Some aspects of the geology and mineralogy of the fluorine-rich skarn at McCullough Butte, Eureka County, Nevada.

A stockwork of veins and their alteration envelopes in Palaeozoic miogeosynclinal sedimentary rocks is exposed on two sides of some Cretaceous two-mica granite porphyry intrusions. The mineralization contains appreciable F, Zn and Be, and minor Mo, W and Sn. From the intrusions, the distal limit of garnet veins comes first, followed sequentially by those of plagioclase, muscovite and quartz veins. Early garnets and pyroxenes are Fe-poor; later garnets are Fe- and Mn-enriched. The skarn is attributed to F-rich fluids from the intrusions reacting with the carbonate host-rock under moderate T, and low P, fO₂, fS₂ and fCO₂ conditions. Total Fe, Cu and Pb contents are low, but acid, F-bearing species and LIL elements are significant.-K.F.