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Victorian Gold: A Sleeping Giant
... Recent studies have demonstrated that alteration around many turbidite-hosted mesothermal gold deposits is more widespread and pervasive than previ-Ž ously thought Crouzet and Tollon, 1980;Stuwe eẗ al., 1988;Annels and Roberts, 1989;Kontak et al., 1990;Miller et al., 1994;Phillips and Hughes, 1995; . Kwak and Li, 1996 . ...
... biquitous feature. However, the intensity and extent of carbonate spotting varies considerably and ranges from several metres at Maldon to kilome-Ž tre wide haloes at Avoca Sandiford and Keays, . 1986 . Although referred to in numerous publications Že.g. Sandiford and Keays, 1986;Binns and Eames, 1989;Cox et al., 1991bCox et al., , 1995Forde, 1991;Phillips . and Hughes, 1995;Molloy et al., 1995 , to date no ( ) detailed study has been carried out into the nature of these carbonate spots. Considering their potential significance as an indicator for mineralisation and pervasive wallrock alteration, a closer look at these enigmatic spots is warranted. ...
The study of wallrock alteration phenomena around slate belt-hosted gold deposits can provide valuable information regarding the physico-chemical conditions of the ore-bearing fluids and the timing relationships between deformation, peak metamorphism, granitoid emplacement and mineralisation. Thus, studying the processes involved in the interaction of the hydrothermal fluids with the host rock is critical in order to constrain genetic models and, ultimately, as a tool for exploration. Largely due to the characteristics of the host rocks, turbidite-hosted mesothermal gold deposits were historically believed to lack such diagnostic alteration haloes and their study has long been neglected. However, recent studies in the Palaeozoic Lachlan Fold Belt in southeastern Australia and in similar tectonic settings elsewhere have demonstrated that alteration around many of these deposits is more widespread and pervasive than previously thought. A review of existing information and new data strongly support the viability of studying hydrothermal alteration as a guide to turbidite-hosted mineralisation and as an indicator of the character of the solutions associated with ore deposition.
... The veins vary in thickness from 2 cm to 1 m and are developed along a strike length of approximately 4 km. The style of the Ondundu mineralisation is very similar to the saddle reef deposits of the Slate Belt in Victoria (Australia; Ramsay and Vandenberg, 1990;Phillips and Hughes, 1995). Jacob (1988, 1991) observed that many ofthe hydrothermal Au deposits in the Damara Orogen are located within isograds defining areas of medium-to high-grade regional high-temperature, low-pressure metamorphism, and that they are distributed around positive gravity anomalies ( Fig. 9.8). ...
Preface. Acknowledgments. Introduction. Part One. 1. The Earth's Internal Structure and Convection in the Mantle. 2. Mantle Plumes and Superplumes, Continental Breakups, Supercontinent Cycles and Ore Deposits. 3. Oceanic Islands, Large Igneous Provinces, Mafic Dyke Swarms, and Intracontinental Alkaline Magmatism. 4. Rifting Processes, Volcano-Sedimentary Basins and the Role of Mantle Plumes. 5. The Planetary and Meteorite Impact Context of Mantle Plumes. Part Two. 6. Intra-Continental Magmatism, Anorogenic Metamorphism, Ore Systems and Mantle Plumes. 7. Direct Links: Magmatic Ore Deposits - Fundamental Features and Concepts. 8. Magmatic Ore Deposits. 9. Indirect Links: Hydrothermal Mineral Deposits. 10. Indirect Links: Sedimentary Rock-Hosted Ore Deposits. Epilogue. Appendix. Index.
... Whereas the majority of the deposits in this anticlinal structure have many Carlin-like features, they are also dominantly distributed along ductile to brittle shear zones. In this latter aspect, they also partly resemble the turbiditehosted orogenic lode-gold deposits in Victoria, Australia (Phillips and Hughes 1995;Ramsay et al. 1998). Along the Mainlue fault, the large Dashui deposit (50 t Au) is hosted by a Triassic carbonate sequence and appears more similar to most recognized Carlin-like deposits. ...
