- © 2017 Society of Economic Geologists.
The Cortez Hills breccia zone, a Nevada Carlin-type gold deposit located in the Cortez district on the Battle Mountain-Eureka trend, formed within a high-grade polylithic breccia, and grades at the center of the breccia zone are locally in excess of 1 oz/ton Au. Sample transects from low grade or below detection (<5 ppb) to high grade were collected and analyzed using petrography, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses to correlate changes in ore pyrite geochemistry and alteration mineralogy with grade and to relate ore deposition to breccia formation.
Ore-related minerals, textures, and analyses are consistent with fluid-rock reaction and replacement at relatively low hydrothermal temperatures (<~260°C) during a single heating and cooling event and over a relatively short time frame, coincident in time and space with regional magmatism. Prior to Carlin mineralization, Roberts Mountains, Wenban, and Horse Canyon Formations were affected by burial diagenesis and low-grade contact metasomatism and metamorphism, producing host rocks composed of calcite, dolomite, and quartz with generally minor dickite, pyrite, chalcopyrite, sphalerite, clinochlore, tremolite, and organic material, and minor to trace K-feldspar, muscovite, and biotite. Acidic ore fluids sulfidized Fe in carbonate rock to form rims of Au- and trace element-rich pyrite on earlier formed pyrite or produced small, generally <10-μm, spheres of fuzzy ore pyrite with no apparent core. Trace elements in ore-stage pyrite are dominated by As with strongly correlated Hg, Tl, and Cu, all greater than Au; lesser and variable Sb; and minor to undetected Ag, Bi, Ti, W, and Te. Ore fluids also variably dissolved calcite and dolomite and replaced carbonate minerals with jasperoid or produced minor pore space. Ore pyrite and jasperoid after calcite are enclosed in illite that replaced dickite, clinochlore, and tremolite and filled minor open space, primarily between ~227° and 277°C for most analyzed samples based on illite chemistry.
As hydrothermal collapse began, ore pyrite chemistry transitioned to an evolved ore-stage chemistry with abundant As, low to undetected Au, lesser Pb + Cu > Sb, and diminished Hg and Tl compared to the ore stage. Evolved ore-stage fluids coincidently precipitated sulfosalt minerals—initially, unusual Cu- and Hg-rich aktashite (Cu6Hg3As4S12) and Tl- and Hg-rich christite (TlHgAsS3) and, later, realgar as Cu and Zn in hydrothermal fluids—and chalcopyrite and sphalerite in the host rocks, respectively, were consumed. Calcite-only veins indicate termination of the hydrothermal system.
Small fragmented clasts of realgar with chemically distinct Au-bearing, evolved ore-stage pyrite rims found in brecciated rock along a faulted dike-Wenban contact provide the first evidence in a Carlin-type gold deposit of precipitation of an Au-bearing pyrite rim on a late ore-stage mineral. These rare rims likely formed in response to fault movement along the dike-Wenban contact or by dike injection that allowed deep, Au-bearing fluids to access a higher and cooler part of the system where late ore-stage minerals had begun to form. These rims further highlight the normal textures in Carlin-type gold deposits where ore-stage pyrite was overgrown by late ore-stage sulfosalt minerals, demonstrating a single hydrothermal event followed by a single cooling event. This paragenesis is consistent with previously modeled deposit formation time frames of ~45,000 to 15,000 years.
Two styles of breccia were identified within the deposit. Replacement, matrix-supported breccia, in which an illite-rich, punky matrix encloses Au-bearing pyrite, jasperoid, and variable relict calcite, formed during intense ore-stage alteration and mineralization. The fine-grained ore and alteration replacement minerals did not require significant open space to form. Later evolved and late ore-stage sulfosalt minerals and calcite that are visible at the hand-sample scale provide the earliest evidence for the development of significant open space. Hydrothermal collapse allowed the incursion of cooler meteoric water into the system, which—along with the retrograde solubility of calcite—provided coupled mechanisms driving significant calcite dissolution, collapse brecciation, and formation of the Cortez Hills breccia zone. Dissolution and collapse brecciation increasingly concentrated ore-stage pyrite as the hydrothermal system transitioned to the late ore stage, contributing to high Au grades.