Characterization of the Gold Hill low-sulfidation epithermal Au-Ag vein system, northern Nye County, Nevada; Implications for continued discoveries in established mining districts
Loading...
Authors
Goetz, Silas L.
Issue Date
2024
Type
Thesis
Language
en_US
Keywords
characterization , epithermal , gold , Gold Hill , Low-sulfidation , Round Mountain
Alternative Title
Abstract
The Gold Hill low-sulfidation epithermal (LSE) vein system is an exceptionally well-preserved and extensive (~400k oz Au (eq.), RMGC internal data, 2023.) Oligocene-age vein deposit located within the Round Mountain mining district of northern Nye County, Nevada, USA. Gold Hill is seven kilometers north of the world-class Round Mountain LSE deposit, which has produced >16.5 million ounces of gold to date. Although there is a close spatial and temporal relationship between Gold Hill and the giant Round Mountain deposit, their genetic relationship has not been well-studied. This study presents new data that characterizes the ore and gangue mineralogy of at least three stages of mineralization at Gold Hill as follows:• Stage I), Monomictic pyrite±marcasite breccias, with angular-subangular 2-20 cm, quartz-adularia altered clasts of tuff of Mt. Jefferson (Tmj) and varying intensities of silicification in the matrix, which includes up to 10% pyrite±marcasite (modal abundance).
• Stage II), white-tan-pink medium-coarse grained, massive-recrystallized quartz veins with pyrite±marcasite±electrum±chalcopyrite.
• Stage III), fine-medium grained crustiform-colloform quartz-chalcedony veins with abundant electrum- pyrargyrite- acanthite- Ag-tetrahedrite (freibergite)- pyrite± arsenopyrite.
Economic stage III Au-Ag mineralization occurs above the base of the boiling zone (~260 m below sinter), as evidenced by abundant macroscopic Au-Ag minerals, and well-developed vigorous boiling textures (e.g. crustiform-colloform banding). EPMA data collected on pyrite suggests that the ore forming fluids associated with stage III quartz veins were >200⁰C and indicate that stages I, II, and III pyrite have elevated precious and base metal concentrations. Pyrite, chalcopyrite and acanthite from the Gold Hill deposit have δ34S values that range from 4.4‰ to +16.6‰, including a pyrite-chalcopyrite mineral pair that yielded a temperature estimate of ~227°C.
250- ~220⁰C fluids appear concentrated along high-angle normal faults that host stages I-III of mineralization and altered tuff of Mt. Jefferson wall rock shows distinct alteration patterns. High temperature alteration assemblages (quartz-adularia-pyrite-illite) occur proximally to both breccia and vein hosted orebodies and quartz-adularia-pyrite-illite alteration pervasively replaces primary feldspars and the groundmass of the rhyolitic Tmj wall rock. Distal from breccia and vein orebodies, alteration consists of a K-feldspar±smectite±calcite±chlorite±quartz±pyrite assemblage that occupies both deeper depths of the deposit and appears distal from inferred upwelling zones (i.e. stage II + III quartz veins). This study proposes that the Gold Hill system may have formed during upward fluctuation of high-temperature ore-forming fluids and indicates a close spatial association between orebodies and intrusive domes and dikes, which suggests a local heat source at depth.
Overall, Gold Hill appears to be a discrete LSE vein system, outboard of the giant Round Mountain deposit, and the genetic characteristics of the Gold Hill LSE deposit differ from that of Round Mountain based on 1) high-temperature Tmj wall rock alteration, which indicates high fluid flux, and appears spatially constrained to breccia and vein orebodies, 2), upward fluctuation of the hydrothermal system with input of both meteoric waters and more deeply sourced ore-forming fluid components, and 3) discrete periods of mineralization that are recorded in stages I-III mineralization and that show clear variation in both metal budget and trapping mechanisms. These genetic characteristics indicate potential for the discovery of comparable vein systems in tectonic settings like that of the giant Round Mountain deposit, but also indicate the importance of understanding LSE hydrothermal alteration profiles in homogenous host rocks (i.e. Tmj), and equally important, this research demonstrates that the presence of one style of epithermal system (e.g. Round Mountain) does not preclude the presence of other styles of epithermal mineralization within the same region, district, or brownfield environment (e.g. Gold Hill), suggesting that exploration in established mining districts needs to consider multiple styles of both alteration and mineralization.