Exploration for Epithermal Gold Deposits

Reviews in Economic Geology 01/2000; 13:245-277.


The successful exploration geologist uses knowledge of geologic relationships and ore-deposit styles, tempered by experience, to interpret all information available from a given prospect in order to develop an understanding of its mineral potential. In the case of exploration for epithermal gold deposits, this understanding can be augmented by familiarity with active hydrothermal systems, their present-day ana-logues. Just as geological skills and exploration experience are the defining elements of a philosophy of exploration, the needs of a company determine, as much as the funding and skills available, which level of exploration it pursues and where: grassroots, early-stage or advanced targets. Epithermal gold deposits have size, geometry, and grade variations that can be broadly organized around some genetic classes and, therefore, influence the exploration approach or philosophy. Nearly 80 years ago, Waldemar Lindgren defined the epithermal environment as being shallow in depth, typically hosting deposits of Au, Ag, and base metals plus Hg, Sb, S, kaolinite, alunite, and silica. Even before this, Ransome recognized two distinct styles of such precious-metal deposits, leading to the conclusion that the two end-member deposits form in environments analogous to geothermal springs and volcanic fumaroles, which are dominated by reduced, neutral-pH versus oxidized, acidic fluids, re-spectively. The terms we use are low-and high-sulfidation to refer to deposits formed in these respective environments. The terms are based on the sulfidation state of the sulfide assemblage. End-member low-sulfidation deposits contain pyrite-pyrrhotite-arsenopyrite and high Fe sphalerite, in contrast to pyrite-enargite-luzonite-covellite typifying highsulfidation deposits. A subset of the low-sulfidation style has an inter-mediate sullidation-state assemblage of pyrite-tetrahedrite/tennantite-chalcopyrite and low Fe sphalerite. Intermediate sulfidation-state deposits are Ag and base metal-rich compared to the Au-rich end-member low-sulfidation deposits, most likely reflecting salinity variations. There are characteristic mineral textures and assemblages associated with epithermal deposits and, coupled with fluid inclusion data, they indicate that most low-sulfidation and high-sulfidation deposits form in a temperature range of about 160" to 270°C. This temperature interval corresponds to a depth below the paleowater table of about 50 to 700 m, respectively, given the common evidence for boiling within epithermal ore zones. Boiling is the process that most favors precipitation of bisulfide-complexed metals such as gold. This process and the concomitant rapid cooling also result in many related features, such as gangue-mineral deposition of quartz with a colloform texture, adularia and bladed calcite in low-sulfidation deposits, and the formation of steam-heated waters that create advanced argillic alteration blankets in both low-sulfidation and high-sulfidation deposits. Epithermal deposits are extremely variable in form, and much of this variability is caused by strong permeability differences in the near-surface environment, resulting from lithologic, structural, and hydra thermal controls. Low-sulfidation deposits typically vary from vein through stockwork to disseminated forms. Gold ore in low-sulfidation deposits is commonly associated with quartz and adularia, plus calcite or sericite, as the major gangue minerals. The alteration halos to the zone of ore, particularly in vein deposits, include a variety of temperature-sensitive clay minerals that can help to indicate locations of paleofluid flow. The areal extent of such clay alteration may be two orders of magnitude larger than the actual ore deposit. In contrast, a silicic core of leached, residual silica is the principal host of high-sulfidation ore. Outward from this commonly vuggy quartz core is a typically upward-flaring advanced argillic zone consisting of hypogene quartz-alunite and kaolin minerals, in places with pyrophyllite, diaspore, or zunyite. The deposit form varies from disseminations or replacements to veins, stockworks, and hydrothermal breccia. During initial assessment of a prospect, the first goal is to determine if it is epithermal, and if so, its style, low-sulfidation or high-sulfidation. Other essential determinations are: (1) the origin of advanced argillic %orresponding author: e-mail, 245 246 HDENQUIST ET AL.. alteration, (i.e., hypogene, steam-heated, or supergene), (2) the origin of silicic alteration (e.g., residual silica or silicification), and (3) the likely controls on grade (i.e., the potential form of the orebody), be-cause this is one of the most basic characteristics of any deposit. These determinations will define in part the questions to be asked, such as the relationship between alteration zoning and the potential ore