Controls on the formation of disseminated- and vein-style low-sulfidation epithermal precious metal deposits

Abstract

Low-sulfidation epithermal deposits are major sources of Au and Ag. They form in the shallowsubsurface (<1.5 km) from near-neutral chloride waters at <300°C. The ore-forming waters are rock- buffered and have a low salinity (<3‒4 wt.% NaCl equiv.). Many low-sulfidation epithermal deposits are characterized by bonanza-type ore zones confined to banded quartz veins and breccia zones and are mined as high-grade, small-tonnage deposits. However, the ore zones in some of these deposits consist of disseminated hypogene sulfides and may be extracted by low-grade, large-tonnage operations. Research at the Castle Mountain low-sulfidation epithermal deposit in California highlighted the importance of lithological controls on the nature of the deposit style. Castle Mountain represents a low- grade, large-tonnage deposit hosted in a Miocene volcanic succession that is dominated by volcaniclastic rocks. The highest gold grades occur where breccia deposits associated with rhyolite flows and domes and vertical breccia pipes interpreted to represent diatreme breccias coincide spatially with extensional faults. These host rocks provided cross-stratal permeability for thermal liquids that precipitated metals primarily through cooling during their upflow. In contrast, bonanza-type precious metal enrichment apparently occurs primarily in competent rocks of flow-dominated volcanic successions. Detailed textural studies on samples collected from bonanza-type ore zones in low-sulfidation epithermal deposits in Nevada, California, and Japan suggest that high-grade precious metals are deposited as a result of flashing of the thermal liquids. This process leads to an efficient precipitation of metals, typically forming ore mineral dendrites, which are hosted by noncrystalline silica formed by homogeneous nucleation in the liquid. The textural observations suggests that the noncrystalline silica that originally makes up the bulk of the mineralized veins recrystallizes to thermodynamically more stable quartz during and after the ore deposition. The combination of field and microanalytical research provided new insights into the mechanisms by which low-sulfidation epithermal deposits are formed. It highlights volcanological and rheological controls on the nature of these deposits as high-grade deposits can only develop in competent host rocks allowing flashing of the thermal liquids to depth. The improved understanding of ore-forming processes has implications to the design of exploration strategies for this deposit type.