Epithermal Gold/Silver Deposits

Transcription

1 Epithermal Gold/Silver Deposits This text is based mostly on the Economic Geology Notes by Ron Morton and the notes by Jeff Hedenquist. See the references at the end of the text and the references for the figures. Definition: The term epithermal was used by Lindgren in 1913 to refer to those hydrothermal ore deposits that formed in a relatively near surface environment. Today we know this is from about 50 meters down to 1,500 meters below the surface with temperatures between 100 and 400 ºC and low pressures. Surface expressions of these near surface deposits are hot springs. Most geologists think of epithermal deposits as being mainly continental and associated with subaerial geothermal systems, however volcanogenic massive sulfide and sedimentary exhalative deposits are certainly in the epithermal group. Ore Minerals, Production, Size: Ore elements associated with epithermal gold/silver deposits include gold, silver, antimony, arsenic, and mercury. The corresponding minerals are native gold and silver, argentite, tennatite, enargite, gold tellurides, electrum, stibnite, and cinnabar. Minor elements are tellurium, copper, zinc, lead, and bismuth represented by chalcopyrite, covellite, sphalerite, galena, and bismuthinite. Most epithermal gold/silver deposits are obviously mined for gold and/or silver with individual deposits varying from gold-rich (silver/gold ratios less than 10) to silver-rich (silver/gold ratios from ). Open pit methods are used for large, low grade deposits (1-2 g/t Au, g/t Ag) with gold in oxidized (supergene) ore being amenable to low cost, heap-leach treatment. Underground mining is generally used for smaller, higher grade deposits ( g/t Au, or greater than 500 g/t Ag). In these deposits, grade control is essential and difficult because gold and silver values can vary dramatically over just a few meters. Tonnages mined often depend on grade with low grade deposits needing more goldbearing rock; generally tonnages will be in the 100 s of millions. Higher grade deposits could have as little as 400,000 tons of ore. Large mining companies typically will not mine a deposit unless it has at least 1 million ounces of gold. Distribution and Age: Most known epithermal deposits are Tertiary in age, which goes along with their near surface level of formation and thus low level of preservation. However, older deposits are being recognized, including those that are metamorphosed and extensively recrystallized. Most deposits are closely associated with convergent plate boundaries (continental and island arcs). Epithermal deposits thus form in areas of active volcanism where there is high heat flow, tectonic activity, and active geothermal systems. Mined deposits represent parts of fossil geothermal systems. Classification and Deposit Types: Based on the nature of the ore forming fluids as well as the type of volcanic host rocks, alteration assemblages, and ore minerals, epithermal gold/silver deposits have been divided into 2 groups: 1) Low sulfidation deposits, also called the adularia-sericite class 2) High sulfidation deposits, these are also called the acid sulfate or alunite-kaolinite pyrophyllite class. The term sulfidation is used here to describe stabilities of sulfur-bearing minerals in terms of sulfur fugacities (partial pressure).

