Geology of the Stateline District, Utah-Nevada

The rocks exposed in the Stateline District, Iron County, Utah and Lincoln County, Nevada, are Tertiary volcanics, which serve as hosts for gold and silver bearing quartz veins. The Miocene rhyolites and dellenites, a possible caldera, cupola-associated rock textures, and the faults and styles of deformations indicate the geologic setting of Stateline is in harmony with the tectonic framework of the southern Great Basin. Four map units, subdivided into nine members, are recognized. X-ray fluorescence analyses for SiO₂, TiO₂, Fe₂O₃, CaO, and K₂O differentiated the map units, while variations in the primary quartz, sanadine, plagioclase, biotite, hornblende, and total crystal content characterized members. Secondary minerals, determined by optical and X-ray techniques, are scarce in the second rhyolites, whereas the first rhyolites have secondary quartz and possibly clays, and the dellenite flows and lithic pumice tuff map unit have secondary calcite. Bixbyite and associated topaz are found in the volcanic rocks of Stateline Mining District. These bixbyites have a lattice parameter of 9.415Å while reference specimens from other localities have parameters of 9.393Å to 9.428Å. Visible alteration and high K₂O content are coincident with the distribution of veins. Gains or losses of major components, and density and porosity changes during alteration are the result of leaching, cavity filling, replacement, and recrystallization processes. Except for the pervasive recrystallization, these processes altered the rocks selectively. Chemical variations, including high SiO₂, TiO₂, Fe₂O₃, and K₂O near one Stateline vein, are comparable with mineralogical variations adjacent to veins in a nearby fluorspar district --Abstract, pages ii-iii