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The various circulating waters which deposit hydrothermal minerals to form quartz veins include: shallow circulating meteoric waters which might deposit only barren quartz with adularia or quartz after platy calcite, composite meteoric-magmatic waters which have entrained metals to deposit lower grade Au mineralisation as disseminated sulphides within quartz veins, and magmatic dominated hydrothermal fluids which deposit the well mineralised sulphide dominant vein portions. The ground waters which contribute towards mixing reactions include: shallow oxygenated meteoric waters, blankets of bicarbonate waters related to CO 2 exsolved from a cooling dacite dome, and low pH waters derived by the oxidation of H 2 S
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Although the early geological literature proposed both mixing and boiling as mechanisms for Au deposition, there has been a more recent emphasis upon mainly boiling to account for Au deposition in low sulphidation epithermal deposits. Extensive field studies and analysis of magmatic arc (Philippine) geothermal systems support theoretical considerat...
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... of high grade ore deposition, typically by reactivation of the host dilatant structures (Corbett and Leach, 1998) and other factors such as host rock competency and style of mineralisation (Corbett, 2007) also contribute towards the development bonanza Au grades. Ore shoots characterised by elevated Au grades may develop at structural intersection such as where ore fluids rising up a normal fault may mix with low pH fluids collapsing down a hanging wall splay ( figure 2; Corbett, 2007). An analogous situation occurs at the Palinpinon geothermal field, Philippines (figure 2.12, Corbett and Leach, 1998) where acid sulphate waters collapsing down a fault zone mixed with pregnant fluids rising up a geothermal drill hole, which became blocked with scale containing base metal sulphides and electrum (Leach, unpubl data;pers. ...
Citations
... Wang et al. 2019). Furthermore, the presence of minerals such as haematite and carbonate, often accompanied by various clays within mineralized zones, can be interpreted as magmatic fluids mixing with oxidized meteoric fluids (Leach 2008). This simultaneous presence highlights the contribution of meteoric fluids to the Bahçecik mineralization. ...
... Intermediate sulfidation epithermal deposits consisting of a mixture of magmatic and meteoric fluids in andesitic rocks that may be cogenetic with deeper porphyry-related intrusions indicate the presence of Fe-poor light sphalerite, while sphalerites in high-sulfidation epithermal deposits are rich in Fe (Shahbazi et al., 2019). Some minerals such as hematite, kaolinite, and carbonate, suggesting that magmatic fluids were mixed with oxidized meteoric water (Leach and Corbett, 2008), are also observed in the Daghkesemen Au-bearing polymetallic deposit. Accordingly, high-sulfidation (HS) epithermal deposits are predominantly magmatic, whereas low-sulfidation (LS) deposits are predominantly meteoric fluids. ...
The Daghkesemen Au-bearing polymetallic vein-type deposit, which is the most important ore deposit of the Kazakh graben located in the northwestern part of the Lesser Caucasus, is hosted by calc-alkaline volcanic rocks (andesite, dacite, basaltic trachyandesite, trachyandesite, rhyodacite and their pyroclastics), subvolcanic rocks (andesite porphyry, dolerite, albitophyre/rhyodacite porphyry), and volcanic breccias. The mineralization is commonly observed as gold-bearing quartz-sphalerite-galena-chalcopyrite veins. The Daghkesemen Au-bearing polymetallic deposit, mostly observed in the propylitic alteration, is commonly composed of primarily sphalerite, galena, chalcopyrite, pyrite, native gold, bornite and magnetite, and secondarily digenite, cerussite, covellite, chalcocite, malachite, azurite, hematite, jarosite, and anglesite. The Daghkesemen Au-bearing polymetallic deposit mostly comprises Zn, Pb, Cu, Au, Ag, Fe and Cd. Homogenization temperatures (Th) of the fluid inclusions in sphalerite and quartz range from 200.1 °C and 281.2 °C and 266.4–266.7 °C, respectively. The low salinities for the fluid inclusions in sphalerite and quartz show a range of 0.7–6.6 wt. % NaCl equivalent and 2.7 to 3.1 wt. % NaCl equivalent, respectively. Eutectic temperatures measured in the fluid inclusions vary between −27.4 and −18.2 °C, indicating H2O–NaCl–KCl system. The δ³⁴SH2S values (−0.8 to −4.4‰) of sulfides (chalcopyrite, sphalerite, galena and pyrite) suggest a magmatic-hydrothermal origin for sulfur. The calculated sulfur isotopic geothermometer is value of 190 °C for sphalerite-chalcopyrite pair. The geochemical, mineralogical, sulfur isotopic and fluid inclusion data of the Daghkesemen Au-bearing polymetallic deposit suggest an intermediate sulfidation epithermal deposit associated with volcanic rocks.
