commonly they form only a few per cent of microcrystalline dolomite(s). Many, if not most, penecontemporaneous dolomites appear to have formed through the mediation of microbes. Virtually all volumetrically large, replacive dolostone bodies are post-depositional and formed during some degree of burial. The viability of the many models for dolomitization in such settings is variable. Massive dolomitization by freshwater-seawater mixing is a myth. Mixing zones tend to form caves without or, at best, with very small amounts of dolomite. The role of coastal mixing zones with respect to dolomitization may be that of a hydrological pump for seawater dolomitization. Reflux dolomitization, most commonly by mesohaline brines that originated from seawater evaporation, is capable of pervasively dolomitizing entire carbonate platforms. However, the extent of dolomitization varies strongly with the extent and duration of evaporation and flooding, and with the subsurface permeability distribution. Complete dolomitization of carbonate platforms appears possible only under favourable circumstances. Similarly, thermal convection in open half-cells (Kohout convection), most commonly by seawater or slightly modified seawater, can form massive dolostones under favourable circumstances, whereas thermal convection in closed cells cannot. Compaction flow cannot form massive dolostones, unless it is funnelled, which may be more common than generally recognized. Neither topography driven flow nor tectonically induced (‘squeegee-type’) flow is likely to form massive dolostones, except under unusual circumstances. Hydrothermal dolomitization may occur in a variety of subsurface diagenetic settings, but has been significantly overrated. It commonly forms massive dolostones that are localized around faults, but regional or basin-wide dolomitization is not hydrothermal. The regionally extensive dolostones of the Bahamas (Cenozoic), western Canada and Ireland (Palaeozoic), and Israel (Mesozoic) probably formed from seawater that was ‘pumped’ through these sequences by thermal convection, reflux, funnelled compaction, or a combination thereof. For such platform settings flushed with seawater, geochemical data and numerical modelling suggest that most dolomites form(ed) at temperatures around 50–80 °C commensurate with depths of 500 to a maximum of 2000 m. The resulting dolostones can be classified both as seawater dolomites and as burial dolomites. This ambiguity is a consequence of the historical evolution of dolomite research.

U-Pb zircon date from the Neoproterozoic Ghaub Formation, Namibia: Constraints on Marinoan glaciation

Article

Sep 2004
GEOLOGY

Dropstone-bearing glaciomarine sedimentary rocks of the Ghaub Formation within metamorphosed Neoproterozoic basinal strata (Swakop Group) in central Namibia contain interbedded mafic lava flows and thin felsic ash beds. U-Pb zircon geochronology of an ash layer constrains the deposition of the glaciomarine sediments to 635.5 ± 1.2 Ma, providing an age for what has been described as a “Marinoan-type” glaciation. In addition, this age provides a maximum limit for the proposed lower boundary of the terminal Proterozoic (Ediacaran) system and period. Combined with reliable age constraints from other Neoproterozoic glacial units—the ca. 713 Ma Gubrah Member (Oman) and the 580 Ma Gaskiers Formation (Newfoundland)—these data provide unequivocal evidence for at least three, temporally discrete, glacial episodes during Neoproterozoic time with interglacial periods, characterized by prolonged positive δ13C excursions, lasting at most ˜50 80 m.y.

The composition of descloizite-mottramite in relation to the trace-element distribution of Pb, Zn, Cu and V in the Otavi Mountain Land, South West Africa/Namibia

Article

Aug 1989
J GEOCHEM EXPLOR

The carbonate rocks of the Otavi Group (ca. 800 Ma) in the northeastern part of South West Africa/Namibia are known to contain Pb, Zn and Cu sulphide deposits with the mining activities at Tsumeb, Kombat and Berg Aukas being the most prominent. Vanadium mineralisation, associated with karst development, is often associated with the sulphide deposits, but occurrences without a sulphide association are also present.The distribution of Pb, Zn, Cu and V in the dolomite and limestone of the Otavi Mountain Land delineates geochemical regions which coincide with the above mineralisation. This coincidence is also manifest in the type of V mineralisation; where high concentrations of Zn occur in the country rocks, descloizite predominates, whilst mottramite occurs when the Cu values are higher.

