Characteristics and Genesis of Epithermal Gold Deposits, David R. Cooke and Stuart F. Simmons, SEG Reviews Vol. 13, 2000, p. 221-244 - 1
Characteristics and Genesis of Epithermal Gold Deposits David R. Cooke and Stuart F. Simmons SEG Reviews Vol. 13, 2000, p. 221-244
Introduction Epithermal gold deposits form in the shallow parts of magma-related hydrothermal systems, commonly in sub-aerial volcanic arcs (Fig. 1). There was little advance in understanding of epithermal deposits until they became a major target of exploration as the price of gold rose in the 1970s and 1980s .
Fig. 1. Schematic diagram showing the positions of low- and high-sulfidation epithermal environments (A and B, respectively) within hydrothermal systems associated with flow-dome volcanic setting of modest relief (modified from Henley and Ellis, 1983). A. The epithermal environment is represented by “y”. Meteoric water circulates deeper than 5 km through convection driven by heat from an underlying, crystallizing magma. At depths of 1 to 2 km below the water table, within the up-flow zone, maximum temperature-pressure gradient are close to hydrostatic boiling (Henley, 1985) . At shallower levels, the hydraulic gradient causes rising fluids to move laterally (thus suppressing boiling through dilution by mixing with local ground waters) to form outflow zones. The three end-member water types that form in the low-sulfidation epithermal environment. Buoyant (floating) chlorite waters form in the hydrothermal plume and are most important for aqueous transport of precious metals. These waters derive from interaction between deeply circulated meteoric water and magmatic components (H 2O, CO2, SO2, H2S, HCl) near the base of the convection cell and subsequent fluid-mineral interactions; their pH is close to neutral. Steam-heated ground waters that gain heat and aqueous constituents from condensation of steam (containing vapor, CO 2, H2S) which separated from deeply boiling chloride water. Acid sulfate steam-heated waters contain high concentration of sulfate that derived from oxidation of H2S in the vadose zone. The distribution of these water types largely depends upon topographically controlled hydraulic gradients. B. The oval-shaped hatched zone represents the epithermal environment. It is located beneath the summit of a modest volcanic cone and above a degassing magma. In this setting, acid fluids dominate and include acid sulfate and acid chloride-sulfate waters. The outflow zone results from the hydraulic gradient or water table (dashed line which almost follows the trace of topography).
Fluid Inclusions Fluid inclusion data are used to deduce the temperatures and compositions of mineralizing fluids along with the formation depths of a deposit. They provide important constraints on the genesis of low-sulfidation deposits. Most fluid inclusion data in this study have been obtained from quartz and calcite. Bonafide primary fluid inclusions typically occur in growth zones within medium- to coarse-grained euhedral quartz crystals (Bodnar et al., 1985). In the epithermal deposits, such material may be difficult to find and data may be obtained mainly from secondary inclusions. Most fluid inclusions contain two phases (liquid + vapor) and are liquid rich. Vapor-rich two-phase inclusions are also common. It is usually impossible to measure either homogenization (Th) or ice-melting temperatures (Tm) from vapor-rich inclusions in epithermal environments (Roedder, 1984; Bodnar et al., 1985). Homogenization temperatures range from 150° to 300° C and ice-melting temperatures range from 0° to -2° C, indicating dilute fluids of generally less than 3.5 wt percent NaCl equivalent (Fig. 4A).
Characteristics and Genesis of Epithermal Gold Deposits, David R. Cooke and Stuart F. Simmons, SEG Reviews Vol. 13, 2000, p. 221-244 - 2
Hedenquist and Henley (1985) showed that ice-melting temperatures between 0° to -1.5° C could be solely (only) attributed to the presence of aqueous carbon dioxide rather than dissolved salts as is commonly interpreted. One other indicator of the gas contribution to ice-melting temperature is trend of Th-Tm data shown in Figure 4A.
Fig. 4. Temperature of homogenization versus temperature of ice melting (apparent salinity) for fluid inclusions from epithermal lowsulfidation. The shaded area between 0.0 to -1.5°C represents the effect of aqueous CO2 on the Tm of water with up to 0.85 m CO2. The inset shows the trajectories (curve) of boiling and mixing for fluids in the Broadlands-Ohaaki geothermal systems (from Hedenquist and Henley, 1985). The parent composition A is similar with to the deep chloride water (~0.75 m CO 2, 0.1 m NaCl), whereas B represents the composition of the peripheral CO2-rich steam-heated water. Curve III represents the effect of gas loss due to boiling for a water of the parent composition. Curve III’ represents the effect of gas loss after mixing with steam-heated CO 2-rich water. Curve II represents the mixing trend between A and B. Curve I represents the boiling trend due to Rayleigh evaporative steam loss of a dilute, gas-poor parent liquid.
In a few cases, Tm significantly lower than -1.5°C were measured, indicating the occurrence of waters with salinities exceeding 10 wt percent NaCl. In deposits where obvious sources of brine, such as evaporates, are absent, these high salinities can be explained by variable amounts of evaporate concentration (steam loss) under opensystems conditions (Simmons and Browne, 1997). Using fluid inclusion data for geobarometry and assuming a hydrostatic boiling point for depth pressure gradient, estimates of the depth of mineralization range from <100 to >500 m below the paleowater table. Since hydrostatic pressure gradients are common in active geothermal systems (Henley, 1985), they are generally assumed in epithermal environments.