... revealed the essential role of sharp temperature gradients for the Comstock Lode Au-Ag deposit, Nevada (Vikre, 1989) and of boiling caused by sudden decompression due to hydrothermal brecciation for the Apacheta Au-Ag deposit, Peru (André-Mayer et al., 2002). However, exploration surveys of numerous Au-Ag deposits have indicated that cooling and mixing of fluids were more important than boiling for ore deposition (Leach and Corbett, 2008). A combination of processes (mixing, boiling, and cooling) was indicated for the Kencana Au-Ag deposit, Indonesia, in which mixing was inferred to be particularly important (Clark et al., 2018). ...
... The homogenization temperature of gangue quartz was as low as approximately 160 °C. These findings suggest the downward flow of acidic water through the LPAH cap into the vein fluid and a mixing of acidic water with hydrothermal fluid (Leach and Corbett, 2008). The H 2 S released from boiling fluids during the middle stage is the most plausible cause of the acidification of the groundwater. ...
... The numerical simulation supported the concept that a cold-water trap, the mixing of hydrothermal fluids with cold groundwater, was essential to induce early stage mineralization in the Fuji vein. The mixing of ascending fluids with various types of groundwater, such as oxygenated groundwater, bicarbonate water, and acid sulfate water, induces effective Au mineralization (Leach and Corbett, 2008). The presence of oxygenated groundwater was suggested by the proximity of the mixing front to the deep aquifer in the early stage. ...
Abstract
Although low-sulfidation epithermal deposits, including the Hosokura deposit (Ag–Pb–Zn) in the Miyagi Prefecture, northern Japan, are globally significant metal sources, their ore deposition mechanisms are not thoroughly understood. This study aimed to construct a genetic ore deposition model for this deposit by focusing on the richest vein, the Fuji, and test the model using a numerical simulation of the formation and transitions of this fossil geothermal system. It was proposed that the mixing of neutral hydrothermal fluids (~260°C) ascending through the vein with oxygenated cold groundwater caused early stage mineralization and the transport of hydrothermal fluids into the surrounding host rocks, forming a hydrothermal alteration halo around the vein that results in silicification, adularia alteration, and argillization. The progress of this alteration decreased the permeability of the halo and generated a mushroom-shaped low-permeability alteration halo (LPAH) in permeable zones surrounding the vein. The low-permeability and insulating properties of the LPAH increased the fluid temperature, the vein fluid boiled, and consequently the middle stage mineralization occurred. After a further decrease in LPAH permeability, hydrothermal brecciation resulting from increasing fluid pressure occurred at the top of the vein, resulting in late-stage mineralization. A TOUGH2 numerical simulation tested these proposed processes: early stage cooling of hydrothermal fluids by mixing with groundwater, the development of vapor in the vein by the formation of an LPAH during the middle stage, and an increase in fluid pressure leading to hydrothermal brecciation during the late-stage. The results of the simulation confirmed that a cold-water trap is an essential process for the generation of low-sulfidation epithermal deposits. Mushroom-shaped LPAHs, silicification zones, and brecciated veins caused by cold-water traps have been found in representative low-sulfidation epithermal deposits in Japan, such as the richest Au deposit, the Hishikari. Therefore, identification of cold-water-trap footprints can contribute to the exploration of this type of deposit.
... Ore shoots characterized by elevated Au grades (including IS and LS) may develop at structural intersection where ore fluids may rise up along a normal fault and mix with acid sulfate fluids (as exemplified by hypogene kaolinite/dickite in IS veins in Mexico, Camprubí and Albinson, 2007) collapsing down along a hanging wall splay (Fig. 2 in Leach and Corbett, 2008). ...
... Gold of lower fineness commonly forms at shallow epithermal levels (Corbett and Leach, 1998;White, 1981). By contrast, gold deposited under low pH conditions will have a high fineness, similar to the high sulphidation deposits in the southwest Pacific (Leach and Corbett, 2008). ...