Measurements relating to the genesis of the Tsumeb pipe, South West Africa

Article

Nov 1970
EARTH PLANET SC LETT

The Tsumeb ore-body is one of a number of steep pipe-like structures that pierce the Otavi Series of the Damara System in South West Africa. These structures lie on a well-defined ENE belt of ring-complexes; included in the latter are carbonatites. Strontium isotope and La and Ce analyses were carried out on 13 carbonate rocks taken from within the pipe and country-rocks. The mean87Sr/86Sr value of ≈0.712 combined with the low La and Ce contents of all the samples does not support a carbonatitic origin for the pipe.The problem of the age of the Tsumeb pipe remains unresolved. The most likely events that could be responsible for the development of the pipe are the post-kinematic Damara granites of Cambrian age or alternatively they could be associated with the magmatic activity responsible for the emplacement of the post-Triassic ring complexes.

An overview of carbonate-hosted mineral deposits in the Otavi Mountain Land, Namibia: implications for ore genesis

Article

Apr 1993
J AFR EARTH SCI

A model of metallogenesis for the carbonate-hosted base metal deposits of the Otavi Mountain Land (Namibia) is proposed based on stratigraphic, mineralogical, metal association, Pb and S isotope systematics and fluid inclusion data. Three mineralising episodes are recognised. The first involved low-temperature and high-salinity basinal fluids, which formed the Berg Aukas-type deposits of Mississippi Valley-Type (MVT) affinity. These are characterised by a ZnPb metal association. The second episode appears to have involved Cu-rich fluids having higher temperatures and lower salinities of possible metamorphic origin. These formed Tsumeb-type deposi ts, characterised by a metal association of CuPbZnAg, locally accompanied by FeMn oxides and silicates. The third and last episode is related to recent weathering, which resulted in the precipitation of V-rich minerals.

Geomorphic evolution of Southern Africa since the Mesozoic

Article

Jan 1987

A map of erosion surfaces, and contour plans of elevations on the more important of these, form the basis of this paper. A consistent datum is provided by the oldest of the surfaces (the African) through the diagnostic deep weathering profiles and duri-crust cappings associated with it. The Great Escarpment was formed following continental rifting. There is conclusive evidence for the co-existence of surfaces of the same age at different elevations inland of and below the Great Escarpment. The onshore evidence of erosion surfaces is correlated with recent data on offshore sedimentation, and reveals that a single cycle of erosion prevailed from the time of rifting to the early Miocene. By the end of this period a gentle pediplain (the African surface) extended across most of S Africa at elevations of 500-600m. Most erosion and scarp recession produced thick late Jurassic and Cretaceous sedimentary sequences, but shelf sedimentation declined during the Tertiary and had virtually ceased by the Oligocene. Modest renewed uplift of 150-300m in the Miocene tilted the continent slightly to the W and initiated a new (Post-African I) landscape cycle. The cycle was terminated near the end of the Pliocene and its relatively short duration resulted in imperfect planation in most areas to levels of no >100-300m below the African surface. A 2nd uplift of major proportions at the end of the Pliocene raised the E interior of the sub-continent by as much as 900m. Major monoclinal warping resulted in the SE hinterland. The ensuing Post-African II cycle is manifested chiefly in deep incision of the coastal hinterland and downcutting along major rivers of the interior. Earlier surfaces were severely deformed and dissected. The resulting sedimentation is evident mainly in the offshore deltas of major rivers. The successive uplifts to which the continent was subject produced major changes in drainage patterns. The preservation of discrete erosion surfaces with separating escarpments over considerable periods not only confirms the episodic nature of uplifts, but indicates a dominance of backwearing over downwearing in the geomorphic evolution of the sub-continent. Some thoughts are offered towards the formulation of a tectonic model which takes account of the evolutionary scheme now established for S Africa. -from Authors

Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with 40Ar/39Ar dating

Article

Apr 2004
GEOCHIM COSMOCHIM AC

(U-Th)/He chronometry of zircon has a wide range of potential applications including thermochronometry, provided the temperature sensitivity (e.g., closure temperature) of the system be accurately constrained. We have examined the characteristics of He loss from zircon in a series of step-heating diffusion experiments, and compared zircon (U-Th)/He ages with other thermochronometric constraints from plutonic rocks. Diffusion experiments on zircons with varying ages and U-Th contents yield Arrhenius relationships which, after about 5% He release, indicate Ea = 163–173 kJ/mol (39–41 kcal/mol), and D0 = 0.09–1.5 cm2/s, with an average Ea of 169 ± 3.8 kJ/mol (40.4 ± 0.9 kcal/mol) and average D0 of 0.46+0.87−0.30 cm2/s. The experiments also suggest a correspondence between diffusion domain size and grain size. For effective grain radius of 60 μm and cooling rate of 10°C/myr, the diffusion data yield closure temperatures, Tc, of 171–196°C, with an average of 183°C. The early stages of step heating experiments show complications in the form of decreasing apparent diffusivity with successive heating steps, but these are essentially absent in later stages, after about 5–10% He release. These effects are independent of radiation dosage and are also unlikely to be due to intracrystalline He zonation. Regardless of the physical origin, this non-Arrhenius behavior is similar to predictions based on degassing of multiple diffusion domains, with only a small proportion (

