GEOMETRY, ALTERATION, AND STYLES OF LOWSULFIDATION EPITHERMAL DEPOSITS
*
Richard Rich ard M. M . Tosdal Tos dal
INTRODUCCIÓN
Epithermal precious and base-metal deposits are classified as lowsulfidation and high-sulfidation types based upon their alteration and sulfide minerals as well as the chemistry of the hydrothermal fluids (White and Hedenquist, 1995). These two classes of epithermal deposits are not usually spatially and temporally associated although there are there some notable exceptions exceptions (Fig. (Fig. 1) (Hedenquist et al., 2001). Highsulfidation systems are dominated by highly acidic fluids and vapors exsolved from a subjacent degassing magma chamber into the volcanic carapace. Much but not all all of the contained metal metal in high-sulfidat high-sulfidation ion deposits is paragenetically late and deposited from less oxidized fluids. Low-sulfidation systems are deposited from dilute near neutral fluids dominated by meteoric water, which largely masks the magmatic fluid (Fig. 2). Geothermal systems associated with volcanic complexes that are being exploited for power generation provide modern analogues to low-sulfidation epithermal deposits (Henley and Ellis, 1983). Considerable advances in the understanding of these deposits derive from studies of the geothermal systems. Low-sulfidation epithermal deposit are characterized generally by high economic grades and low tonnages (<30 Mt) (e.g. Orcopampa, Hishikari) with the metalliferous rock being concentrated in discrete veins flanked by altered wall rocks (Hedenquist et al., 1996). There are, however, some large tonnage world-class low-sulfidation deposits (50*
Mineral Deposit Research Unit University of British Columbia
120 Mt) but with low grades (around 1 g/t Au). Round Mountain (Nevada, U.S.A.), where grades are <1 g/t Au, is one example of a giant deposit of this type. As part of the IV Seminario Internacional sponsored by the Universidad Nacional de Ingenieria, this section reviews briefly the geometry, alteration, and styles of low-sulfidation epithermal deposits. Detailed overviews of epithermal gold deposits by Cooke and Simmons (2000) and Hedenquist et al. (2000) provide an excellent resource for exploration geologists interested in learning more or refreshing their knowledge of these types of deposits.
GEOMETRY DEPOSITS
OF
LOW-SULFIDATION
EPITHERMAL
Low-sulfidation epithermal deposits are shallow level deposits (< 1 km depths) associated with high-level intrusive or volcanic complexes (Fig. 2). They are found in dome fields, diatremes, pyroclastic and sedimentary rocks as well as pre-volcanic rocks that underlie volcanic supracrustal sequences. Disseminated ore minerals are generally uncommon except where permeable strata enhanced lateral fluid flow. Most deposits are veins, vein swarms, and breccia of tectonic, magmatic, or hydrothermal origin. They are localized along faults active during hydrothermal circulation. Fluid flow was channeled along faults and interconnected fracture systems, and is commonly up to several km’s from any magmatic source (Cooke and Simmons, 2000). Low-sulfidation-related hydrothermal systems have a large vertical extent (Fig. 3), and the resultant deposits may extend several kilometers in strike length. Siliceous sinter deposits dominate surficial environments, and represent the surface discharge zone. Silica deposition as opal forms the sinter. They are barren of economic mineralization, although Hg has been mined from some sinters above a low-sulfidation deposits (e.g. McLaughlin; Sherlock et al., 1995). Near sinter deposits, acid vapors generated by steam heated ground waters thoroughly leach rocks or sediments resulting in porous rocks dominated by kaolinite-alunite-opaline silica-native sulfur. Beneath the steam-
heated zone lies a tabular chalcedony-rich horizon, which represent replacement of rocks just beneath the water table. With time, opal and to a lesser extent chalcedony recrystallizes to quartz, making distinguishing a chalcedony replacement blanket along a paleowater table from a surface sinter deposit difficult. Distinguishing between these two environments is critical to an exploration program. The single diagnostic feature is the presence of vertical structures due to algal growth and evaporation (White et al., 1989). Mineralized rock lies immediately beneath sinter deposits or may lie at variable depths beneath the surface (Sherlock et al., 1995; Cooke and Simmons, 2000; Hedenquist et al., 2000). With depth, deposits become more focused and restricted to the fluid channels, and wall-rock alteration becomes telescoped in width (Fig. 3). Early stage wall-rock alteration may isolate the hydrothermal fluid from the effects of wall rock interaction. (Gibson et al., 1991). Subsequent paragenetic variations within the vein mineralogy reflect the input of fluids with subtle but important differences in fluid composition that may have economic implication. Within any deposit, the mineralized horizon is restricted to a vertical distance of 100 to 200 meters. The hydrodynamics of the geothermal system, fluid chemistry, and tectonics during fluid flow determine the depth of the mineralized zone. Flow of high temperature waters is driven by buoyancy and subtle differences in mean stress. It is largely vertical, although lateral fluid flow was important in some deposits.
