Stream sediment exploration for gold and silver in Nevada — application of an old prospecting method using modern analytical techniques

Abstract

The Nevada Bureau of Mines and Geology has completed a mineral resource assessment of the Nellis Air Force Range (NAFR), an area of about 12,000 km 2 in southern Nevada that has essentially been withdrawn from mineral entry since the early 1940s. The NAFR is mainly underlain by Proterozoic and Paleozoic miogeoclinal rocks, Tertiary volcanic rocks, and Quaternary continental deposits. It contains all or part of 36 historic mining districts, about 18 of which are precious metals districts with known but minor gold and silver production. Most of the precious metals mineralization is hosted by Tertiary volcanic rocks and includes both low-sulfidation and high-sulfidation deposit types. Assessment of potential for metallic mineral deposits in the NAFR was carried out using background rock sampling, stream sediment sampling, and remote sensing, followed by examination and sampling of historic mining districts and alteration areas. Silt and float chip sampling were selected as the most efficient methods following a pilot stream sediment sampling study in regional mining districts that also included panned concentrate analyses. Threshold values for anomalous samples were determined from background rock samples. Float chip samples, which consist of small fragments of visibly mineralized and altered rock (generally limonitized and silicified) collected from stream sediment sampling sites, were determined to be the most effective in the differentiation of favorable areas for gold and silver deposits. Ag, As, and Au were present in anomalous amounts in dry stream beds draining more than most of the areas that were determined to have high potential on the basis of subsequent rock sampling. Hg, Mo, and Te were also found to be effective precious metal pathfinders in many cases. Silt sampling was clearly less effective. Ag, As, and Au appear to be the best indicators of potential in silt samples. On the basis of this work, combined silt and float chip sampling is recommended for gold and silver exploration in Tertiary volcanic rocks.