A FIELD-BASED GEOCHEMICAL AND PETROGRAPHIC STUDY OF THE FLUIDS PRESERVED WITHIN THE HARRISON PASS PLUTON WITH IMPLICATIONS FOR THE FLUID ORIGIN OF CARLIN-TYPE GOLD DEPOSITS

The origin and evolution of the ore-bearing hydrothermal fluids responsible for the formation of Carlin-type Au deposits (CTD’s) in NE Nevada have not been conclusively determined. Many studies have attempted to characterize the geochemistry of the fluids that formed the deposits on the Carlin Trend. They indicate a complex, open-system evolution with multiple fluid inputs, and invoke several genetic models for different deposits. However, few studies have directly investigated potential sources for these fluids. The Harrison Pass Pluton (HPP) is a ~36 Ma, multi-phase, calc-alkaline, granitoid intrusion in the southern Ruby Mountains. It is ~80 km SE along strike with the temporally associated Au deposits along the Carlin Trend. The uplifted and exposed HPP serves as a viable analog for plutons of similar age, size, and style of emplacement that have been geophysically imaged beneath the Carlin Trend.

This study defines the spatial and geochemical parameters of fluid exsolution and migration within the HPP during intrusion. Preliminary results indicate a potential genetic link with spatially and temporally associated CTD’s along the Carlin Trend. Mapping vein systems, miarolitic cavities, and different alteration facies on the exposed oblique section of the tilted pluton has helped to constrain the nature, distribution, and conditions of fluid activity within the HPP. Multiple generations of pegmatitic quartz veins, miarolitic cavities, and zones of alteration are found throughout the HPP. Petrography, fluid inclusion microthermometry, stable isotope analysis, and lithogeochemistry have been used to characterize these fluids. Microthermometry and petrography of fluid inclusions within veins and altered rocks have defined the compositions, densities, and salinities of the fluids within the HPP and have yielded estimates of pressure and temperature conditions during intrusion. Comparison between these data and existing geochemical characterizations of CTD's provides insights into the degree and nature of meteoric inputs into ore fluids. This study provides an important component in understanding the role of regional plutonism in the formation of CTD ore fluids, and may ultimately support the proposed mixing of magmatic and meteoric fluids as a mechanism for Carlin-type ore-fluid evolution.