2 Low Sulfidation Epithermal Gold/Silver Deposits: Historically most gold production from volcanic rock-hosted epithermal deposits have come from this type. Regional Setting: This deposit type, in terms of tectonic setting, is related to back-arc basins, continental margins, and island arcs. The deposits form at shallow depths and are associated with regional faults that control volcanism and are relatively long-lived. For example the Comstock, Oatman, Bullfrog, and Tonapah epithermal districts line up along what is called the Walker Lane in Nevada and the deposits in the San Juan Mountains are related to the Rio Grande Rift. Local Setting: Low sulfidation deposits are commonly associated with calderas and, to a lesser extent, stratovolcanoes. However, in the case of calderas, it needs to be realized that just because you have identified a caldera does not mean you will necessarily find a gold deposit. In fact most calderas contain no gold mineralization. In the San Juan Mountains, only a third of the known calderas are mineralized. Within calderas the ore deposits occur: a) Within permeable rock units like ash flow tuffs, dome breccias, debris flows and especially where these units are in proximity to synvolcanic faults. Thus, margins of calderas and dome-flow complexes are common environments. b) In the case of stratovolcanoes, the gold mineralization is typically associated with faults along the flanks of the volcano and permeable rock units such as air fall deposits, debris flows, and ash flow tuffs. Host rocks are dominantly Tertiary to Quaternary rhyodacites, rhyolites, and andesites. Rhyolitic host rocks tend to be enriched in molybdenum, tungsten, fluorine, niobium, and tin. It has been proposed that these epithermal deposits represent the upper parts of porphyry molybdenum deposits but that claim has not been proven. In some rare cases volcanic sediments (moat-type) host the ore. Mineralogy: The mineralogy of the ore is characterized by veins of adularia and sericite with quartz. Other common associate minerals are chlorite, rhodochrosite, rhodonite, barite, and fluorite. Pyrite is ubiquitous and may be accompanied by cinnabar, arsenopyrite, and/or stibnite. The ore minerals are electrum (gold with 20% silver), native gold, native silver, gold tellurides, acanthite, tennatite, chalcopyrite, and tetrahedrite. These occur as replacements of the host rocks and in individual veins or vein stockworks. In general, these deposits contain more silver than gold though there are exceptions like Round Mountain, a 178 million ton deposit with oz gold (300,000 ozs) and no silver. Gold grades vary from <1 to >100 g/ton; typically, gold is fine-grained and mostly invisible. Silver, when present, ranges from 1 to more than 5 oz/ton. Minor sphalerite and galena are often associated with these deposits. Alteration: At and above the water table, the rocks tend to be silicified and alunite often is present with the quartz. The silicification is due to boiling of the hydrothermal fluid, and precipitation of quartz as silica sinter. Below the water table, clay minerals (kaolinite), sericite, and manganese carbonate are the common alteration products. This alteration gives way outward to a zone of epidote, albite, and carbonates with or without pyrite and chlorite. This alteration type is similar to propylitic alteration in porphyry copper deposits, is quite extensive, and may have formed prior to the ore deposition. Quartz is variable in appearance ranging from banded to crustiform and cockscomb; varieties include chalcedony, amethyst, and chert. Zonation of Ore minerals:

3 Above the water table to the surface, there may be native gold and/or silver, with minor stibnite and cinnabar. This will be a low grade zone with the ore minerals occurring as disseminations and fracture fillings. At and just below the water table (boiling zone), one can find native gold and silver, gold tellurides, stibnite, and arsenopyrite. These minerals occur dominantly as fracture fillings with some replacement ore. This is often the high-grade ore zone. Below the water table, we can find electrum, acanthite, tetrahedrite and arsenopyrite as fracture fillings and replacement ore. Deeper zones (>600 m) may have galena, sphalerite, chalcopyrite, and/or molybdenite. Paleodepth of ore fluid: From geological reconstructions and fluid inclusion studies, the low sulfidation deposits form between 100 to 1000 m below the surface. The ore fluids have low salinity (1-5% NaCl), a near-neutral ph, and contain CO 2, H 2 S, and NaCl as the dominant species. The temperatures of the fluid were between 200 and 300 ºC The composition of the fluid is similar to many active geothermal systems dominated by meteoric water and magmatic gas such as the Broadlands in New Zealand. The amount of magmatic gas/fluid is between 10-30% and thus the fluid represents a mixture of meteoric water and magmatic hydrothermal fluid. High Sulfidation Epithermal Gold/Silver Deposits Tectonic Setting: These deposits occur in continental margins, island arcs, and back-arc basins. They form in active volcanic areas where intrusions are near the surface; thus, like the low sulfidation gold/silver deposits, they are associated with stratovolcanoes and calderas. Depositional Environment and Geological Setting: Within calderas, the deposits commonly occur along the margins or near synvolcanic structures within host rocks that range from dome-flow complexes and ash flow tuffs to hyalotuffs associated with tuff rings and maars. These high sulfidation deposits are postulated to represent the near surface expression of porphyry copper systems. Mineralogy: These deposits are characterized by alunite [KAl 3 (SO 4 ) 2 (OH) 6 ] [white, powdery, finegrained earthy mineral], enargite, and pyrite with or without covellite, quartz, and barite. The ore and gangue occurs as disseminations, fracture fillings, and replacements. Vein ore is subordinate to these other types. Ore minerals are native gold, enargite, tennatite, argentite, tetrahedrite, bismuthinite, and gold tellurides. Alteration: Characterized by extensive leaching of the volcanic rocks with ore most often associated with the more intense alteration. From the surface down to the water table, the alteration produces alunite, kaolinite, and pyrophylite with native sulfur and some cinnabar as well as quartz (chalcedony). There may be extensive zones of silification at and just below the water table. The ore minerals occur beneath this silicified zone and the alteration here is sericite-alunite-kaolinite-quartz (opal, chert), which gives way laterally to kaolinite and montmorillonite that grades out into a propylitic alteration zone. With depth, pyrophylite and diaspore take the place of kaolinite. The ore minerals found here are enargite, native gold, electrum, stibnite, with minor native silver. This gives way downward to tetrahedrite, tennatite, bismuthinite, and chalcopyrite.

