Ore classification and breccia formation in the 144 Zone gold deposit: A chemical replacement model, Bare Mountain range, Nevada

The 144 Zone is an oxidized, breccia-hosted, disseminated gold deposit that formed along the contact between Early Cambrian Carrara Formation limestone and overlying Bonanza King Formation dolomite within the Bare Mountain range in southern Nevada. Gold mineralization occurs within a breccia body that contains a variety of breccia types. Research goals of this project included classifying clay, oxide and other minerals as well as breccia types to identify the habits of gold mineralization and the minerals associated with gold. Research was also aimed at determining the paragenesis of Au mineralization and brecciation in the 144 Zone. Underground mapping provided spatial relationships between breccia types, host rocks and alteration assemblages. Clay minerals and the effects of a post-ore oxidation event have made it difficult to identify primary mineral assemblages; however, samples of different breccia types collected along transects from low to no Au to high grade were analyzed using transmitted and reflected light petrography, applied reflectance spectroscopy, secondary electron microscopy, and electron probe microanalysis to characterize mineral assemblages, identify clay and oxide minerals, and locate gold.;Two main breccia types have been identified. Breccia type 1 (BT1) has clasts composed of dolomite, phengitic dolomite and quartz with goethite cemented by a quartz-rich matrix; breccia type 2 (BT2) has clasts consisting of dolomite, phengitic dolomite and quartz with goethite cemented by a matrix of dominant phengite and goethite and minor quartz. Neither breccia type has a preferred association with high gold grades.;Clast and matrix compositions and textures show that both breccia types formed simultaneously by selective replacement of the lower-most Bonanza King dolomite. Fluid-rock reactions transformed the dolomite into having the appearance of clasts. Quartz replacement of dolomite and precipitation of pyrite, Au and phengite produced the matrices of BT1 and BT2.;Gold is most closely associated with cubic goethite that replaced pyrite, but is also closely related to anhedral goethite. Gold is located in pitted zones of goethite and along the contact of goethite with quartz. Quartz is typically associated with and encloses Au-bearing goethite.;Clay minerals include relatively common high-temperature phengite with minor to trace kaolinite, montmorillonite, smectite, and phengitic illite. Although phengite is the most abundant clay, it is not consistently spatially associated with gold at the sample scale, though it may be at the deposit scale.;The geologic constraints that characterize gold mineralization in the 144 Zone can be applied to exploration throughout Bare Mountain for additional gold that formed under similar conditions. This study suggests that the important geological controls were the Carrara-Bonanza King contact, the presence of reactive lowermost Bonanza King dolomite, and the presence of a quartz latite dike. Locating other localities of this contact at fault or dike intersections that could have provided conduits for gold-bearing fluids can provide future drill targets.