The research proposed here concerns sediment-hosted micron gold (SHMG) deposits, which are the largest fossil hydrothermal systems in the Great Basin (Fig. 1) and the most important sources of gold in the U.S. The project will focus on determination of: 1) deposit-scale zoning, which could provide information on the source of SHMG mineralizing fluids, as well as the temperature, pressure and crustal depth at which they formed. 2) the age of these deposits, which will clarify their relation to both magmatic heat and water, and the tectonic evolution of the Great Basin. Most of the study will be carried out on the newly discovered Post SHMG deposit, which extends to depths of over 2500 ft. and contains large volumes of ore that have not been overprinted by weathering and oxidation. Zoning studies will include mineralogical, fluid inclusion and stable isotopic measurements. Radiometric age measurements will use an experimental evacuated quartz tube/total degassing 40Ar-39Ar method to analyse less 1 mg of alteration mineral separates and these ages will be checked by conventional 40Ar-39Ar age determinations on igneous and alteration minerals in pre- and post-ore dikes, which bracket mineralization at Post. Our results could yield a depth-zoning classification for SHMG deposits, which could be used both in exploration and in regional tectonic studies.