The gold deposits within the Latimojong Metamorphic Complex of Sulawesi, Indonesia, including Awak Mas and Salu Bulo, are estimated to host 50 tonnes Au with an average grade of 1.41 g/t. They are located within the metamorphic basement consisting of pumpellyite- to greenschist-facies metasedimentary and metavolcanic rocks, where gold precipitated in quartz veins that fill north-south striking normal faults and extensional fractures. The mineral assemblage is dominated by pyrite, chalcopyrite, galena, minor tetrahedrite-tennantite and sphalerite; gold is electrum with a low silver content (Au:Ag ratio of 8.5 to 9.1). Albite, dolomite-ankerite, siderite, chlorite, and white mica are the main alteration minerals. Quartz, albite and carbonate veins hosting H2O-bearing fluid inclusions with minor aqueous-carbonic phases (CO2 ± N2) were detected (mole fraction <0.15). Raman microspectroscopy, microthermometry, and crush leach analysis of gold-bearing quartz veins and associated host rocks provide evidence for processes resembling those described for epizonal gold mineralization. The gold bearing fluids have salinities between 1.4 and 7.3 eq. mass% NaCl and were trapped in quartz at about 180–250 °C and <1.27 kbar, corresponding to depths less than 5 km. Trapping conditions of barren veins are about 190–390 °C and <1.15 kbar with salinities ranging from 2.2 and 6.1 eq. mass% NaCl. Halogen and alkali ratios (Na/Cl/Br/I) from crush leach analyses correspond to deposits originating from metamorphic fluids with a strong albitization signature during ore formation. Isothermal decompression during the retrogression stage mobilized large volumes of fluids, leading to significant gold mineralization within the Awak Mas District.
... Boiling, mixing and/or cooling of the hydrothermal solutions are considered the main mechanisms controlling ore deposition in shallow epithermal environments (e.g., Hedenquist et al., 2000;Simmons et al., 2005;Leach and Corbett, 2008). Among these three, most researchers recognize boiling as the main process affecting the pH, fluid composition and mineral solubility in the hydrothermal solutions, and therefore controlling ore deposition (e.g., Skinner, 1997;Simmons et al., 2005;Canet et al., 2011;Moncada et al., 2012). ...
... Boiling is also the most cited ore-controlling process in the low-to intermediate-sulfidation epithermal veins from the Deseado Massif (e.g., Cerro Vanguardia Mine, Manantial Espejo Mine, Cerro Negro Project, La Josefina Project; see Schalamuk et al., 2005;Echavarría et al., 2005;Wallier, 2009;Permuy Vidal, 2014;Moreira and Fernández, 2014). Mineralogical and fluid inclusion evidence of a boiling system includes ore-bearing colloform-banded textures with amorphous silica, platy calcite (commonly later replaced by quartz), adularia, and coexisting liquid-rich and vapor-rich fluid inclusions, among others (Hedenquist et al., 2000;Simmons et al., 2005;Leach and Corbett, 2008). Except for the presence of widespread adularia across most of the paragenetic scheme (stages 3 to 7, Fig. 3), veins at the Martha mine lacks most of these features, suggesting non-boiling conditions for mineral deposition during the main hydrothermal phase. ...
... Cooling and dilution involving different types of meteoric-dominant waters (e.g., deep-circulating ground waters, shallow oxygenated ground waters, bicarbonate waters and low pH acid sulfate waters; see Leach and Corbett, 2008) have been reported as an important orecontrolling process in epithermal deposits such as Veta Madre (Mexico, Mango et al., 2014), Buckskin Mountain (USA, Vikre, 2007), Creede district (USA, Plumlee, 1994), Tayoltita (Mexico, Conrad et al., 1992), and several other low-sulfidation epithermal deposits around the world (Hedenquist and Lowenstern, 1994;Hedenquist et al., 2000;Simmons et al., 2005;Leach and Corbett, 2008), however this represents the first mention of these processes for any epithermal deposit at the Deseado Massif. ...