Gold deposits of the western Qinling belt occur within the western part of the Qinling-Dabie-Sulu orogen, which is located between the Precambrian North China and Yangtze cratons and east of the Songpan-Ganzi basin. The early Paleozoic to early Mesozoic orogen can be divided into northern, central, and southern zones, separated by the Shangdan and Lixian-Shanyang thrust fault systems. The northern zone consists of an early Paleozoic arc accreted to the North China craton by ca. 450 Ma. The central zone, which contains numerous orogenic gold deposits, is dominated by clastic rocks formed in a late Paleozoic basin between the converging cratonic blocks. The southern zone is characterized by the easternmost exposure of Triassic sedimentary rocks of the Songpan-Ganzi basin. These Early to Late Triassic turbidities, in part calcareous, of the immense Songpan-Ganzi basin also border the western Qinling belt to the west. Carlin-like gold deposits are abundant (1) along a westward extension of the southern zone defined by a window of early Paleozoic clastic rocks extending into the basin, and (2) within the easternmost margin of the basinal rocks to the south of the extension, and in adjacent cover rocks of the Yangtze craton. Triassic and Early Jurassic synkinematic granitoids are widespread across the western Qinling belt, as well as in the Songpan-Ganzi basin. Orogenic lode gold deposits along brittle-ductile shear zones occur within greenschist-facies, highly-deformed, Devonian and younger clastic rocks of the central zone. Mainly coarse-grained gold, along with pyrite, pyrrhotite, arsenopyrite, and minor base metal sulfides, occur in networks of quartz veinlets, brecciated wall rock, and are disseminated in altered wall rock. Isotopic dates suggest that the deposits formed during the Late Triassic to Middle Jurassic as the leading edge of the Yangtze craton was thrust beneath rocks of the western Qinling belt. Many gold-bearing placers are distributed along the river systems that flow south from the lode-bearing central zone. Carlin-like gold deposits have only been identified during the last decade in the southern zone of the western Qinling and in the northeastern corner of the Songpan-Ganzi basin. The deposits mainly contain micron-diameter gold in arsenical pyrite; are characterized by the common occurrence of cinnabar, stibnite, realgar, and orpiment; exhibit strong silicification, carbonatization, pyritization, and decalcification dissolution textures; and are structurally controlled. The lack of reactive host lithologies may have prevented development of large (>100 tonnes of gold), stratigraphically-controlled orebodies, which are typical of the Carlin deposits in the western USA. These deposits are hosted by Triassic turbidities and shallow-water carbonates, and an early Paleozoic inlier in the Songpan-Ganzi basin that extends in an east-west belt for about 300 km. Rather than true "Carlin" deposits, these Carlin-like deposits may be some type of shallow-crustal (i.e., epithermal) hybrid with features intermediate to Nevada-style Carlin deposits and the orogenic gold deposits to the immediate north. These Carlin-like deposits also overlap in age with the early Mesozoic orogenic gold deposits and, therefore, also formed during the final stages of collision between the cratons and intermediate basin closure.
... Ž 5.5 million oz being the largest of these Phillips . and Hughes, 1995 . These three districts, most of the other one million ounce districts, and many of the smaller ones are within what has been informally termed the Ballarat zone of central Victoria. ...
More than 420 million oz of gold were concentrated in circum-Pacific synorogenic quartz lodes mainly during two periods of continental growth, one along the Gondwanan margin in the Palaeozoic and the other in the northern Pacific basin between 170 and 50 Ma. These ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary-rock dominant terranes. The auriferous veins contain only a few percent sulphide minerals, have gold:silver ratios typically greater than 1:1, show a distinct association with medium grade metamorphic rocks, and may be associated with large-scale fault zones. Ore fluids are consistently of low salinity and are CO2-rich.
The path to a major discovery is far from uniform, and every discovery is different. Anecdotally it is about the seventh owner of a property who makes the big discovery, and to extend this tale, it is important not to be amongst the first six, nor waiting in line to be number eight. The siting of the so-called discovery hole is greatly over-rated in its importance, and the critical moments may have been much earlier and possibly protracted. The critical moment can even come well after drilling—there are multiple examples of a discovery having been drilled and not recognised until a new owner comes along. In this setting, the challenge of management is to engender conviction well before the big intersection is obvious and announced to the world, and this is best done using all scales of geology. Fosterville Deeps, in the Victorian Gold Province, is a major gold-only discovery in the 21st century, and the input of three decades of external research played a significant role in understanding the opportunities at depth. Transfer of the research findings led to a change of direction of the company and a major discovery.
In Chapters 2, 3 and 4 we have examined how doming of the crust, its rupturing and formation of a rift basin are processes that can be linked to mantle plumes. The East African Rift System, and its Red Sea and Gulf of Aden extensions are a modern example of this phenomenon; the Mid-Continent Rift System in North America is an ancient example. Another ancient example is the Damara-Ribeira rift systems, which subsequently evolved to ocean floor spreading, resulting in the separation of South America from southwestern Africa (Damara hotspot junction, discussed in Chapter 4).
