CONTRASTING STYLES OF EPITHERMAL PRECIOUS-METAL MINERALIZATION IN THE SOUTHWESTERN NEVADA VOLCANIC FIELD, USA

The southwestern Nevada volcanic field contains epithermal precious-metal deposits hosted by Miocene volcanic rocks and pre-Tertiary sedimentary rocks with production + reserves greater than 60 t of gold and 1 50' t of silver. The volcanic rocks consist predominantly of ash-flow tuffs erupted between 15 and 7 Ma during three major magmatic stages: the main stage (ca. 15-13 Ma); the Timber Mountain stage (ca. 13-9 Ma); and the late stage (ca. 9-7 Ma). Hydrothermal activity and precious-metal mineralization in the southern part of the field took place between ca. 13 and 8.5 Ma, coinciding with portions of all three magmatic stages. Regional extension during this period produced imbricate normal and detachment faulting that provided structural control for some of the mineralization. Contrasts in the style and geochemistry of mineralization, together with stratigraphic and radiometric age data and differences in geologic setting reflect the variable nature of hydrothermal activity during development of the southwestern Nevada volcanic field. During the main magmatic stage, silver-rich vein mineralization of the adularia-sericite type occurred in an intermediate volcanic center at Wahmonie. Secondary high-salinity fluid inclusions in felsic subvolcanic intrusions. a trace element suite that includes bismuth and tellurium, and geophysical data support the presence of a buried porphyry-type magmatic system at Wahmonie. Hydrothermal activity at Bare Mountain took place during the main magmatic stage, and may have continued into the Timber Mountain magmatic stage. Bare Mountain contains gold-rich, disseminated Carlin-type deposits with high arsenic. antimony, mercury and fluorine in sedimentary and igneous rocks. In northern and eastern Bare Mountain, mineralization is associated with felsic porphyry dikes that contain secondary high-salinity fluid inclusions. A genetic relationship between porphyry magmatism and shallow Carlin-type gold deposits seems likely at Bare Mountain. Sedimentary-rock-hosted mineralization at Mine Mountain is spatially associated with a thrust fault and was apparently deposited, in part, by a hydrothermal system active during the Timber Mountain magmatic stage. The silver: gold ratio is high and base-metal, arsenic, antimony, mercury and selenium contents are verv high. Mine Mountain mineralization shares features with vein and disseminated silver deposits at Candelaria, Nevada. Gold-silver deposits in the areally extensive Bullfrog district comprise the largest known precious-metal resource in the volcanic field. They are mainly quartz-carbonate +/- adularia veins with alteration and mineralization styles similar to other adularia-sericite-type deposits in the Great Basin. Deposits in the Rhyolite area and at the Gold Bar mine have very low contents of arsenic and mercury compared to other epithermal deposits in the Great Basin, although copper and antimony are locally elevated. Similarities in mineralization style and assemblages. which include two occurrences of the rare gold-silver sulfide uytenbogaardtite, indicate deposition under similar conditions in different parts of the district. Hydrothermal activity in the Bullfrog district was coeval with extensional tectonism and may have continued from the Timber Mountain stage into the late magmatic stage. Mineralization at some deposits in the Bullfrog and Bare Mountain districts is spatially associated with, and, in part, structurally controlled by a regional detachment fault system. However, significant differences in age, mineralization style and geochemistry indicate that mineralization in the two districts is unrelated.