zone, and will guide further exploration and eventual drilling, if warranted. Observations in the field must focus on the geologic setting and structural controls, alteration mineralogy and textures, geochemical anomalies, etc. Erosion and weathering must also be considered, the latter masking ore in places but potentially improving the ore quality through oxidation. As information is compiled, reconstruction of the topography and, hence, hydraulic gradient during hydrothermal activity, combined with identifica-tion of the zones of paleofluid flow, will help to identify ore targets. Geophysical data, when interpreted carefully in the appropriate geological and geochemical context, may provide valuable information to aid drilling by identifying, for example, resistive and/or chargeable areas. The potential for a variety of related deposits in epithermal districts has exploration implications. For example, there is clear evidence for a spatial, and in some cases genetic relationship between high-sulfidation epithermal deposits and underlying or adjacent porphyry deposits. Similarly, there is increasing recognition of the potential for economic intermediate sulfidation-state base metal k Au-Ag veins adjacent to high-sulfidation deposits. By contrast, end-member low-sulfidation deposits appear to form in a geologic envi-ronment incompatible with porphyry or high-sulfidation deposits of any economic significance. The expla-nation for these empirical metallogenic relationships may be found in the characteristics of the magma (e.g., oxidation potential) and of the magmatic fluid genetically associated with the epithet-ma1 deposit. For effective exploration it is essential to maximize the time in the field of well-trained and experi-enced geologists using tried and tested methods. Understanding the characteristics of the deposit style being sought facilitates the construction of multiple working hypotheses for a given prospect, which leads to efficiently testing each model generated for the prospect, using the tools appropriate for the situation. Geologists who understand ore-forming processes and are creative thinkers, and who spend much of their time working in the field within a supportive corporate structure, will be best prepared to find the epithermal deposits that remain hidden.

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Available from: Jeffrey Hedenquist, Feb 25, 2015
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    • "IOCG deposits are arguably the most studied and potentially best known and understood ore deposit type (Seedorff et al., 2005), and their relationships with the skarn environment have been appreciated for many years (Einaudi et al., 1981). Only in the last decade , however, have the physicochemical connections with the high-and intermediate-sulfidation epithermal environment within and around overlying lithocaps been clarified (Hedenquist et al., 2000). Breccias, veins, disseminations and massive lenses with polymetallic enrichments are genetically associated with A-to Itype granite (Pollard, 2006), alkaline–carbonatite stocks, and crustal-scale fault zones (Groves et al., 2010). "
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    ABSTRACT: GIS-based 2D prospectivity modelling of three greenfields geological regions of Western Australia, namely, the West Arunta Orogen, West Musgrave Orogen and Gascoyne Province, was implemented for a range of deposit types including orogenic and intrusion-related gold, volcanic sediment-hosted base-metal sulfides, magmatic nickel-copper and magmatic platinium group elements sulfides, iron-oxide copper gold, tin-tungsten, igneous and metamorphic related rare earth elements , surficial uranium and unconformity-related uranium . Conceptual mineral systems models were generated to identify the targeting criteria. The inputs to the models were the spatial proxies derived from 1:100,000 to 1:500,000 scale public domain data. The results showed similar prospectivity patterns for all of the targeted deposit types except sediment-hosted uranium and surficial uranium deposit types. Once a favourable geodynamic architecture is established, it can sustain different mineral systems and produce diverse deposit types depending on the nature of ligands in the source regions and physical-chemical environment in the trap regions through repeated reactivation in the subsequent geological history. A model is proposed to explain the formation of different deposit types at different stages of tectonic evolution of a province. The implication for GIS-based 2D prospectivity modelling at the scale of geological region is that the prospectivity model may not be deposit type specific. Further, prospectivity modelling should be carried out sequentially at progressively finer scales (regional- to district- to camp-scale), using only the targeting criteria that are relevant at the specific scale to delineate targets for specific deposit types.
