Monday, November 9, 2015

Epithermal High Sulfidation Gold Deposits

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Ephitermal deposits schema
Epithermal gold and silver deposits of both vein and bulk-tonnage styles are known by a variety of largely synonymous terms (Table 1; Sillitoe and Hedenquist, 2003). They may be broadly grouped into high sulfidation (HS), intermediate-sulfidation (IS), and low-sulfidation (LS) types based on the sulfidation states of their primary sulfide assemblages (Hedenquist et al., 2000). In addition to their mineralogical differences, epithermal deposits also form in distinction tectonic settings (Sillitoe and Hedenquist, 2003) Most Epithermal High Sulfidation Gold Deposits are generated in mildly extensional to neutral calc-alkaline andesitic-dacitic arcs, although a few major deposits also occur in compressive arcs characterized by the suppression volcanic activity.

By contrast, rhyolitic rocks lack appreciable HS deposits. Highly acidic fluids form advanced argillic lithocaps over porphyry systems. The lithocaps may host subsequent HS mineralization, which itself is due to higher-pH, moderate- to low-salinity fluids. IS epithermal deposits occur in a broadly similar spectrum of andesitic-dacitic arcs, but commonly do not possess such a close connection with porphyry copper deposits as does the HS type. Most LS deposits, including a large proportion of the world’s bonanza veins, are associated with bimodal (basalt-rhyolite) volcanic suites in a broad spectrum of extensional tectonic settings (Sillitoe, 2002).

See also : Stratigraphy of Bali : Related Volcanic Deposits

Vuggy quartz is typical, but not a determining characteristic, of Epithermal High Sulfidation Deposits. Zones of residual, vuggy quartz have halos of advanced argillic quartz-alunite alteration and roots of pyrophyllite and/or sericite. These zones typically contain disseminated pyrite and >95 wt% SiO2, and form bodies that flare out upwards and/or preferentially replace a lithologic unit. In many cases, these bodies lack base- and precious-metals and constitute a barren lithocap of advanced argillic alteration (Sillitoe, 1995).

In other cases, after the leaching stage, copper and gold were introduced to form epithermal Au-(Cu) deposits with abundant sulfides. A good example is the Lepanto HS deposit, which overlies the Far Southeast porphyry deposit in Luzon, Philippines (Hedenquist et al., 1998). An important IS vein deposit, Victoria, is also located adjacent to Lepanto, and is slightly younger in age. The principal copper minerals in HS deposits are enargite, luzonite and/or famatinite, indicating a high sulfidation state. A typical sequence of mineral deposition is pyrite + enargite?? luzonite, followed by chalcopyrite?? tennantite?? sphalerite?? galena + pyrite. Also post-dating the enargite assemblage is the gold stage, consisting of electrum and gold tellurides, as at Lepanto, Goldfield, Nevada, and El Indio, Chile. Porphyry- related base-metal veins and HS epithermal deposits share many common features, and some deposits appear to have a close relationship (Einaudi et al., 2003).

See also : Tin Mineral Products and Consumption Globally 2015

During initial assessment of a prospect, the first goal is to determine if it is epithermal, and if so, its style, LS, IS, or HS. Observations in the field must focus on the geologic setting and structural controls, alteration mineralogy and textures, geochemical anomalies, etc. Determination of the alteration zonation is critical at this step. Other essential determinations include: the origin of advanced argillic alteration, i.e., hypogene, steam-heated or supergene, the latter two with blanket morphology, and the origin of silicic alteration (e.g., residual silica or silicification). In addition, the likely controls on grade must be determined, i.e., the potential form of the ore body. Deposit form is one of the most basic characteristics of any deposit, and in the epithermal environment form may be highly variable. This is caused by strong permeability differences in the near-surface environment, resulting from lithologic, structural and hydrothermal controls.

The form of  HS deposits varies from replacement or dissemination to vein, stock work and hydrothermal breccia. These and other determinations will help to define the relationship between host rocks, structure, alteration zoning, and the location of the potential ore zone. Such understanding will guide further exploration and eventual drilling, if warranted. Erosion and weathering must also be considered, the latter masking ore in places but potentially improving the ore quality through oxidation. Geophysical data, when interpreted carefully in the appropriate geological and geochemical context, may provide valuable information to aid drilling by identifying resistive and/or chargeable areas.

See also : Skarn and Polymetallic Carbonate

The potential for a variety of related deposits in epithermal districts has exploration implications. There is clear evidence for a spatial, and in some cases genetic relationship between HS epithermal deposits and underlying or adjacent porphyry deposits. Similarly, there is increasing recognition of the potential for economic IS base-metal ± Au-Ag veins adjacent to HS deposits.

History of nomenclature for divisions of epithermal deposit types
Table 1: History of nomenclature for divisions of epithermal deposit types.
Note: CAPITALIZED names used in this presentation
From Sillitoe and Hedenquist, 2003. See this paper, as well as Einaudi et al., 2003, for sources of references listed.

Selected References:
Arribas, A., Jr., 1995, Characteristics of high-sulfidation epithermal deposits, and their relation to magmatic fluid, in Thompson, J.F.H., ed., Magmas, fluids, and ore deposits: Mineralogical Association of Canada Short Course, v. 23, p. 419-454.

Einaudi, M.T., Hedenquist, J.W., and Inan, E.E., 2003, Sulfidation state of hydrothermal fluids: The porphyry-epithermal transition and beyond, in Simmons, S.F, ed., Understanding crustal fluids: Roles and witnesses of processes deep within the earth, Giggenbach memorial volume: Society of Economic Geologists and Geochemical Society, Special Publication, in press.

John, D.A., 2001, Miocene and early Pliocene epithermal gold-silver deposits in the northern Great Basin, western USA: Characteristics, distribution, and relationship to magmatism: Economic Geology, v. 96, p. 1827-1853.

Hedenquist, J. W., Arribas, A., and Reynolds, T. J., 1998, Evolution of an intrusion-centered hydrothermal system: Far Southeast-Lepanto porphyry and epithermal Cu-Au deposits, Philippines: Economic Geology, v. 93, p. 373-404.

Hedenquist, J. W., Arribas, A., Jr., and Gonzalez-Urien, E., 2000, Exploration for epithermal gold deposits: Reviews in Economic Geology, v. 13, p. 245-277.

Lindgren, W., 1933, Mineral deposits, 4th edition: New York and London, McGraw-Hill Book Company, 930 p.

Sillitoe, R. H., 1995, Exploration of porphyry copper lithocaps, in Mauk, J.L., and St. George, J.D., eds., Pacific Rim Congress 1995, Auckland, Proceedings: Parkville, Australasian Institute of Mining and Metallurgy, p. 527-532.

Sillitoe, R.H., 1999, Styles of high-sulphidation gold, silver and copper mineralization in the porphyry and epithermal environments, in Weber, G., ed., Pacrim ’99 Congress, Bali, Indonesia, 1999, Proceedings: Parkville, Australasian Institute of Mining and Metallurgy, p. 29-44.

Sillitoe, R.H., 2002, Rifting, bimodal volcanism, and bonanza gold veins: Society of Economic Geologists Newsletter, no. 48, p. 24-26.

Sillitoe, R.H., and Hedenquist, J.W., 2003, Linkages between volcanotectonic settings, ore-fluid compositions, and epithermal precious-metal deposits, in Simmons, S.F, ed., Understanding crustal fluids: Roles and witnesses of processes deep within the earth, Giggenbach memorial volume: Society of Economic Geologists and Geochemical Society, Special Publication, in press.

Written by: Jeffrey W. Hedenquist, Consulting economic geologist, Ottawa, editor by Flyshgeost.
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