The complexity of high-grade ore-shoot formation is addressed through a detailed study of the Martha mine epithermal deposit (Deseado Massif, Patagonia, Argentina). The mine was active from 2002 to 2012, producing over 21 million oz of silver from an intermediate sulfidation vein system hosted in a pyroclastic sequence erupted during Upper Jurassic times (40Ar–39Ar age: 157.6 ± 1 Ma). The Martha deposit is composed of more than 15 different veins, 8 of which show high grade ore-shoots (locally up to 45,000 g/t Ag). Veins are hosted in a sinistral transtensive horst structure delimited by two NW-SE trending first-order veins (up to 600 m long and 5 m thick). Inside this horst, a series of E-W trending veins (up to 450 m long and 2.5 m thick) developed as second-order structures in response to growth and linkage of the two overlapping first-order vein structures. Ore is characterized by a complex paragenetic sequence of Ag-As-Sb sulfosalts and Cu-Pb-Zn sulfides, with an adularia-quartz-illite gangue. Hydrothermal alteration is zoned outward and rarely extends for more than 15 m away from the vein walls; five alteration zones were defined: an adularia zone close to the veins, an intermediate halo characterized by illite and illite-smectite zones, and a distal smectite zone, additionally a late and supergene kaolinite zone can be found near the surface in close association with the mineralized structures. High grade “bonanza” ore-shoots are the result of the interplay of a series of structural, hydrothermal (mostly physicochemical) and supergene processes. The mineralization evolved through three main phases: a main hydrothermal tectonically controlled phase, a late tectonic-hydrothermal phase and a post-mineral supergene phase. The main hydrothermal phase was the result of mineral deposition from low salinity (0.5 to 3.5 wt. % NaCl eq.) near-neutral to weakly alkaline chloride aqueous hydrothermal solutions with estimated temperatures ranging from 215.5° to 316.5° C, and isotopic composition suggesting high proportions of meteoric waters (δ18OH2O from -8.1‰ to -2.7‰). Relatively constant fluid inclusion liquid-to-vapor ratios and a trend of decreasing Th and δ18OH2O with increasing time suggest that cooling and dilution were the main mechanisms controlling ore deposition. These physicochemical conditions, combined with an efficient network of structural conduits (vein intersections, jogs and step-over zones) were the first step in the construction of the high grade ore shoots that characterize the vein system. 40Ar–39Ar dating in adularia crystals constrains the hydrothermal activity to the Upper Jurassic (156.5 ± 0.9 Ma). Afterwards, a late tectonic-hydrothermal phase occurred through the development of irregularly-shaped bodies of massive fault breccias and foliated cataclasites related to tectonic reactivations of the vein system in synchronism with the waning stages of the hydrothermal system. The interplay between deformation and fluid circulation resulted in mechanical and chemical remobilization of the previously deposited Ag-rich sulfosalts by lower temperature (138° to 168 ° C) and higher salinity fluids (7.7 and 9.9 wt. %. NaCl eq.), possibly reflecting a late magmatic input into the waning system. This late tectonic-hydrothermal phase strongly contributes to increase the grades in some veins, and therefore is considered an essential process in order to build the highest-grade portions of the ore-shoots found at the mine. Finally, a post-mineral supergene phase defined a 20 m thick oxidized zone close to the surface, followed by a 20-30 m thick secondary enrichment zone below it, developed in response to the circulation of descending cold meteoric waters. Despite its small size and vertical extension, the secondary enrichment level at Martha mine constitutes an important economic factor for the mining operations as it helped to increase the silver grades in the shallower portion of the vein system. The Martha mine vein system constitutes an excellent example of how the interplay of hydrothermal processes controlling mineral deposition, structural factors building up an efficient plumbing system, tectonic ore-remobilization, and late supergene secondary enrichment processes, are combined in order to build “bonanza”-grade ore shoots in the epithermal environment.
... In addition, the data in Fig. 14 support the existence of a miscibility gap in fahlores first predicted by O' Leary and Sack (1987) and confirmed by Sack (2005) and Chutas and Sack (2004). Precipitation of ore minerals due to decreasing temperature in hydrothermal systems is generally the result of either conductive loss of heat from the system or from mixing with a lower temperature fluid ( Leach and Corbett, 2008). Cooling has also been described in some deposit as the main deposition process (e.g. ...
