GRANITE, PORPHYRIES, HYDROTHERMAL ACTIVITY AND VOLCANISM IN THE YERINGTON DISTRICT, NEVADA

The Middle Jurassic Yerington batholith, exposed in cross section due to steep westward Cenozoic tilting, consists of early intrusions of quartz monzodiorite and quartz monzonite, intruded by a central pluton of porphyritic granite. Parts of the floor of some of these intrusions are exposed in the eastern part of the district, and volcanic equivalents of some are exposed to the west. Granitic porphyries, some with associated porphyry Cu deposits, occur in the roof of the porphyritic granite. At least some of the porphyries that are directly associated with porphyry Cu mineralization terminate upward, and clearly did not vent. In the western part of the district some porphyries, with only weak pyritic, ±hematitic mineralization, can be followed to the Jurassic surface, where they apparently fed lavas and tuffs in the middle Jurassic volcanic section exposed there. This volcanic section consists of an andesite-dacite lower unit (Artesia Lake volcanics), which is overlain by a sequence of felsic porphyritic volcanics (Fulstone Spring volcanics). A SHRIMP U-Pb age of 167.8 ±1.2 Ma was measured on zircons from a flow just above the base of the Fulstone section, which appears to have been fed by a dike of granite porphyry below. Within analytical uncertainty this age overlaps a previous bulk zircon age of 168.5 ±0.4 Ma for a granite porphyry, and along with geologic relationships and similar petrography and chemistry, indicates genetic ties between the lower part of the Fulstone volcanics and the porphyries. Feldspar destructive alteration, which formed late in the upper parts of the porphyry Cu systems, is also strongly developed in much of the Artesia Lake volcanics. In the contact zone with overlying Fulstone, conglomerates with altered Artesia clasts are themselves strongly altered locally. Strong alteration occurs only locally in the Fulstone, decreasing in abundance and intensity upward. The observations suggest that the earliest parts of granite and porphyry emplacement, and associated Cu mineralization and alteration, was accompanied by a lull in volcanism, but that as volcanic activity again increased, hydrothermal activity decreased.