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Exploration for Epithermal Gold Deposits

01/2000; 13:245-277.

ABSTRACT 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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    ABSTRACT: Successful exploration for mineral deposits requires tools that the explorationist can use to distinguish between targets with high potential for mineralization and those with lower economic potential. In this study, we describe a technique based on gangue mineral textures and fluid inclusion characteristics that has been applied to identify an area of high potential for gold-silver mineralization in the epithermal Ag-Au deposits at Guanajuato, Mexico. The Guanajuato mining district in Mexico is one of the largest silver producing districts in the world with continuous mining activity for nearly 500 years. Previous work conducted on the Veta Madre vein system that is located in the central part of this district identified favorable areas for further exploration in the deepest levels that have been developed and explored. The resulting exploration program discovered one of the richest gold-silver veins ever found in the district. This newly discovered vein that runs parallel to the Veta Madre was named the Santa Margarita vein. Selected mineralized samples from this vein contain up to 249 g/t of Au and up to 2,280 g/t Ag. Fluid inclusions in these samples show homogenization temperatures that range from 184 to 300°C and salinities ranging from 0 to 5 wt.% NaCl. Barren samples show the same range in homogenization temperature, but salinities range only up to 3 wt.% NaCl. Evidence of boiling was observed in most of the samples based on fluid inclusions and/or quartz and calcite textures. Liquid-rich inclusions with trapped illite are closely associated with high silver grades. The presence of assemblages of vapor-rich-only fluid inclusions, indicative of intense boiling or “flashing”, shows the best correlation with high gold grades.
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    ABSTRACT: Fluid inclusion petrography and vein mineral textures indicative of boiling have been characterized in 855 samples from epithermal precious metals deposits along the Veta Madre at Guanajuato, Mexico. Mineral textures and fluid inclusions characteristic of fluid immiscibility or boiling, including colloform quartz, plumose/feathery/flamboyant quartz, lattice-bladed calcite and lattice-bladed calcite replaced by quartz, as well as coexisting liquid-rich and vapor-rich fluid inclusions and assemblages of vapor-rich only inclusions, have been identified in mineralized samples from the Veta Madre. Most samples studied were assayed for Au, Ag, Cu, Pb, Zn, As and Sb, and were divided into ore grade and sub-economic samples based on the gold and silver concentrations. For silver, samples containing > 100 ppm were classified as ore grade, and ore grade gold samples contained > 1 ppm Au. The feature that is most closely associated with ore grades of both gold and silver is colloform quartz that was originally precipitated as amorphous silica, and this feature also shows the largest difference in average grade between samples that show colloform texture (178.8 ppm Ag and 1.1 ppm Au) and those that do not exhibit this texture (17.2 ppm Ag and 0.2 ppm Au). Statistical analysis of the data confirmed the petrographic observations that indicated that colloform quartz is the feature that has the greatest predictive power for distinguishing between ore grade and sub-economic samples. For both Ag and Au, there is no significant difference in average grade of samples containing coexisting liquid-rich and vapor-rich fluid inclusions or assemblages of vapor-only inclusions and those that do not, suggesting that fluid inclusion evidence for boiling is not correlative with ore grades. This result is consistent with the fact that most forms of silica that are precipitated during boiling do not trap useful fluid inclusions. The results of this study suggest that mineral textures and fluid inclusions provide complementary information that should both be used in exploration for epithermal precious metal deposits. Metal grades and boiling intensity of samples collected along a traverse perpendicular to the Veta Madre and above known economic mineralization are both low at relatively short distances away from the vein and increase as the vein is approached. This suggests that mineralogical and fluid inclusion evidence for boiling are restricted to the immediate vicinity of, and increase in the direction of, mineralized veins and may be used in exploration to establish vectors towards vein systems that may host precious metal mineralization. Previous studies of epithermal systems show that the Ag and Au mineralization zone is most often located at or above the bottom of the boiling zone. In this regard, the presence of abundant evidence for boiling that is observed in the deepest levels of the Veta Madre that have been sampled suggests that additional precious metal mineralization may be present beneath the deepest levels that have been explored.
    Journal of Geochemical Exploration 03/2012; 114:20-35. · 1.95 Impact Factor

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