The Patricia ore deposit represents an unusual example of economic Zn-Pb-Ag mineralization at the northernmost end of the Late Eocene-Oligocene metallogenic belt in Chile. It is hosted by volcano-sedimentary units, which are typically tuffaceous and andesitic breccias. The ore body consists of a set of subvertical E-W vein systems developed under a sinistral strike-slip regime that included transtensive domains with generalized extensional structures where the ores were deposited. The deposit is divided into two blocks by a set of NNW-ESE-trending reverse faults, which uplifted the eastern block and exhumed thicker and deeper parts of the deposit. At least 200 m of volcanosedimentary pile hosting the mineralization has been eroded in this block. By contrast, the western block exposes a shallower part of the system where cherts, amorphous silica and jasperoids occur. Three main stages of mineralization have been defined: (1) pre-ore stage is characterized by early quartz, pyrite and arsenopyrite, (2) base-metal and silver stage; characterized by sphalerite (6 to 15
mol.% FeS), galena, chalcopyrite, pyrrhotite and Ag-bearing minerals (freibergite, polybasite, stephanite, pyrargyrite, freieslebenite and acanthite) and (3) post-ore stage; characterized by late quartz, kutnohorite and minor sulfides (arsenopyrite, sphalerite, pyrite, galena, Ag-bearing minerals and Pb-sulfosalts). Whole-ore geochemistry shows two groups of elements that are positively correlated; 1) Ag-Cd-Cu-Pb-Zn related to the base metal sulfides and 2) Au-As-Ge-Sb-W related to arsenopyrite and pyrite. Hydrothermal alteration is pervasive in the outcropping mineralized areas, including silicification and locally, vuggy silica textures. At depth, chloritic and sericitic alteration is developed along vein selvages and is superimposed to the regional propylitic alteration. Fluid inclusions indicate that the base-metal ores were deposited from 250-150º C moderate salinity fluids (1-9 wt % NaCl). The pre-ore stage is characterized by a saline fluid (6-22 wt % NaCl) and between 210 and 250ºC whereas the post-ore stage has salinity of 4-8 wt % and temperature from 175 to 215ºC. Cooling was the mechanism of ore mineral precipitation in the Patricia deposit, although mixing of
fluids could have occurred in the pre-ore stage. Mineralogical, geochemical and fluid inclusion evidence is consistent with an intermediate sulfidation (IS) epithermal deposit type. This study highlights the high potential for hidden economic mineralization at depth in the western block and for extension of the ore body both to the south and to deeper levels in the eastern block of the Patricia ore deposit.
The Sofía-Julia-Valencia vein system, located in the Andacollo mining district in central west Argentina, is hosted by ENE-WSW oriented strike-slip faults which are the result of reactivation of normal faults affecting Carboniferous to Jurassic rocks during Upper Cretaceous-Paleogene. These veins contain a total resource of 22,900 Oz of gold with 5.5-6.7 g/t AuEq. Geologic mapping and a U-Pb age of 71±1Ma in zircon, obtained in an altered and mineralized dacitic dyke of the district, allowed to associate the mineralizing event to the Naunauco Andesitic belt magmatism (Upper Cretaceous-Paleogene) and to the Cretaceous-Paleogene Metallogenic Belt of the Andes in southwestern Argentina. The ore bodies are made up of multiple veins and veinlets that, from oldest to youngest, correspond to: (1) scarce early quartz+pyrite+molybdenite+iron poor-sphalerite veinlets, (2) quartz+epidote+calcite±albite (apatite+rutile+titanite+light rare earth elements bearing phosphates) associated with quartz+biotite, epidote (actinolite)+chlorite+calcite, with pyrite+pyrrhotite±chalcopyrite±(iron rich-sphalerite), marcasite veins. These veins are cut and reopened by (3) polymetallic veins and veinlets formed by quartz+sericite±carbonates (chlorite), with iron-gold rich sphalerite+silver rich-galena+chalcopyrite+pyrite, native gold±arsenopyrite±(pyrrhotite, bornite, argentite). Pyrite (4) and (5) carbonate+framboidal pyrite veinlets cuts all the previous ones. Multistage carbonate generation brecciate and cut previous veins and veinlets. Quartz shows granular, comb textures and some calcites developed platy textures. Four hydrothermal alteration types affected the veins host rock: (1) patches of early potassic alteration; (2) widespread propylitic alteration with disseminated sulfides; (3) later phyllic alteration overlapped to the previous ones; and (4) late supergene alteration. The sphalerite and chlorite composition in the veins (1 and 2) along with their mineral assamblages indicates they were formed by initially alkaline fluids (e.g., feldspar stable) with intermediate sulfur and oxygen fugacity and mesothermal temperature conditions (~400-240 °C), that evolved to conditions of lower sulfur (e.g., pyrrhotite stable) and oxygen fugacity, temperature