GEOCHEMICAL AND MINERALOGIC CHARACTERISTICS Low-sulfidation epithermal deposits are deposited from near-neutral pH, chloride-rich waters that have extensively interacted with meteoric water. Sulfur and carbon are derived from magmatic fluids, with the dominant sulfur species being reduced H 2S(aq). Temperatures of ore deposition at depth are around 300°C, and diminish as the fluid rises, with temperatures of mineral deposition being controlled by the hydrostatic pressure gradient. Boiling is the most effective means of ore deposition. Fluid mixing is important locally, and usually happens late
as the hydrothermal collapses. Fluid salinity are low (<3.5 wt.% NaCl equivalent). High dissolved gas (CO2, H2S) concentrations can increase the depths of boiling. Depth and temperature controls alteration mineral assemblages and their distribution. Base- and precious metal ore is associated with quartz or chalcedony plus lesser but variable amounts of adularia and or Kmica (illite or sericite), calcite and (or) rhodochrosite, chlorite and pyrite. Rhodonite, roscoellite, fluorite, anhydrite, hematite are present in some deposits. Early stage adularia may be subeconomic or barren in some deposits (Orcopampa; Gibson et al., 1991). Lattice textures of calcite pseudomorphed by quartz are common. Veins are generally crustiform and colloform bands. The latter were probably deposited as amorphous silica (Fournier, 1985) that crystallize to quartz with time. In deeper veins, comb textured quartz dominates. Multiple opening episodes typify veins, thereby imparting a repetitive paragenetic mineral sequence. From vein margins and outward, adularia is replaced by K-mica, and quartz decreases (Fig. 3). At depth, propylitic assemblages form a halo of variable width lateral to the veins. Beneath the silica sinter in the shallow parts of the geothermal system and above the mineralized rocks, rocks are altered to clay (smectite, interlayered smectite-illite, illite, chlorite, and kaolinite) and carbonate minerals. Veins in these intervals are filled with carbonate minerals deposited from the CO 2-rich steamheated bicarbonate waters (Fig. 2) (Simmons et al., 2000). Ore minerals likewise vary with depth. In shallow deposits (<300 m), cinnabar, stibnite, pyrite or marcasite, arsenopyrite, Au-Ag selenides, Se sulfosalts, pyrrhotite, and Fe-rich sphalerite are recognized (Hedenquist et al., 2000 and references therein). Metals and anomalous elements in these deposits are Au-Ag-As-Sb-Se-Hg-Tl with low Ag/Au, and limited base metals (<0.1-1%). In deeper deposits (>300 m), mineral assemblages of pyrite, Au-Ag-bearing sulfides, and sulfosalt minerals, variable Fe-poor sphalerite, galena, chalcopyrite, and tetrahedrite or tennantite. Metals and anomalous elements include Ag-Au-Pb-Zn with anomalous Ba-Mn-Se, and high Ag/Au with 2-10% and locally as much as 20% of the vein being base metal minerals. A vertical transition from
“deeper” and base metal rich to “shallower” base metal deficient deposits has not been demonstrated, although predicted in early models of lowsulfidation deposits (e.g. Buchanan, 1981). Such a transition is now thought to be unlikely based upon the differences in fluid chemistry required by the metal and mineral assemblages (Hedenquist et al., 2000; see below). John et al., 1999 (see also Hedenquist et al., 2000, 2001) proposed that low-sulfidation deposits form from two distinct fluids that are related to the oxidation states of parental magmas. They link the magmas and exsolved fluids to distinctive tectonic environments. Sulfide poor deposits dominated by precious metals and that are base metal deficient appear to be associated with reduced magmas commonly found in bimodal basal-rhyolite volcanic sequences. These deposits also have shallow depths of formation. In contrast, convergent margin andesitic arcs are typified by highly oxidized and water rich magmas. Deposits formed in these environment can have significant base metals associated with precious metals and appear to have formed at greater depths.
