EPITHERMAL GOLD DEPOSITS BRUCE E. TAYLOR Geological Survey of Canada, 601 Booth Street, Street, Ottawa, Ontario K1A 0E8 Corresponding author’s email:
[email protected] Abstract Epithermal Au (±Ag) deposits form in the near-surface environment, from hydrothermal systems typically within 1.5 km of the Earth’s surface. They are commonly found associated with centres of magmatism and volcanism, but form also in shallow marine settings. Hot-spring deposits and both liquid- and vapour-dominated geothermal systems are commonly associated with epithermal deposits. Epithermal Au deposits are commonly consider to comprise one of three subtypes: high sulphidation, intermediate sulphidation, and low sulphidation, each denoted by characteristic alteration mineral assemblages, occurrences, textures, and, in some cases, characteristic suites of associated geochemical elements (e.g. Hg, Sb, As, and Tl). Base metal (Cu, Pb, and Zn) and sulphide minerals may also occur in addition to pyrite and native Au or electrum. In some epithermal deposits, notably those of the intermediate-sulphidation subtype, base metal sulphides may comprise a significant ore constituent. Canadian Au production from epithermal deposits has been minor (<5%), compared to that from transitional and intrusion-related Au deposits, or to other lode Au deposits. The shallow origin of epithermal Au deposits makes them more susceptible to erosion, and, accordingly, epithermal Au deposits have represented a high-grade, readily mineable, exploration target largely in Tertiary and younger volcanic centres, including the Cordillera. However, a number of older epithermal Au deposits have also been discovered, including several Proterozoic examples in Canada. Thus, older terranes need not be excluded entirely from exploration. Modern geothermal and volcanic systems provide natural laboratories for the study of epithermal deposits, guiding theoretical models and laboratory experiments, and expanding our understanding of po tential environments and vectors to mineralized systems. Yet, a principal, unanswered question still remains: do rich Au deposits form from Au-rich sources, or from exceptionally efficient mechanisms or processes of Au pr ecipitation?
Résumé Les gîtes d’or (+/- argent) épithermaux se forment dans le milieu subsuperficiel, de manière caractéristique à moins de 1,5 km de la surface de la Terre. Ils sont couramment présents dans des centres magmatiques ou volcaniques à la surface du globe, mais se forment également dans des cadres marins de faible profondeur. Des dépôts de sources thermales ainsi que des systèmes géothermiques tant à dominante liquide qu’à dominante vapeur sont couramment associés aux gîtes épithermaux. Deux sous-types de gîtes sont définis d’après les minéraux d’altération communs qu’ils renferment: les gîtes à quartz-(kaolinite)-alunite (QAL) formés à partir de fluides acides avec un important apport magmatique et les gîtes à adulaire-séricite (ADS) formés à partir de fluides quasi neutres en grande partie composés d’eau météorique. Des métaux communs peuvent être présents dans l’un ou l’autre sous-type. La production canadienne d’or tirée de gîtes épithermaux est mineure (< 5%), mais ailleurs dans le monde, cette classe de gîtes à teneur élevée et facilement exploitables constitue une cible d’exploration. Les gîtes épithermaux profonds ou associés à des intrusions ont fourni un e plus importante contribution à la production canadienne d’or. La faible profondeur à laquelle se forment les g îtes d’or épithermaux fait en sorte qu’ils sont davantage susceptibles d’érosion et ils ont par conséquent tendance à être découverts dans des terrains plus récents. De toute évidence, un enfouissement rapide a préservé un certain nombre de gîtes d’or épithermaux dans des terrains plus anciens, dont on trouve des exem ples dans des roches du Protérozoïque. Ces occurrences permettent d’afficher un certain optimisme quant à l’exploration de terrains plus anciens au Canada. Les systèmes géothermiques contemporains constituent des laboratoires naturels d’étude des gîtes épithermaux ainsi que des guides pour l’élaboration de modèles théoriques et d’expériences à mener en laboratoire. De telles études permettent continuellement d’améliorer la compréhension et fournissent des guides pour l’exploration; chacune contribuant à répondre à l’importante question qui reste sans réponse : les riches gîtes se forment-ils à partir de sources riches ou sont-ils attribuables à des facteurs déterminés par un processus de précipitation ou de concentration?