4 Paleodepth of Formation: m below the surface Hydrothermal fluids have a low ph (<1-3), are saline (10-30%), and dominated by H 2 S, HCl, H 2 SO 4, and SO 2 These are characteristic of magmatic-hydrothermal fluids that enter the near surface environment with little or no interaction with meteoric water Surface expression of these low ph waters are: Solfataras, acid lakes Abundant native sulfur, realgar, and orpiment Examples include White Island, New Zealand, and Osorezan, Japan Hot Spring Deposits: Hot spring gold deposits, which include siliceous (chert) sinters and brecciated lower zones, cap many modern geothermal systems and may be associated with either low or high sulfidation deposits. Their surface formation makes them very vulnerable to surface weathering and erosion and thus their preservation is poor. In these deposits, ore was precipitated within 100 meters of the surface due to the direct interaction of the hydrothermal fluid with surface waters. The surface manifestations are mud pots, solfataras, thermal pools, and siliceous sinters. Deposits tend to occur as disseminations and veins and may be associated with hydrothermal breccias. Gold mineralization is native gold and electrum with associated arsenic, antimony, mercury, thallium, silver, and sometimes tungsten. Today some active geothermal fields are precipitating gold at well heads of geothermal power stations (Wairki), and at the surface in hotsprings (Osorezan, Japan). References Bethke, P.M., Rye, R.O., Stoffregen, R.E., and Vikre, P.G., 2005, Evolution of the magmatic-hydrothermal acidsulfate system at Summitville, Colorado: Integration of geological, stable isotope, and fluid inclusion evidence: Chemical Geology, v. 215, p Bonham, J.F., Jr., 1988, Models for volcanic-hosted precious metal deposits; a review, in: Schafer, R.W., Cooper, J.J., and Vikre, P.G., eds., Bulk Mineable Precious Metal Deposits of the Western United States: Geological Society of Nevada, p Cooke, D.R., and Simmons, S.F., 2000, Characteristics and genesis of epithermal gold deposits: Society of Economic Geologists Reviews, v. 13, p Giggenbach, W., 1992, Magma degassing and mineral deposition in hydrothermal systems along convergent plate boundaries: Economic Geology, v. 87, p Hagemann, S.G., and Brown, P.E.eds., Gold in 2000, Reviews in Economic Geology v. 13, Society of Economic Geologists Hedenquist, J.W., and Roberts, P.J., eds., Guide to Active Epithermal (Geothermal) Systems and Precious Metal Deposits: Mineral Deposits, Monograph 26, 211 p Sillitoe, R.H., and Bonham, H.F., Jr., 1984, Volcanic landforms and ore deposits: Economic Geology, v. 79, p Sillitoe, R.H.,1993, Epithermal models: genetic types, geometrical controls and shallow features, in: Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., and Duke, J.M. eds., Mineral Deposit Modeling: Geological Association of Canada, Special Paper 40, p Simmons, S.F., White, N.C., and David, J., 2005, Geological Characteristics of Epithrmal Precious and Base Metal Deposits, in: Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and Richards, J.P., eds., Economic Geology 100th Anniversary Volume: The Economic Geology Publishing Company, p

5 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 White, N.C., and Hedenquist, J.W., 1990, Epithermal environments and styles of mineralization: variations and their causes, and guidelines for exploration, in: Hedenquist, J., White, N.C., and Siddeley, G. eds., Epithermal Gold Mineralization of the Circum-Pacific, Geology, Geochemistry, Origin and Exploration, II: Journal of Geochemical Exploration, v. 36, p Wood, D.G., Porter, R.G., and White, N.C., 1990, Geological features of some Paleozoic epithermal gold occurrences in northeastern Queensland, Australia, in: Hedenquist, J., White, N.C., and Siddeley, G., eds., Epithermal Gold Mineralization of the Circum-Pacific, Geology, Geochemistry, Origin and Exploration, II: Journal of Geochemical Exploration, v. 36, p