Geochemical Exploration for Gold Through Transported Alluvial Cover in Nevada: Examples from the Cortez Mine

Abstract

Geochemical orientation surveys were completed over covered Carlin-type gold deposits in the Cortez mine area with the expressed aim of identifying and evaluating exploration methods to discover Au ore under transported alluvial cover in Nevada. Orientation tests were designed to assess the utility of geochemical applications at various scales of exploration, both drill targets at the deposit scale and "footprints" associated with deposits at the district scale. Detailed surveys were completed over the covered Gap deposit, located adjacent to the giant Pipeline deposit. Both Carlin-type gold mineralization and earlier, spatially associated, base metal skarn mineralization at Gap were located by soils, soil gas, and vegetation. Loam soils at 6-to 12-cm depth provided a consistent and uniformly available sample medium. Gold ore under 10 m of cover in the northern portion of the Gap deposit was readily detected by analysis of Au by fire assay and ultra trace aqua regia methods in the-80 mesh fraction of these loam soils. Arsenic anomalies occur over the northern end and over the main ore zone at Gap, where there is 25 to 50 m of alluvial cover. Zinc concentrations in soil show the most coherent spatial relationship with underlying Au ore. Tests of MMI-B and Enzyme Leach selective leaches did not result in significant enhancement of the anomalies relative to aqua regia. CO2 and O2 in soil gas indicate faults and underlying mineralized carbonates at Gap, where weathering reactions likely generated CO2 from acid reaction with carbonate. Elevated Au and As in mixed sagebrush and shadscale occur over most ore zones, with the highest Au concentrations over the main ore zone rather than the shallowly buried northern zone. Like soils, elevated Zn in vegetation shows the most coherent spatial relationship with underlying ore. Assays from 1,859 drill holes show a 4-to 5-km 2 "footprint" at the basin gravel-bedrock unconformity centered on the Pipeline Carlin-type gold deposit, where samples of basal alluvium provide a large, coherent >50 ppb Au anomaly. Enriched As, Tl, K, and F in alkaline groundwater sampled from monitoring wells surrounding the Pipeline open pit provide a ≥5-km 2 hydrogeochemical footprint. Higher As and Tl concentrations occur down gradient from Pipeline indicating Carlin-type Au mineralization is the source of the enrichment. The surface metal anomalies are consistent with upward migration of metals through fractured alluvial cover. Likely metal transport mechanisms include barometric pumping of gases or seismic pumping of groundwater. Upward diffusion of metals through the thick vadose zones in northern Nevada is not a viable process. Surface anomalies over Gap and other deposits appear to be mature and may have developed over millions of years. In such mature anomalies, much of the metal that migrated from underlying bedrock is probably hosted in resistant secondary minerals, which are more readily dissolved by aqua regia than by various selective leaches. Sampling of soil gas and soils is appropriate ...

Barren ground depressions, natural H2 and orogenic gold deposits: Spatial link and geochemical model

“…Difference in aqueous species migration in the reduced column compared to its surroundings is also proposed for explaining the anomalous element concentration "rabbit-ear" pattern measured in soils (Smee, 1984). High CO 2 and CH 4 concentration and low O 2 concentration anomalies in soil air are known as markers of concealed mineralization (McCarthy et al, 1986;Pauwels et al, 1999;Lovell, 2000;Zhang, 2000;Polito et al, 2002;Muntean and Taufen, 2011). These anomalies have been related to the release of gas during fluid inclusion breakdown, biological respiration and to the alteration of the carbonate gangue.…”

Diversity in Carlin-Style Gold Deposits

“…Detailed studies of ore-stage arsenian pyrite show that trace element and sulfur isotope zoning formed from temporally discrete ore fluids fed by separate structures (Barker et al, 2009;Longo et al, 2009;Muntean et al, 2009). Recent research also focused on enlarging the target for Carlin-type gold deposits by looking for zoning patterns surrounding Carlin-type gold deposits, including lithogeochemistry (Patterson and Muntean, 2011) and halos of depleted δ 18 O in carbonate oxygen isotopes (Barker et al, 2013;Hickey et al, 2014a;Vaughan et al, 2016), as well as signatures in transported alluvial and sedimentary cover and groundwater (Muntean and Taufen, 2011;Cluer, 2012). In chapter 2, Muntean (2018) presents a mineral systems approach to exploring for Carlintype gold deposits in Nevada.…”

A review of metal transfer mechanisms through transported cover with emphasis on the vadose zone within the Australian regolith

“…For example, Alpers et al (1990) studied the stable isotopic composition of soil CO 2 over the Crandon deposit and found that anomalous concentrations reported by McCarthy et al, (1986) were derived from root respiration rather than from mineralization. Despite the complex and multiple origins of soil gases, numerous gas species (e.g., CO 2 , O 2 , Hg, Rn, He, sulfur compounds, and light hydrocarbons) appear to be related to buried mineral deposits (Carr et al, 1986;Klusman, 1993;Hale, 2000;Pauwels et al, 1999;Polito et al, 2002;Klusman, 2009;Muntean and Taufen, 2011;Noble et al, 2013). Specific ore bodies have been shown to contain varying amounts of light hydrocarbons having C chains of C 1 -C 9 and include methane, ethane and propane (Disnar, 1990;Hulen and Collister, 1999;Polito et al, 2002).…”