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, hedenquist@aol.com 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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Article: Exploration for Epithermal Gold Deposits

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    • "Many of the characteristics of the Mazarrón deposit, including host rock type, vein style, mineral associations and hydrothermal alteration halos (Morales-Ruano and Fenoll, 1990; Arana et al., 1993; Oyarzun et al., 2011; this paper) support its epithermal nature consistent with formation within the shallow part of a magmatic–hydrothermal system. Nevertheless, there is a lack of consensus as to its classification (low sulfidation, Hedenquist et al., 2000; intermediate sulfidation, Arribas, 2004; high sulfidation, Oyarzun et al., 2011), due to a shortage of data and the introduction of new terminology and classification schemes (Sillitoe and Hedenquist, 2003). Volcanic-hosted epithermal deposits such as the Mazarrón deposit usually have a magmatic sulfur source (Ohmoto and Goldhaber, 1997). "
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    ABSTRACT: The present study shows the existence of textural, mineralogical, microthermometric and isotopic data that evidences processes such as thermochemical sulfate reduction (TSR) that are not typical features of volcanic-hosted epithermal deposits formed at shallow submarine and/or subaerial environments. Geological evidence and mineralogical, fluid inclusion temperature and geochemistry data suggest that the Mazarrón Zn-Pb-Cu-Ag-Fe deposit is a volcanic-hosted epithermal deposit. It is hosted by dacites with different alteration halos: K-Ar dating of illite from the argillic alteration gives an age of 9.7 ± 1.2 Ma, which suggests that mineralization took place very close to the volcanism. The fluid inclusion study in the ore veins shows epithermal temperatures (between 190 and 260 °C), but moderate salinities, between 12 and 18 wt.% NaCl eq., compatiblewith mixing between surface waters and polygenic deep hydrothermal fluids. Evolution of the hydrothermal activity shows the existence of three stage of mineralization: in the first and last stages, with Fe-sulfides as main phases, mineralizationwas mainly formed by interaction between basinal fluids that leached Triassic sulfates (+ surface waters), and C-bearing reduced deep waters. During interaction of both fluids, the sulfur was subjected to extreme and geologically unusual fractionation under TSR conditions, with ranging from -2.9 to +28.4‰ δ34S, reaching exceptional values up to+53‰δ34S. The intermediate stage is mainly formed by sphalerite-galena-chalcopyrite- fahlore bearing sulfides; this event is compatiblewith the influx of a magmatic fluid discharge. Finally, this study suggests that Mazarrón could be classified as an intermediate sulfidation deposit.
    Ore Geology Reviews 01/2016; 72:1102-1115. DOI:10.1016/j.oregeorev.2015.10.005 · 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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    • "Coexisting primary VL and LV populations with variable vapor-to-liquid ratios, existence of chalcedony and colloform quartz (White and Hedenquist, 1990;Hedenquist, 2000) are potential indicators of boiling in the hydrothermal system (e.g., Thiersch et al., 1997; White and Hedenquist, 1995; Ronacher et al., 2000; Simmons et al., 2005;). Moreover, the coexistence of L and V primary and pseudo secondary inclusions in quartz (I) suggests that boiling occurred repeatedly during breccia and veins formation(e.g., Bodnar et al., 1985; Ronacher et al., 2000; Yilmaz et al., 2010). "
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