Abstract

Large areas of Miocene volcanic rocks in the western part of the Virginia Range, Washoe and Storey Counties, Nevada, were altered to quartz, alunite, and clay-dominated mineral assemblages, and all significant precious metal and mercury deposits throughout the range, including the Comstock Lode, Ramsey, and Talapoosa districts, occur in or adjacent to extensively altered andesites and dacites of the Miocene Alta and Kate Peak Formations. Radioisotopic analyses of alunite and precious metal vein minerals bracket the ages of quartz-alunite alteration at approximately 16 to 9 Ma, and vein mineralization at approximately 14 to 10 Ma, closely corresponding to Kate Peak Formation eruptive rock ages that range from approximately 15 to 10 Ma. Oxygen, deuterium, and sulfur isotope analyses of alunite, quartz, and water extracted from fluid inclusions in quartz-alunite altered rocks in the western part of the Virginia Range, and fluid inclusion microthermometry, show that the altering fluids were composed of magmatic water and mixtures of meteoric and magmatic water, and that the alteration took place in the temperature range approximately 205 degrees to 265 degrees C. Fluid inclusion water isotopic analyses indicate that Comstock Lode district veins were also deposited by a mixed magmatic-meteoric water fluid, and that subsurface altered intrusions, such as the molybdenite-bearing diorite in the Washington Hill alteration zone, can be identified by isotopic analyses of water in surface samples. Sources and thermal characteristics of fluids involved in quartz-alunite alteration and gold mineralization in the Ramsey district, similar to that at Goldfield, Nevada, and for other deposits in the Virginia Range, have not been determined, but mixingof magmatic and meteoric water at moderate hydrothermal temperatures may have been important in localizing ore.

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