The Palaeozoic succession of Victoria represents a major world gold province with a total production of 2500 t of gold (i.e. 78 million oz). On a global scale, central Victoria represents one of the most gold mineralized areas outside the Witwatersrand of South Africa, and remains the prime example of a ‘slate belt’ gold province (also known as ‘turbidite-hosted’, or ‘shale-greywacke’ gold province). Production came from quartz veins (1000 t), modern placers (1200 t) and palaeoplacers (300 t). Up to 7000 mines produced gold from quartz veins; however, the 168 mines of this type which produced over 1 t Au contributed 68% of the primary gold production. Twelve goldfields have each produced 30 t Au (approximately one million ounces) from all sources, with Bendigo (697 t), Ballarat (408 t) and Castlemaine (173 t) being the largest: these twelve goldfields have contributed 71% of the combined primary and secondary production. Gold mining commenced in 1851, declined dramatically around 1914, and has not increased significantly since. The Palaeozoic succession of Victoria is part of an extensive fold belt along much of eastern Australia (i.e. the Tasman Orogenic Belt). In Victoria, the succession is dominated by Cambrian to Early Devonian clastic metasedimentary rocks that have undergone major upright deformation and low-grade metamorphism, culminating in the Middle Devonian (Tabberabberan) Orogeny. North-south inliers of Cambrian metavolcanic- and metasedimentary rocks associated with regional fault zones divide the province into several geologically distinct zones. In central Victoria, there are particularly well-developed acid volcanic complexes of Late Devonian age, and synchronous peraluminous granites which represent high-temperature crustal melting. Tertiary to Quaternary basalts have covered some of the Palaeozoic gold deposits, and have also covered several rich gold placer deposits of Cainozoic age. Larger goldfields, including Bendigo and Ballarat, occur in Ordovician slate and greywacke sequences, but a wide range of metamorphosed sedimentary and igneous rock types are host to goldfields. Most, if not all, primary deposits demonstrate structural control, although the controlling structural features differ between deposits. Wallrock alteration is strongly influenced by host rock: in metasedimentary rocks, it is limited and subtle, but it is more pervasive in mafic and felsic igneous rocks. Carbonates, muscovite and pyrite are the most widespread alteration minerals in these rocks, representing CO2, K and S addition. Enrichment of As within gold deposits is common, with Sb important in some small deposits, whereas base metals are only abundant locally, and Bi, W, Mo and Te show strong spatial association with granites and are rarely associated with economic gold deposits. Granites are an important component of the Palaeozoic succession in Victoria. No large deposits are within granites, but a few goldfields, including Maldon (65 t Au), are within contact aureoles of either S or I type granites of differing degrees of fractionation and silica content. A single-stage, but protracted and possibly diachronous, period of gold introduction near the time of the Middle Devonian (Tabberabberan) deformation can explain geological relationships at many gold deposits. The possibility that this event was slightly older (i.e. Silurian or Early Devonian) in the west of the state cannot be precluded from the available evidence. The mineralizing event appears linked to the same thermal event as diorite (‘lamprophyre’) dykes and graite intrusions, acidic and basaltic volcanism, deformation and regional metamorphism. The limited amount of fluid chemistry data from Victorian gold deposits suggests low-salinity fluids which contained CO2, and variable methane, with homogenization temperatures from 150 to 300°C. The sulphur isotopic compositions of pyrite are mostly +1 to +5‰, with some values slightly above + 10‰ Mineralizing temperatures are inferred to be near 300°C, and pressure around 1–2 kbar. Sulphide-rich gold deposits in the east of Victoria, many of which contain significant base metals, contrast with the rest of the province (which contains mostly gold deposits with lower sulphide content), and this eastern area should possibly be viewed as separate from the Victorian gold province, per se. The benefits to Victorian gold mining coming from the 1980's global gold boom have been insignificant compared to the historical importance of the Victorian gold province. There seems little justification for assuming that all the large deposits have been discovered, and there is reason to believe new deposit styles may emerge to complement the limited current exploration activity which is focussed on the search for extensions and repeats of known deposit types. A major advance in scientific understanding of these oft neglected deposits could kindle a major exploration and production revival.
World Gold 91, Aust. Inst. Min. Metal., Melbourne, 237-245.
This batholith is a high-level discordant, composite granitic intrusion in Victoria, Australia. Textural evidence indicates early crystallization of biotite (6 analyses), spinel (1), garnet (3), plagioclase and some quartz, with the bulk of the quartz occurring later together with cordierite (7 analyses) and K-feldspar. Modal analyses of six rock types and whole-rock analyses of seven rock types with corresponding CIPW norms are also given. The low inclusion content and other evidence suggest that partial melting of sedimentary rocks did not produce the batholith, even though the rocks are S-type granitic rocks. The rocks are believed to have originated through partial melting caused by intrusion of mafic magma into the lower crust (M.A. 80-3880)-S.A.K.
