Geochemical evaluation of a mineralized fossil hot spring system, Eureka County, Nevada

Abstract

In 1980, Chevron Resources Company discovered anomalous gold and silver in a massive silica vein set near the Beowawe hot springs, Nevada. Surface exploration revealed low but consistent gold values associated with the chalcedony veins. The veins are hosted by flat-lying Miocene dacites, and structurally juxtaposed fine-grained Ordovician sedimentary rocks of the Valmy Formation. Preliminary analyses of the relict hydrothermal system revealed sparse mineralogy. The system is dominated by quartz, pyrite, and clay minerals; base metal sulfides and fluid inclusions are absent. Analyses of precious metal data reveals that significant mineralization occurs in the wallrock despite sharp vein contacts, lack of brecciation, and apparent lack of alteration. Although elevated values of arsenic do occur, anomalous values of As, Sb, and Ti do not occur outside of mineralized zones, and are not useful as pathfinder elements. In 1983, Chevron drilled seven exploration holes, encountered only weak precious metals, and dropped the property. Analyses of drill cuttings reveal widespread advanced argillic alteration and secondary iron oxides. Xray analyses of clays indicate the system is dominated by kaolinite with subordinate montmorillonite and illite/sericite. Secondary potassic alteration takes the form of sericite rather than adularia. Higher Ag/Au ratios in the Valmy Formation are likely the result of residual silver in the sedimentary rocks rather than hydrothermal zoning. Base metals, although very weak, indicate zoning trends and are inversely correlative with gold. Major elements shifts are roughly conformable to the fluid mobility of those elements. Si and K are generally enriched; Ca, Mg, Fe, Na, and Mn generally depleted. These metasomatic reactions produced moderate increases in mass and volume ranging from 10% to 30%. Overall, the system is dominated by, 1) pervasive silicification, 2) hydrolytic leaching and the production of argillic alteration products, and 3) a general fluid redistribution of major elements between the two disparate lithologies.