(U-Th)/He Dating: Techniques, Calibrations, and Applications

Article

Jan 2002

Kenneth A. Farley

The possibility of dating minerals by the accumulation of ^4He from U and Th decay has been recognized for many years (e.g., Strutt 1905), but in the century since the idea was first conceived, the method has rarely been applied successfully. After several investigations of (U-Th)/He dating of various minerals (e.g., Damon and Kulp 1957; Fanale and Kulp 1962; Damon and Green 1963; Turekian et al. 1970; Bender 1973; Leventhal 1975; Ferreira et al. 1975) the technique was essentially abandoned as yielding unreliable and usually low ages, presumably as a result of diffusive He loss possibly associated with radiation damage. In 1987, Zeitler and coworkers rekindled interest in the method by proposing that in the case of apatite, He ages might be meaningfully interpreted as ages of cooling through very low temperatures. Laboratory diffusion data presented by these authors indicated a closure temperature of about 100ºC, a value supported by more recent studies (Lippolt et al. 1994; Wolf et al. 1996b; Warnock et al. 1997). Consistent with this interpretation Wolf et al. (1996a) found that apatite He ages increase systematically with sample elevation in a mountain range, as expected for exhumation-induced cooling through a low closure temperature. Based on the strength of these results and additional laboratory (Farley 2000) and natural (Warnock et al. 1997; House et al. 1999; Stockli et al. 2000) constraints on He diffusivity, recent attention has focused on applications of apatite He thermochronometry. There is also renewed interest in He dating of other U- and Th-bearing minerals both for dating mineral formation and for thermochronometry. For example, Lippolt and coworkers have undertaken detailed studies of He diffusion and dating of various phases, most notably hematite formed in hydrothermal systems (Lippolt and Weigel 1988; Wernicke and Lippolt 1992; Lippolt et al. 1993; Wernicke and Lippolt 1994a,b). Here I present an overview of recent techniques, calibrations, and applications of the (U-Th)/He dating method; Hurley (1954) provides an excellent summary of earlier work in this field. Much of this paper focuses on apatite, because the He behavior and requisite analytical techniques are better established for this phase than for other target minerals, such as zircon and titanite. Similarly, much of this paper concerns He diffusivity behavior required for thermochronometric applications, yet recent work is also considering applications to direct dating, for example, of young tephras (Farley et al. 2001).

SREX. A new process for the extraction and recovery of strontium from acidic nuclear waste streams

Article

Feb 1991

A new process for the extraction and recovery of strontium from acidic waste streams is described. In this process, called SREX (for Strontium Extraction), strontium is extracted from acidic (greater-than-or-equal-to 1 M HNO3) solution using a 0.20 M solution of di-t-butylcyclohexano-18-crown-6 in 1-octanol. Extracted strontium is readily stripped from the organic phase using either water or dilute (< 0.05 M) HNO3. Tests of the process on a synthetic dissolved sludge waste solution show that only strontium, barium, and technetium are appreciably extracted by the crown ether. Prolonged exposure of the process solvent to nitric acid at elevated temperatures or to less-than-or-equal-to 50 Wh/L gamma radiation from a Co-60 source produces essentially no deterioration in its performance. Benchtop batch countercurrent extraction experiments show that 99.7% of the strontium initially present in a feed solution can be removed in only three extraction stages.

A Neoproterozoic Snowball Earth

Article

Aug 1998

Paul F. Hoffman

Isotopic dating of low-grade metamorphic shales in northern Namibia (South West Africa) and implications for the orogenic evolution of the Pan-African Damara-Belt

