Ore and Gangue Mineral Paragenesis of the Cortez Hills Carlin-Type Gold Deposit, Nevada: Evidence for Coincident High-Grade Gold Deposition and Collapse Brecciation

摘要

The Cortez Hills Carlin-type gold deposit (CTGD), located on the Battle Mountain-Eureka trend of Northern NV, is hosted in a conical shaped polylithic breccia whose central axis strikes parallel to the imbricate Voodoo fault system. Gold grades at the center of the Cortez Hills Breccia Zone (CHBZ) are locally in excess of an ounce per ton. Gold mineralization within the refractory ore at Cortez Hills shares many characteristics with other well studied CTGDs. However, new observations during this study have recognized 1) fragmented realgar that are rimmed by gold-bearing iron-sulfide minerals in four samples, 2) a Hg- and Tl-rich late-ore stage characterized by rare minerals, and 3) textural relationships show that ore-stage mineralizing fluids enhanced porosity through decarbonatization, resulting in contemporaneous brecciation and Au mineralization.The primary objectives of this thesis project were to produce a paragenesis and characterize mineralization at the Cortez Hills deposit with an emphasis on the CHBZ in order to test the hypothesis that the CHBZ contains typical Carlin-type mineralization and alteration. The data collected indicate that the CHBZ refractory ore exhibits the following paragenetic relationships. 1) Diagenetic pyrite was deposited in passive margin carbonate sediments. 2) A pre-ore suite of minerals apparently associated with low-grade contact metamorphism includes sphalerite, chalcopyrite, and tremolite. 3) Ore stage alteration minerals include illite, ore-stage jasperoid, and Au- and trace element-rich iron sulfides. 4) Hg- and Tl-rich minerals including aktashite (Cu6Hg3As4S12) and christite (TlHgAsS3) precipitated during a late-ore stage directly following the precipitation of Au-bearing iron sulfides. 5) Late- to post-ore stage minerals include realgar and calcite that are associated with cooling and collapse of the hydrothermal system.As a secondary goal, data were collected to test the hypothesis that Carlin-type mineralization occurred contemporaneously with brecciation throughout the CHBZ; understanding this relationship is necessary to understand how the CHBZ formed. Transects through host rocks that transition from negligible to high Au concentrations show a succession from primarily recrystallized calcite into rocks that contain Au-bearing iron sulfides, illite, fine grained calcite, and insoluble residual material precipitated in dissolution seams. As Au concentrations continue to increase within the ore body, host rocks that contain dissolution seams transition into heavily brecciated rocks with strongly mineralized clasts and a lightly mineralized matrix. Late-ore stage realgar precipitated in open space throughout the breccia, which is evident because it commonly conforms to euhedral crystal faces and cements zones of intense fracturing and areas of high porosity. This study concludes that the dissolution seams reveal Carlin ore fluid pathways through the carbonate host rocks, and extensive fluid-rock interaction along these seams ultimately resulted in brecciation.Late-ore-stage realgar precipitation in open space throughout the breccia signifies that brecciation terminated with cessation of the ore fluid incursion throughout the majority of the CHBZ. However, an area within the CHBZ contains fragmented realgar clasts with Au-bearing iron-sulfide rims, indicating localized mineralization both post- and pre-brecciation. The consistent association between fragmentation and mineralization of realgar clasts indicates that the same process that fragmented the realgar also allowed the ore fluid to be reintroduced to previously mineralized rocks. Reactivation along a fault may have fragmented the realgar and reduced pressure sufficiently to allow the Au-bearing fluid to encounter an area that was previously cooler and collapsing, thus mineralizing the same area twice.