    Ore Geology Reviews 06/2015; 71. DOI:10.1016/j.oregeorev.2015.06.007 · 3.56 Impact Factor
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    • "The Chah Zard deposit is a pipe-like breccia deposit (Kouhestani et al., 2012) that has several features similar to the LS to IS epithermal deposits as discussed by Einaudi et al. (2003), Gemmell (2004), Hedenquist et al. (2000), Sillitoe and Hedenquist (2003), and White and Hedenquist (1990). It is situated in the central part of the Urumieh-Dokhtar Magmatic Arc (UDMA) approximately 100 km southwest of the city of Yazd, central Iran (Fig. 1). "
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    ABSTRACT: The Chah Zard gold–silver deposit, in the central part of the Urumieh-Dokhtar Magmatic Arc (UDMA) of Iran, is a breccia-hosted low- to intermediate-sulfidation epithermal deposit with a resource of ~ 2.5 Mt averaging 1.7 g/t Au and 12.7 g/t Ag. Gold and silver mineralization occurs in breccia and veins associated with a 6.2 ± 0.2 Ma volcanic complex. Microthermometric measurements on quartz- and sphalerite-hosted, two-phase liquid-rich fluid inclusions indicate that the mineralization may have taken place between 260 and 345 °C, from a moderately saline hydrothermal fluid (8.4–13.7 wt.% NaCl equiv.). First ice-melting temperatures between − 37 and − 53 °C indicate that the aqueous fluids contained NaCl, CaCl2 ± MgCl2 ± FeCl2. Coexisting liquid-rich and vapor-rich fluid inclusions in quartz and sphalerite provide evidence for boiling in ore-stage breccia and veins. Additionally, the occurrence of adularia and bladed calcite in high-grade ore zones and the presence of hydrothermal breccias and chalcedonic quartz are consistent with boiling. Calculated δ18O values of water in equilibrium with quartz (+ 3.4 to + 13.1‰) suggest that the fluid may have had a magmatic source, but was 18O-depleted by mixing with meteoric water. The average calculated δ34SH2S values are − 0.2‰ for pyrite, + 0.2‰ for chalcopyrite, − 1.0‰ for sphalerite and − 0.2‰ for galena. The δ34SH2S values are consistent with a magmatic source for sulfur. Gold deposition at Chah Zard is inferred to have been largely caused by boiling, although fluid mixing and/or wall rock reactions may also have occurred. After rising to a depth of between 970 and 440 m, the fluid boiled, causing deposition of fine-grained quartz, and sealing of the hydrothermal conduit. Episodic boiling in response to alternating silica sealing and hydraulic brecciation was responsible for ore deposition. Gold and silver may have precipitated due to the destabilization of HS− complexes, caused by the boiling-off of H2S to vapor, whereas the dilution and/or cooling of hydrothermal fluids led to the precipitation of base metals.
    Ore Geology Reviews 03/2015; DOI:10.1016/j.oregeorev.2013.06.003 · 3.56 Impact Factor
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    • "Sillitoe (1997) suggested that epithermal deposits and porphyry deposits derived from the same thermal system and could occur at the same time. Hedenquist et al. (2000) suggested the intermediatesulfide (IS) type between the high-sulfide (HS) and low-sulfide types (LS). Corbett (2002) tried to reveal the inner links between epithermal deposits, porphyry Cu–Au deposits and skarn deposits with a uniform model. "
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    ABSTRACT: The Ciemas gold mining area is located in the Sunda arc volcanic rock belt, West Java, Indonesia. Ore bodies are associated with Miocene andesite, dacite and quartz diorite porphyrite. To constrain ore genesis and mineralization significance, a detailed study was recently conducted examining these deposits, which included detailed field observation, petrographic study, petrochemistry, sulfur isotope analyses, zircon U–Pb dating, and fluid inclusion analysis. The results include the following findings. 1) Ore types have been identified as porphyry, a quartz–sulfide vein, and structure-controlled alteration rocks. 2) In host rocks, zircon LA–ICP-MS U–Pb dating of quartz diorite porphyrite, amphibole tuff breccia and andesite yield ages of 17.1 ± 0.4 Ma, 17.1 ± 0.4 Ma and 17.5 ± 0.3 Ma, respectively. 3) Fluid inclusions in the quartz from ore are given priority to liquid and gas–liquid phases, and their components are of the NaCl–H2O system with homogenization temperatures of 240–320 °C, salinities of 14–17%, densities of 0.85–0.95 g/cm3, and fluid pressure values between 4.1 and 46.8 MPa, corresponding to metallogenic depths from 150 to 1730 m. Fluid characteristics are identified as similar to those of high sulfur epithermal deposits. 4) The sulfur isotopic compositions are notably uniform, the δ34S values of wall rocks range from 3.71 to 3.85‰, and the δ34S values of ores vary from 4.90‰ to 6.55‰. The sulfur isotopic composition of ores is similar to that of the wall rocks, indicating a mixed origin of mantle with a sedimentary basement. 5) The trace element patterns of different ore types are similar, which indicates that they originate from the same source. Au deposits primarily occurred during the late magmatic activity. Finally, we have set up the regional metallogenic model, confirming that this gold deposit in the Sunda arc volcanic rock belt belongs to a metallogenic system from porphyry to epithermal type.
    Ore Geology Reviews 01/2015; 64(1):152–171. DOI:10.1016/j.oregeorev.2014.07.003 · 3.56 Impact Factor
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