GSN Elko Chapter Meeting

Description

Contact: justin.b.milliard@gmail.com

The Elko GSN Chapter is excited to have a GSN Elko scholarship recipient return to our stage. Atticus Proctor will be presenting his ongoing, nearly completed research focused on Characterizing the Deep High-Grade Fourmile Gold Deposit in the Cortez District, Nevada. To tie into his talk, please notify any students you may know to apply for our GSN Elko Scholarships for the 2022-2023 academic year at https://www.gsnv.org/chapters/elko/. We have yet to receive applicants and really want to see the money available get to use doing geologic research.

Western Folk Life Center

Thursday, March 17th

Food and drinks @ 6 pm and talk to begin at 7 pm.

Presenter: Atticus Proctor, Center for Research in Economic Geology at the University of Nevada, Reno

Title:  “Breccia Bodies in the High-Grade Fourmile Carlin-Type gold deposit, Nevada: Implications for understanding the Paragenesis of High-Grade Gold Ore”

MEETING SPONSOR:  IDS

Abstract: 

The Fourmile gold deposit is a Carlin-type deposit discovered in the Cortez mining district, Nevada, USA in 2015. Ore is hosted in deep (>600 meters) breccia bodies. Gold is associated with multiple breccia types, occurring as sub-millimeter native gold and in arsenian pyrite rims. The purpose of this study is to understand the complex series of hydrothermal alteration, gold deposition, and brecciation events using time-space relationships gathered from analysis of breccia textures.

Breccias are timelines constraining the relative ages of at least four superimposed hydrothermal events. Early calcite veining and calc-silicate mineral growth are followed by metasomatic bleaching that forms a halo around the east-verging (30-50°) Sadler reverse fault, suggesting that faulting pre-dated pre-Carlin metasomatism. Subsequent base metal mineralization, indicated by calcite veins containing base metals crosscutting pre-Carlin metasomatized rocks, was succeeded by sub-millimeter native gold mineralization, observed in rocks only within the Sadler fault bleached halo. Breccia relationships show Carlin-style decarbonatization, silicification, and deposition of pyrite with arsenian rims followed. Mineralized breccias carry the highest gold grades where the Sadler fault intersects high-angle (60-75°) west dipping normal faults, which are typified by breccias with milled to subangular mineralized clasts. Late calcite brecciation and realgar signal the most recent hydrothermal event. Understanding the role of overprinted brecciation and hydrothermal systems is likely the key to revealing causes of Fourmile’s high grade and the genesis of Carlin-type deposits. Paragenetic observations suggest the unusually high-grade ore at Fourmile is the result of multiple hydrothermal events, ranging from the Jurassic-Cretaceous to Eocene.