Article

Nov 1979
PRECAMBRIAN RES

The fine mineral fractions (< 2 μm) from pelitic sediments of the molasse-type Mulden Group in the Pan-African Damara belt of Namibia were dated by the Rb-Sr and K-Ar methods. The sediments contain two major parageneses of metamorphic origin which can be related to two separate low-grade regional tectono-thermal events of anchizonal intensity at about 535 (545) Ma∗ and 455 (465) Ma respectively.A depositional model age for the top of the Mulden Group is about 550–560 (565–570) Ma and it is possible that the Precambrian-Cambrian boundary in northern Namibia may be placed at the base of the Mulden sequence.The above metamorphic events are reflected by mineral and whole-rock ages throughout the Damara belt and are therefore of regional significance. They are part of Clifford's (1967) Damaran Episode which is now more complex than previously suggested and which postdates an earlier tectonic event of preMulden age, provisionally dated at about 650 (665) Ma.We propose to abandon the use of the terms Damaran and Katangan episodes in their original connotation for two chronologically and regionally distinct Pan African events as defined by Clifford (1967) and suggest to restrict these names to the tectono-thermal domains of the Damara and Katanga belts whose orogenic development was broadly coeval.

Fundamentals of Noble Gas Thermochronometry

Article

Jan 2005

The ideal geochronometer would be a universally stable phase that quantitatively retains both parent and daughter isotopes. Though a few mineral systems such as zircon U-Pb dating come reasonably close to this ideal, most minerals are incompletely retentive of daughter-product nuclides under crustal conditions. The mechanisms by which the daughter product can be lost from minerals include dissolution–reprecipitation reactions (e.g., salt; Obradovich et al. 1982), recrystallization (e.g., micas undergoing deformation; Chopin and Maluski 1980), and diffusive loss (e.g., 40Ar degassing of K-feldspar; Foland 1974). The latter mechanism is perhaps the most common source of discrepancy between a radiometric mineral date and the age of the rock from which it formed. Geochronologists have learned to turn this non-ideal behavior to their advantage and we now understand that most mineral ages from exhumed crustal rocks act in effect as kinetic thermometers sensitive to geologically-induced thermal effects. Such apparent ages are a measure of the temperature range over which daughter product ceased to be lost from a crystal, with intracrystalline diffusion usually acting as the rate-limiting process. Consider the case in which a mineral sample containing a radioactive parent element experiences a complex thermal evolution, possibly involving heating as well as cooling. Within the sample, daughter product is continually produced by radioactive decay and lost by diffusion at natural boundaries. Although random at the scale of an individual atom, both diffusion and radioactive decay are highly predictable processes over longer and larger scales involving many particles. Coupled with the strong temperature dependence of diffusion, the elegant mathematics of the production-diffusion relationship make it possible to recover information about the thermal history experienced by such a sample, simply by knowing the amount of daughter product remaining in the mineral following cooling, or even better, by knowing the distribution of daughter product within the …

Determination of Uranium and Thorium in Zircon, Apatite, and Fluorite: Application to Laser (U-Th)/He Thermochronology

Article

Dec 2005

We have developed a methodology for (U-Th)/He thermochronology on a variety of mineral species. With many laboratories initiating research in the area of (U-Th)/He thermochronology, we recognize that there may be interest in a review of analytical procedures for uranium and thorium determination in single crystals of apatite, zircon, rutile, and fluorite. Uranium and thorium are both determined by inductively coupled plasma mass-spectrometry using an isotope dilution method. While standard and spike solutions can be purchased, their isotopic composition and the concentration of the standard solution need to be verified. Digestion procedures for apatite and fluorite are relatively straightforward, but zircon decomposition requires the use of pressure vessels or fusion. Matrix effects are shown to have an insignificant effect on isotope ratios, although isobaric interferences, particularly of PtAr+ on U isotopes, can be a problem. We include complete thermochronology datasets for replicate analysis of Durango apatite, Yucca Mountain fluorite, and an Australian megacryst zircon.

An Eburnean base metal source for sediment-hosted zinc-lead deposits in Neoproterozoic units of Namibia: Lead isotopic and geochemical evidence