Definition
Simplified Definition Epithermal deposits of Au (± Ag) comprise veins and disseminations near the Earth’s surface (≤1.5 km), in volcanic and sedimentary rocks, sediments, and, in some cases, also in metamorphic rocks. The deposits may be found in association with hot springs and frequently occur at centres of young volcanism. The ores are dominated primarily by precious metals (Au, Ag), but some deposits may also contain variable amounts base metals such as Cu, Pb, and Zn. Scientific Definition Epithermal Au deposits are a type of lode deposit (e.g. Poulsen, 1996; Poulsen et al., 2000) consisting of economic
concentrations of Au (± Ag and base metals). These deposits form in a variety of host rocks from hydrothermal fluids, primarily by replacement (i.e. by solution and reprecipitation), or by open-space filling (e.g. veins, breccias, pore spaces). The form of deposits originating by open-space filling typically reflects that of the structural control of the hydrothermal fluids (planar vs. irregular fractures, etc). The deposits are commonly young, generally Tertiary or Quaternary. They may be of similar age as their host rocks when these are volcanic in origin, or (typically) younger than their host. Early workers (e.g. Lindgren, 1922, 1933; Emmons, 1924) emphasized a broad depth-zoning classification of hydrothermal metal deposits in which epithermal deposits were interpreted to have formed in a ‘shallow’ regime, regime, qual-
Taylor, B.E., 2007, Epithermal gold deposits, in Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p. 113-139.
B.E. Taylor
itatively on the order of ≤1500 m. Depth is, however, challenging to quantify, except where a datum (e.g. the Earth’s surface) may be recognized, or a stratigraphic reconstruction reconstruction made. Depth may be inferred, for example, from a depth-to boiling curve, plus evidence of boiling in fluid inclusions and an estimated formation temperature based on a geothermometer (e.g. trace element or stable isotope distributions), or, simply, simply, from textures indicative of boiling-induced supersaturation (e.g. quartz pseudomorphs of bladed calcite). In any case, the generally accepted shallow origin of epithermal Au deposits is a central (and genetically important) characteristic. This environment is marked by rapid changes in temperature and pressure of the hydrothermal fluids that may be accompanied by boiling and mixing with other fluids, causing changes in pH and oxidation state and, consequently, precipitation of Au (and other metals). Epithermal deposits may also be characterized by the presence of other volatile accessory elements (Hg, Sb, Tl, etc.). Many geologists today regard ‘epithermal’ Au deposits to be primarily associated with continental volcanism or magmatism, although similar processes of ore deposition occur in other, near-surface environments (e.g. seafloor volcanogenic massive sulphide (VMS) deposits, submarine volcanic arc systems, and non-volcanic vein deposits; see Gosselin and Dubé, 2005a-d; Dubé et al., 2007). These other deposits, especially those with similar characteristics from marine environments, may also be considered epithermal deposits in a broad sense. Many extensive reviews of epithermal deposits, among them Buchanan (1981), Hayba et al. (1985), Berger and Bethke (1986), Heald et al. (1987), White and Hedenquist (1990), Arribas (1995), Richards (1995), Hedenquist et al. (1996, 2000), and Cooke and Simmons (2000) have provided a wealth of background information. This chapter will largely focus on the continental environment, emphasizing young deposits in the Canadian Cordillera, as well as examples of epithermal Au deposits found in older terranes elsewhere in Canada. Epithermal Au deposits are distinguished on the basis of the sulphidation state of the sulphide mineralogy as belonging to one of three subtypes: (1) high sulphidation (previously called quartz-(kaolinite)-alunite, alunite-kaolinite, enargite-Au,, or high sulfur: Ashley, enargite-Au Ashley, 1982; Hedenquist, 1987; Bonham, 1988); (2) intermediate sulphidation (Hedenquist et al., 2000); or (3) low sulphidation (previously called adularia-sericite). High-sulphidation subtype deposits usually occur close to magmatic sources of heat and volatiles, and form from acidic hydrothermal fluids containing magmatic S, C, and Cl. Low-sulphidati Low-sulphidation