Abstract

Quartz at the Betze-Post deposit, the largest Carlin-type gold system in the world, was examined to better understand ore fluid properties and sources that generated Carlin-type gold deposits. Detailed petrography and paragenesis investigations distinguished textural relationships, luminescence, various generations of quartz, and fluid inclusion populations; microthermometry, chemical analyses of quartz, and in situ and conventional oxygen isotope analyses of quartz provided information about hydrothermal fluid conditions as well as fluid sources and ore formation processes.

Pre-, syn-, late-, and post-ore stages of quartz were distinguished during petrographic studies and confirmed by cathodoluminescence analyses. Ore-stage jasperoid and late-ore drusy quartz are spatially associated with Au-bearing pyrite; both lack luminescence. Two generations of post-ore drusy quartz overgrew ore and late-ore quartz and exhibit bright and multiply zoned luminescence. Electron probe microanalyses determined that nonluminescing ore and late-ore quartz have elevated concentrations of Al, in contrast to post-ore luminescent quartz that contains low trace element concentrations. Fluid inclusion microthermometry determined that ore-stage fluids had low salinities, generally <5 wt % NaCl equiv, and were trapped at 180° to 240°C.

The δ18O values, calculated for inclusion fluids and determined for quartz using in situ ion probe and conventional analyses, indicate that the isotopic composition of hydrothermal fluids became 18O depleted with time, from the ore stage through the late-ore stage to the post-ore stage. The δ18O composition of each stage of quartz also varies spatially across the Betze-Post deposit, and δ18OVSMOW values of ore-, late-ore, and post-ore stage quartz became more depleted with increasing distance from the master Post fault. These patterns are interpreted to result from increasing dilution of the 18O-enriched ore fluid by unevolved meteoric fluids over time and with increased ore fluid transport distance to the west, away from the Post fault, the master fault that transmitted hydrothermal fluids to the deposit.

Oxygen isotope results are consistent with a low fluid/rock ratio during upward transport of the ore fluid via the Post fault. Below the deposit, some ore fluids were diverted into the JB Series faults, associated with the Betze ore zones, and the Shalosky fault, associated with the Screamer ore zone in the western part of the deposit. Ore fluids were increasingly diluted by meteoric fluids with distance as ore fluids traveled away from the Post fault and over time as different stages of quartz precipitated. Acidic, 180° to 240°C ore fluids accessed the level of the deposits where they reacted with the Devonian Popovich Formation and precipitated nonluminescing, Al-enriched ore-stage jasperoid and late-ore drusy quartz, along with Au and trace element-rich pyrite. Fluid-rock reaction, dissolution, and replacement of silty carbonate host rocks may have produced Al that substituted for Si in non-luminescing jasperoid and late-ore stage quartz. As the hydrothermal ore system began to cool and collapse, late-ore drusy quartz and realgar precipitated in open space as ore-stage pyrite precipitation declined. The hydrothermal system collapsed with meteoric water flooding; post-ore minerals, including luminescent, Al-poor post-ore drusy quartz and later calcite and barite, precipitated either from latest Carlin-related fluids, neutralized by infiltration of meteoric waters, or as part of a later, separate hydrothermal event.

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