Article

May 2004

The lead isotopic composition of galena from the Neoproterozoic sediment-hosted Zn-Pb sulphide deposit at Rosh Pinah and the oxidised Zn deposit at Skorpion in the Pan-African Gariep Belt, southwestern Namibia, as well as that of galena from minor occurrences in the wider Rosh Pinah ore province was investigated and is compared with that of other major sediment-hosted base metal ore deposits hosted by Neoproterozoic strata in southwestern Africa and Brazil. The isotope data were supplemented by a geochemical provenance study of the argillitic host rock to the Rosh Pinah deposit and its stratigraphic equivalents. The pre-orogenic Rosh Pinah deposit and the inferred progenitor for the secondary Skorpion deposits have very similar Pb isotopic compositions. In contrast, syn-orogenic deposits (e.g. Tsumeb-type) show a larger proportion of radiogenic Pb from the country rocks that were infiltrated by orogenic ore fluids. In all examples studied, the ore Pb has elevated 207Pb/204Pb ratios compared to Pb that evolved according to average crustal Pb growth models. The isotopic composition of the ore Pb is in agreement with that of the oldest crustal component known from the pre-Gariep basement, i.e. an Eburnean (c. 2.0 Ga) volcanic arc, best preserved in the Richtersveld Terrane, that represents the largest volume of post-Archaean juvenile crust in southern Africa. Erosion of that arc provided the main sediment source for the metasedimentary siliciclastic host to the Rosh Pinah deposit. Derivation of the Pb, and by analogy Zn, from this Eburnean volcanic arc is therefore inferred for the formation of Rosh Pinah-type syn-rift, early diagenetic replacement mineralisation. The calculated μ2- and ω2-values for the Rosh Pinah ore province (around 10.2 and 42, respectively) are higher than predicted by conventional Pb crustal growth models and are similar to those found in the wider region of southern Africa (Otavi Mountain Land) and eastern Brazil. The obtained data highlight not only that calculated Pb model ages may have no geochronological significance but might also point to a common crustal evolution of this part of SW-Gondwana.

The role of evaporites in the genesis of base metal sulphide mineralisation in the Northern Platform of the Pan-African Damara Belt, Namibia: Geochemical and fluid inclusion evidence from carbonate wall rock alteration

Article

Apr 2000

The Otavi Mountain Land is a base metal sulphide ore province in northern Namibia where deposits are hosted by platform carbonates of the Otavi Group in a foreland fold-and-thrust belt on the northern edge of the Pan-African Damara Belt. Deposits have been classified as the Berg Aukas- or Tsumeb-types, based on differences in ore association, stratigraphic position and geochemistry of ores and gangue carbonates. Mineralisation at these deposits is accompanied by carbonate alteration in the form of dolomite and calcite veins, carbonate recrystallisation, calcitisation and carbonate silicification. Based on cathodoluminescence imaging, trace and rare earth element (REE), O and C isotope, and fluid inclusion data, a series of carbonate generations, constituting wall rock alteration around the Tsumeb and Kombat (Tsumeb-type) and Berg Aukas (Berg Aukas-type) deposits, was established. Similar data obtained on the recently discovered Khusib Springs deposit indicate a strong affinity to Tsumeb-type deposits. Tsumeb-type deposits are distinguished from Berg Aukas-type deposits by having trace element and REE concentrations that are significantly higher in the alteration products compared to the carbonate host rocks. Only around Tsumeb-type deposits a relative enrichment in light REE is noted for the hydrothermal carbonate generations that are cogenetic with the main stage of mineralisation. Microthermometric results from fluid inclusions in carbonate alteration phases and associated quartz indicate relatively high salinity (17–23 wt% NaCl equivalent) for the main mineralising and subsequent sulphide remobilisation stages at the deposits investigated. Estimated mineralisation temperatures are significantly higher for Tsumeb-type deposits (370–405 °C) with early sulphide remobilisation in Tsumeb at 275 °C, whereas they are lower at Berg Aukas (up to 255 °C). Fluid inclusion leachate analysis suggests that most of the observed salinity can be ascribed to dissolved, predominantly Ca- and Mg-carbonates and chlorides with subordinate NaCl. Na-Cl-Br leachate systematics indicate a derivation of the fluid salinity from the interaction with evaporitic rocks en route. Tsumeb-type mineralisation is interpreted to be derived from fluids expelled during Pan-African orogeny in the more intensely deformed internal zones of the Damara Belt further south. When the high salinity fluids reached the carbonate platform after having scavenged high concentrations of base metals, base metal sulphide precipitation occurred in zones of high porosity, provided by karst features in the carbonate sequence. Results obtained for the Berg Aukas-type deposits emphasise their derivation from basinal brines, similar to Mississippi Valley-type deposits, and confirm that mineralisation of the Berg Aukas- and Tsumeb-types are both spatially and temporally distinct.

Genesis of Vanadium Ores in the Otavi Mountainland, Namibia

Abstract

No full-text available

Recommended publications

Ion microprobe U-Pb ages on detrital zircon grains from the Ghanzi Group: Implications for the ident...

Measuring Baseline Agriculture-Related Sustainable Development Goals Index for Southern Africa

Three New Species of Brachystelma (Apocynaceae, Asclepiadoideae, Ceropegieae) from South Tropical an...

A study of the variability of the river flow of the southern Africa region