Geochemistry of pyrite and whole rock samples from the Getchell Carlin–type gold deposit, Humboldt County, Nevada

Carlin-type gold deposits were not recognized as a new major type of gold deposit until the 1960’s. The Getchell deposit, which is now known to be a Carlin-type gold deposit, was discovered in 1934 (Joralemon, 1951). This deposit is located in north central Nevada (Fig. 1). Much is known about the physical characteristics of Carlin-type gold deposits (Cline et al., 2005). An unusual characteristic of these deposits is that free gold is generally not present. Instead, gold occurs as sub-micrometer particles in the mineral pyrite or marcasite. The marcasite and pyrite that are gold bearing commonly occur as rims on gold-free pyrite or marcasite cores. These rims typically contain, in addition to gold, elevated arsenic, antimony, thallium, mercury, and copper. We have samples of ore from the Getchell deposit that contain metals that are not typically present in these deposits. For example, silver, which is typically very minor, is as abundant as gold. It is currently unclear if this silver is in the pyrite rims with the gold and other hydrothermally transported metals, or if the silver is in other minerals in the rock. If this is the case, this would indicate another source for the silver. In this study we are conducting petrographic examinations and chemical analyses of pyrites to quantify gold and other metals. First we used the microscope to look at polished thin sections of the samples. Pyrite crystals were identified that had visible rims giving us a location to analyze for gold and other trace metals. Second, we are using an electron microprobe to quantitatively determine elements and their abundances in pyrite and marcasite cores and rims. We expect to see a suite of elements typical of the Carlin-type systems, such as: Au, Sb, Hg, As, Cu, and Tl in the pyrites. Our study is determining the chemistry of four types of pyrite that have been identified, to locate gold, silver, and other trace metals. Our analyses will indicate whether or not silver occurs with the other Carlin metals, or if it is alternatively present in other minerals in the rock