STRUCTURAL SETTING Low-sulfidation deposits are common along fault systems filled by veins that may extend up to several kilometers or as discrete veins of limited strike-length over large distances. Regardless of strike length, the unifying characteristic is that deformation is synchronous with hydrothermal fluid flow, and that space is made dynamically for the vein growth. Extensional or strike-slip environments associated with magmatic activity are most favorable for low-sulfidation deposits (summarized by Cooke and Simmons, 2001), although locally some deposits are formed in unique settings within fold-and-thrust belts (Tosdal et al., 1996). Within each low-sulfidation deposit there is a dominant vein orientation from which finite strain axes during mineralization can be inferred. The locally inferred strain axes are consistent with regional strain patterns. Because of the syn-mineral deformation and longdimensions to the systems, a differential horizontal stress must have
characterized the region during the hydrothermal system. Tosdal (2001) proposed that syn-hydrothermal deformation was crucial to the formation of low-sulfidation epithermal deposits. Deformation enhances the escape of hydrothermal fluids from the magmatic-hydrothermal system around an intrusive complex at depth (Fournier, 1999), and the lateral transport, interaction, and neutralization of the magmatic-derived fluids through interaction with wall rocks and with meteoric fluids.
CONCLUDING REMARKS Low-sulfidation epithermal deposits are a major source of precious metals and to a limited extent of base metals. These deposits form from near surface circulation of near-neutral pH and chloride rich geothermal fluids associated with shallow level magmatism. Alteration and ore mineral are a product of fluid composition as well as the depth of
formation within the geothermal system. Understanding these variations both laterally and vertically are crucial as it is the early recognition of critical physical criteria which can contribute to the success of an exploration program focused on these deposits.
REFERENCED CITED Buchanan, L.J., 1981, Precious metal deposits associated with volcanic environments in the Southwest: Arizona Geological Society Digest 14, p. 237-262. Cooke, D.R., and Simmons, S.F., 2000, Characteristics and genesis of epithermal gold deposits: Reviews in Economic Geology, v. 13, p. 221-244.
Fournier, R.O., 1999, Hydrothermal processes related to movement of fluid from plastic into brittle rock in the magmatic-epithermal environment: Economic Geology, v. 94, p. 1193-1211. Fournier, R.O., 1985, The behavior of silica in hydrothermal solutions: Reviews in Economic Geology, v. 2, p. 45-72. Gibson, P.C., Noble, D.C., and Larson, L.T., 1990, Multistage evolution of the Calera epithermal Ag-Au vein system, Orcopampa district, southern Peru: First results: Economic Geology, v. 85, p. 1504-1519. Hedenquist, J.W., Izawa, E., Arribas, A., Jr., and White, N.C., 1996, Epithermal gold deposits: Styles, characteristics, and exploration: Poster and booklet, Resource Geology Special Publication 1, 17 p. Hedenquist, J.W., Arribas R., A., and Gonzalez-Urien, E., 2000, Exploration for epithermal gold deposits: Reviews in Economic Geology, v. 13, p. 245-277. Hedenquist, J.W., Claveria, R.J.R., and Villafuerte, G.P., 2001, Types of sulfide-rich epithermal deposits and their affiliation to porphyry systems: Lepanto – Victoria – Far Southeast deposits, Philippines, as examples: II Congresso Internacional de Prospectores Y Exploradores, Lima, Perú. Henley, R.W., and Ellis, A.J., 1983, Geothermal systems, ancient and modern: Earth Science Reviews, v. 19, p. 