on subtype fluids are thought to be near-neutral, dominated by meteoric waters, but containing some magmatic C and S. In addition, some geologists also refer to ‘hot-spring’ deposits as an additional subtype of epithermal deposit that may form as surface expressions of hydrothermal systems, typically of the low-sulphidati low-sulphidation on subtype sometimes associated with acidic, steam-heated alteration zones. See Henley et al. (1984), Hayba et al. (1985), Heald et al. (1987), Hedenquist (1987), Bonham (1988), Berger and Henley (1989), and Panteleyev (1996a-c) for discussions and original definition of these terms. Hedenquist et al. (2000) is recommended for a more recent and very com prehensive summary of current usage, classification, and deposit characteristics characteristics.. 114
The Blackdome and most of the deposits in the Toodoggone River camp in British Columbia, and the Mt. Skukum deposit, Yukon, are among the best Canadian exam ples of volcanic-host volcanic-hosted, ed, low-sulphidation subtype epithermal Au deposits (Table 1). The Cinola deposit, British Columbia (Champigny and Sinclair, 1982), hosted by sedimentary rocks, is an example of a low-sulphidati low-sulphidation on hot-spring deposit (in particular, its upper part). High-sulphidation deposits are less well represented in Canada, but include the volcanichosted Al deposit, Toodoggone River camp, British Columbia and the metamorphosed Hope Brook deposit, Nova Scotia (Table 1). Numerous areas of advanced argillic alteration mineral assemblages and associated Au prospects formed by high-sulphidation systems of Neoproterozoic age are also known in the Burin Peninsula, Newfoundland (e.g. Hickey’ss Pond: O’Brien et al., 1999). The locations of these Hickey’ and other Canadian epithermal deposits or districts, and selected deposits elsewhere in the world, are shown in Figures 1 and 2, respectively, and include, especially, those deposits or districts noted in the text. Numerous examples of both low-sulphidation and highsulphidation deposits in volcanic and sedimentary host rocks exist world wide, especially in younger volcanic terranes. Classic examples of the high-sulphidation subtype include Summitville, Colorado (Bethke et al., 2005) and Nansatsu, Japan (Hedenquist et al., 1994). The Creede district, Colorado (e.g. Heald et al., 1987) and Hishikari, Japan (Izawa et al., 1990; Hayashi et al., 2001, and references therein) are good examples of volcanic-hosted low-sulphidalow-sulphidation subtype deposits. Others are noted below. Diagnostic Features of Epithermal Gold Deposits Geological, mineralogical, and geochemical features of epithermal Au deposits are listed for each of three deposit subtypes in Table 2. Distinctive features typically present include key alteration mineral assemblages (low sulphidation: sericite, adularia, kaolinite, calcite, rhodochrosite, Fechlorite, quartz; high sulphidation: alunite, kaolinite, pyro phyllite, sericite, adularia (illite), chlorite, barite; Table 2), ore mineral assemblages (low sulphidation: electrum, HgSb-As sulphides, base metal sulphides; high sulphidation: native Au, electrum, tellurides, base metal sulphides; Table 2), geological evidence for shallow emplacement: sinter deposits, fluid inclusion or textural evidence (e.g. lamellar calcite, or their quartz pseudomorphs) for boiling, hydrothermal breccias and eruption deposits, open-space crustiform veins, and marked 18O depletion of wall rocks. Vertical zoning of alteration minerals, lower Au:Ag ratios in electrum with depth, and spatial and temporal separation separa tion of Au and abundant base metals are also characteristic charac teristic (though not universal) of epithermal Au deposits. Although most known epithermal Au deposits are Tertiary Tertiary in age, the mineralogical and geological characteristics characteristics noted above have led to the recognition of much older epithermal deposits, including recrystallized and deformed examples in metamorphic terranes. Although high-sulphidation-related alteration is distinctive, corroboration of an epithermal setting by low 18O/16O ratios can, particularly for the low- sul phidation subtype, provide a unique record of alteration by meteoric waters, one that survives metamorphism and requires a shallow origin. For example, the Carolina Slate
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