Experiments were carried out on four compositions from central Victoria, Australia, S-type volcanic plutonic suites in the ranges 700o-900oC, 1-7 kbar, a H2 = 0-1.0, fO2 = or < QFM, but the results of only one composition, Strathbogie (granite 889), are reported in detail (chemical and modal analyses). Early crystallization of low-F biotite with plagioclase (up to 3 kbar) and quartz (at higher P) requires > 2.5 wt.% H2O in the melt. A wide field of crystallization for orthopyroxene was found, but its rarity in S-type granitic rocks is probably due to peritectic reaction to biotite with decreasing T. Cordierite crystallizes near the liquidus at 1-2 kbar, but does not develop above the solidus at P = or > 4-5 kbar. Thus, cordierite is not likely to be a restite phase. Almandine-rich garnet becomes a near-liquidus phase at P = or > 4-5 kbar. Integration of geothermometric and geobarometric calculations and crystallization sequences indicates that early-crystallizing garnet, biotite, orthopyroxene and plagioclase are present at T = or < 850oC, P = or > 4 kbar, in melts with 3-5 wt.% H2O. Experiments also showed that Al2SiO5-saturation is unnecessary to produce mildly peraluminous melts; thus mildly peraluminous sedimentary rocks may produce peraluminous melts. It is believed that peraluminous melts are undersaturated with respect to H2O, which is derived from hydrous minerals, principally precursor biotite. In less aluminous precursor rocks biotite may persist to higher T. Biotite-orthopyroxene partial melting in granulite-facies rocks in the range 5-8 kbar yields granitic liquids with 3-5 wt.% H2O. Experiments with rhyodacite starting materials produced peraluminous glasses (3 analyses) in quenched runs from 750o-850oC.-S.A.K.
Mesothermal, gold-bearing quartz veins are widespread within allochthonous terranes of Alaska that are composed dominantly of greenschist-facies metasedimentary rocks. The most productive lode deposits are concentrated in south-central and southeastern Alaska; small and generally nonproductive gold-bearing veins occur upstream from major placer deposits in interior and northern Alaska. Oreforming fluids in all areas are consistent with derivation from metamorphic devolatilisation reactions, and a close temporal relationship exists between high-T tectonic deformation, igneous activity, and gold mineralization. Ore fluids were of consistently low salinity, CO2-rich, and had 6~SO values of 7%o12%o and 6D values between -15%o and -35%o. Upper-crustal temperatures within the metamorphosed terranes reached at least 450-500 ~ before onset of significant gold-forming hydrothermal activity. Within interior and northern Alaska, latest Paleozoic through Early Cretaceous contractional deformation was characterised by obduction of oceanic crust, low-T/high-P metamorphism, and a lack of gold vein formation. Mid-Cretaceous veining occurred some 50-100 m.y. later, during a subsequent high-T metamorphic/magmatic event, possibly related to extension and uplift. In southern Alaska, gold deposits formed during latter stages of Tertiary, subduction-related, collisional orogenesis and were often temporally coeval with calc-alkaline magmatism. KEYWO~DS: mesothermal gold, Alaska, fluid inclusion~ stable isotope, geochronology, orogensis.
The succession, metamorphism, geochronology and major structure of the Mona Complex, Arvonian and Lower Palaeozoic rocks of N Wales are outlined. This results in a model which relates early (pre-Ordovician) NW-verging structures to the closing of an early Caledonian ocean and obduction of deep water sediments, slide breccia and basic volcanic rocks on to the sediments previously metamorphosed, locally at high temperatures, and intruded by granites in central and N Anglesey.
The pattern of folds, cleavages, thrusts and finite strain states in N Wales are described. A maximum of 43 km shortening between the Menai Straits and the Welsh Borderlands is estimated across the more highly deformed part of central Snowdonia, during end-Silurian deformation. The amount of shortening decreases to the northeast (Denbighshire Basin) and south (Harlech Dome) ends of the arc. This is attributed to compression of Snowdonia against a rigid block situated in the Berwyn Hills. The SE-verging structures are thought to pass downwards, with listric geometry, into a decollement zone at some unknown depth in the basement, which accommodated Caledonian crustal shortening across Wales, and must itself either descend into the mantle or root in any Caledonian suture beneath the Irish Sea.
Low-salinity, high-temperature (>200°C), reducing fluids are poorly represented in ore-forming processes, yet appear to account for 90% of the primary gold mined to date. Metamorphism of chlorite-calcite-albite-quartz assemblages at the greenschist-amphibolite facies boundary occurs around 480° ± 20°C (for 3-5 kbar) and produces large volumes of low-salinity, H 2O-CO 2 fluid similar in composition to those recorded in many gold deposits. Widespread pyrite in the above assemblage typically leads to elevated levels of dissolved sulfur. Such conditions are ideal for gold transport as a molecular Au-S complex and can lead to deposits rich in gold relative to silver and base metals. Low-salinity fluids produced by metamorphic devolatilization provide a common theme embracing the formation of many Archean greenstone, slate belt, Witwatersrand, and potentially epithermal gold deposits. -from Authors