Characterization and Paragenesis of the Deep High-Grade Fourmile Gold Deposit in the Cortez District, Nevada
Authors
Proctor, Atticus
Issue Date
2022
Type
Thesis
Language
Keywords
arsenian pyrite , breccias , Carlin , Cortez , Fourmile , Native gold
Alternative Title
Abstract
The Fourmile gold deposit is a Carlin-type deposit discovered in the Cortez mining district, Nevada by Barrick Gold Exploration Inc. in 2015. The deposit is hosted within carbonate slope facies rocks in the footwall of the Roberts Mountains thrust. Ore is hosted in four deep (>600 meters) breccia bodies within the fault-propagated anticline of the east-verging Sadler reverse fault, specifically where the Sadler fault intersects the high angle (55-65°) west-dipping normal Anna fault corridor. Gold is commonly associated with 16 breccia types that have three genetic mechanisms. Gold occurs as sub-millimeter free gold and in arsenian pyrite rims, similar to other Carlin-type deposits. This study used detailed core logging, petrography, scanning electron microscopy, microprobe analyses and geologic cross section interpretation to characterize and construct a paragenesis of breccias, alteration, and gold mineralization at Fourmile. Early metamorphism recrystallized carbonate country rocks and formed metamorphic calc-silicate mineral assemblages. Subsequent pre-Carlin hydrothermal alteration took advantage of permeability along the low-angle west-dipping Sadler thrust and created a halo of bleaching around this fault. Base metal mineralization overprints pre-Carlin hydrothermally altered rocks, as evidenced by pyrite replacing metamorphic calc-silicate mineral sites in bleached pre-Carlin hydrothermally altered rocks. An additional hydrothermal event, likely linked to an intrusion, deposited native gold and bismuthinite and silicified Fourmile rocks only within the halo of bleached rocks surrounding the Sadler fault, suggesting it formed before high-angle structures. High-angle fault brecciation in the Anna fault corridor postdates base metal and native gold mineralization based on fresh and bleached rounded and milled clasts that have been brecciated multiple times. Geologic cross section analysis shows ore at Fourmile is controlled by the Sadler fault, Anna fault corridor, the contact between the Devonian Horse Canyon and Devonian Wenban 8 subunit, and the contact between the Devonian Wenban 8 and 5 subunits. Fault-breccia-hosted gold mineralization is dominant along fault zones and collapse brecciation is prevalent at lithologic contacts. Gold grades typically increase where more generations of re-brecciation are interpreted. Re-brecciated breccias are typically observed in fault zones that have been overprinted by collapse brecciation and in collapse breccias where multiple acidic decarbonatizing fluid pulses led to the formation of multiple generations of cavity-filling breccias. Breccia cross cutting relationships, petrography, and geologic cross sections suggest three pulses of Carlin ore-stage impacted the rocks at Fourmile. Carlin ore-stage gold is hosted in arsenian pyrite, and pyrite grains with fuzzy rims typically hold more gold than rims with blocky rims. Late calcite veining fills void spaces in breccias at Fourmile and over-pressured calcite-bearing fluids are interpreted to form some hydrothermal calcite breccias. The rocks at Fourmile record the relative ages of one metamorphic and four different super-imposed hydrothermal events. Metamorphism was followed up by pre-Carlin hydrothermal alteration, base metal mineralization, native gold deposition, and Carlin-style gold deposition and alteration. Paragenetic observations suggest the unusually high-grade ore at Fourmile is the result of multiple hydrothermal events, ranging from the Jurassic-Cretaceous to Eocene.
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Citation
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License
Creative Commons Attribution 4.0 United States