J.W. Hedenquist, et al., report that epithermal ore deposits form at shallow depth. Lindgren based this conclusion on ore mineralogy and related textures, plus geologic reconstructions .. more recent studies indicate that most epithermal ores form over the temperature range of 150 to less than 300 degrees C, from the surface to as deep as 1 to 2 km. Based on their observations and those from the literature, they summarize the principal characteristics of epithermal mineralization in which gold is the dominant economic metal.
They highlight their brief summary with case studies of eight deposits that represent two end-member styles of epithermal gold deposits ... high sulfidation (HS) and low sulfidation (LB). These examples represent the range in depth that epithermal deposits form, from just below the surface to more than 1 km depth.
The two deposit styles form from fluids of distinctly different chemical composition in contrasting volcanic environments. The ore of HS deposits is hosted by leached silicic rock associated with acidic fluids generated in the volcanic-hydrothermal environment. The presence of high-sulfidation state sulfide minerals indicates high-oxidation. states typical of acidic hypogene fluids. In contast, the fluid responsible for formation of LS ore veins is similar to waters tapped by drilling beneath hot springs into geothermal systems. Low-sulfidation state minerals form from these reduced, neutral-pH waters.
This poster has two purposes ... 1) to introduce students of economic geology to one of the principal environments of formation of hydrothermal ore deposits, and 2) to initiate discussion among exploration geologists regarding the essential nature of epithermal deposits. They stress the basic characteristics of epithermal systems, especially those that can be recognized easily in the field, as these characteristics reflect ore-forming processes. Interpretations of such processes can then be used to guide mineral exploration.