ORE CLASSIFICATION OF PSEUDOBRECCIA ORE IN THE 144 ZONE GOLD DEPOSIT: A CHEMICAL REPLACEMENT MODEL, BARE MOUNTAIN RANGE, NEVADA

Abstract

Abstract The 144 zone is a pseudobreccia-hosted, disseminated gold deposit that formed in the middle to late Cambrian Bonanza King dolostone along an unconformity with the underlying early to middle Cambrian Carrara limestone at Bare Mountain, southern Nevada. Underground mapping revealed spatial relationships between breccia types, host rocks, and alteration assemblages that are related to gold mineralization. Samples were collected along transects from low- to high-grade Au and were analyzed using petrography, applied reflectance spectroscopy, scanning electron microscopy, and electron probe microanalysis to characterize mineral assemblages and evaluate gold deportment. Two breccia types are identified. Breccia type 1 clasts consist of dolomite, dolomite with phengite, and quartz cemented in a quartz-rich matrix. Breccia type 2 has similar clasts of dolomite, dolomite with phengite, and quartz, but the matrix is phengite dominant. Neither breccia type has a preferred association with gold, which occurs with goethite that replaced pyrite in both breccias. Clast and matrix compositions and textures show that the two breccia types formed at the same time by selective dissolution and replacement of the lowermost Bonanza King dolomite. Fluid-rock reaction transformed massive dolomite into pseudobreccia. Quartz replacement of dolomite plus the precipitation of pyrite, Au, and phengite yielded the 144 zone pseudobreccia matrix. The geology that characterizes gold mineralization in the 144 zone can be applied to exploration throughout Bare Mountain. Other localities where the same stratigraphic contact is cut by silicic dikes of similar age provide drill targets in the mining district.