1-50. John, D.A., Garside, L.J., and Wallace, A.R., 1999, Magmatic and tectonic setting of late Cenozoic epithermal gold-silver deposits in northern Nevada, with an emphasis on the Pah Pah and Virginia Ranges and the northern Nevada rift: Geological Society of Nevada Special Publication no. 29, p. 65-158. Sherlock, R.L., Tosdal, R.M., Graney, J.R., Losh, S., Jowett, E.C., and Kesler, S.E., 1995, Origin of the McLaughlin Mine sheeted vein complex: Metal zoning, fluid inclusion and isotopic evidence: Economic Geology, v. 90, 2156-2181. Simmons, S.F., Arehart, G., Simpson, M.P., and Mauk, J.L., 2000, Origin of massive calcite veins in the Golden Cross, low-sulfidation epithermal Au-Ag deposit, New Zealand: Economic Geology, v. 95, p. 99-113. Tosdal, R.M., 2001, Is there a tectonic influence on whether highsulfidation or low-sulfidation epithermal deposits dominate a
metallogenic episode in a magmatic arc?: Proceedings of the Hydrothermal Odyssey Meeting, Townsville, Australia. Tosdal, R.M., Sherlock, R.L., Nelson, G.C., Enderlin, D.A., and Lehrman, N.J., 1996, Precious metal mineralization in a fold and thrust belt: The McLaughlin hot spring deposit, northern California, in Coyner, A.R., and Fahey, P.L., eds., Geology and ore deposits of the American Cordillera: Geological Society of Nevada Symposium Proceedings, Reno/Sparks, Nevada, April, 1995, p. 839-854. White, N.C., and Hedenquist, J.W., 1995, Epithermal gold deposits: Styles, characteristics, and exploration: Society of Economic Geologists Newsletter, no. 23, p. 1, 9-13. White, N.C., Woode, D.G., and Lee, M.C., 1989, Epithermal sinters of Paleozoic age in north Queensland, Australia: Geology, v. 17, p. 718-722.
Figure 1. Global distribution of significant low-sulfidation and high-sulfidation epithermal deposits. Modified from Hedenquist et al. (2000) and Antonio Arribas (written communication, 2001).
Significant epithermal deposits
Low-sulfidation High-sulfidation
Figure 2. Simplified magmatic-hydrothermal framework of low-sulfidation epithermal deposits. The low-sulfidation epithermal environment lies at the top of a fluid up-flow zone driven by heat associated with a deeper magmatic body. Below 1 km, maximum temperature and the pressure gradient approximates hydrostatic boiling, whereas these are suppressed at shallower level, where the hydraulic gradient can force lateral movement of fluids and increased interaction with ground waters, thus diluting the magmatic fluid. Three fluids are present in the systems. 1. Buoyant chloride fluids form the hydrothermal plume rising above a cooling magma chamber. This fluid transports most of the metals, and is a mix of magmatic and deeply circulating meteoric water. 2. CO2-rich steam-heated ground water that is heated by condensation of steam that separates from the boiling deeper chloride fluids. 3. Acid-sulfate steam-heated waters. Adapted from Cooke and Simmons (2000, and references therein). LS. low-sulfidation.
acid sulfate steam-heated waters mud pools, fumaroles
CO2 - rich steamheated waters cold groundwaters recharge
o
250 C o
o
300 C
Neutral chloride LS waters o
400 C m k 2
2 km
chlorite waters boiling springs, silica sinter
magma
200 C
Figure 3. Schematic cross section of a low-sulfidation system emphasizing the variable form with depth, and showing typical alteration mineral zonation patterns around veins. Adapted from Buchanan (1981) and Hedenquist et al. (2000).
Steam-heated (kaolinite - alunite - native S - opaline silica) Hot Spring
Water table
Sinter
Chalcedony blanket Smectite / mixed layer clay ± chlorite
Vein ore
Disseminated ore
50 - 100 Sericite/illite ± adularia meters 0
50 - 100
Chlorite - calcite ± epidote