MICROTEXTURES OF PRECIOUS METAL-BEARING QUARTZ VEINS FROM LOW-SULFIDATION EPITHERMAL DEPOSITS: EVIDENCE FOR THE OCCURRENCE OF FLUID FLASHING

Low-sulfidation epithermal precious metal deposits form in the shallow crust, at typically less than 1 km below surface, from near neutral, low-salinity fluids. These deposits frequently contain ore minerals in dendritic arrays within colloform banded quartz-rich veins. High-magnification optical microscopy and micro-XRF mapping show that ore minerals occur within certain bands that are commonly less than one millimeter thick. Examples from the McLaughlin deposit in California and the Sleeper deposit in Nevada show that these bands containing the ore minerals are composed of microspherical opal-A, or have been partially recrystallized to quartz. The quartz that formed as a result of maturation of the noncrystalline opal-A is microcrystalline and characterized by mosaic texture. Original ore mineral textures also can be modified as the opal-A is converted to quartz. The primary textures in the vein samples studied suggest that the deposition of the mineralized bands occurred rapidly under nonequilibrium conditions. It is proposed that precipitation occurred during discrete events of fluid flashing. The production of variable amounts of vapor during these events destabilizes precious metal-carrying complexes and triggers the supersaturation of silica in the coexisting liquid.