The occurrence of gold at the Getchell mine, Nevada

Abstract

The Getchell mine, owned by the Getchell Gold Cooperation, is located in northwestern Nevada along the east flank of the Osgood Mountains approximately 70 km northeast of Winnemucca. The deposit comprises a Carlin-type gold deposit in which mineralization is associated with breccias and replacement zones within altered sedimentary rocks. The paragenesis of the mineralized zones can be sub-divided into at least eight discrete hypogene assemblages and at least two supergene events. Broadly the mineralization changes from sulfide skarn mineralization through a quartz–pyrite deposited in association with ductile-brittle deformation through to replacement-style sediment-hosted micron gold mineralization. This later phase of mineralization is characterized by widespread occurrence of realgar and orpiment. There are at least three major modes of occurrence for gold at Getchell that include: sub-micron- to micron-sized grains incorporated into quartz, sulfides and other minerals; chemically held gold within arsenian rims to pyrite grains; and discrete free native gold grains in breccias. The first mode appears to be the most common in material from the open pits and vein mineralization. The second appears to be associated with pyrite-rich replacement deposits in shales typical of Turquoise Ridge and deeper parts of the Getchell Main Underground (GMU) mine. The third appears to be a rare occurrence largely restricted to reactivated breccias in the GMU mine and is probably negligible in terms of bulk tonnage for the deposit. The characterization of mineralogical variations between the different paragenetic stages can be used to refine interpretation of lithogeochemical exploration. Additionally, an understanding of recovery from the different ore types allows weighting to be placed on the recoverable value of an ore during the exploration stage.