RE-OS ARSENOPYRITE GEOCHRONOLOGY OF THE HOMESTAKE GOLD DEPOSIT, BLACK HILLS, SOUTH DAKOTA, AND IMPLICATIONS FOR CHRONOMETER CLOSURE TEMPERATURE

The Homestake gold deposit, containing >40 Moz of gold, is the largest known Proterozoic orogenic gold deposit, and is one of the largest individual gold deposits known worldwide. Situated within the northern portion of the Laramide Black Hills uplift, South Dakota, the Homestake deposit formed during the Paleoproterozoic collision between the Archean Wyoming and Superior Provinces. Existing age constraints from high closure-temperature geochronometers bracket mineralization between 1746 Ma and 1715 Ma. Re-Os arsenopyrite geochronology was performed on coarse arsenopyrite crystals closely associated with gold. Multiple Re-Os analyses yield a precise Model 1 isochron age of ca. 1730 Ma that falls within the existing age constraints and dates sulfide mineralization directly. Comparison of this Re-Os age with ages from other geochronometers with well-constrained closure temperatures allows for estimation of a minimum Re-Os arsenopyrite closure temperature. Host rocks proximal to the deposit are of upper greenschist to lower amphibolite grade, and regional studies show that peak prograde metamorphic temperatures attained 30-40 Ma prior to mineralization at Homestake were in the 500ºC – 600ºC range. Elevated temperatures (400º-500ºC) remained until intrusion of granites at 1715 Ma, which caused widespread thermal overprinting (T=480º-650ºC) of earlier metamorphic assemblages. ⁴⁰Ar/³⁹Ar analysis of micas derived from selvages to gold-bearing veins at Homestake yield plateau ages of 1304 and 1220 Ma. These dates, which clearly postdate ore emplacement, likely reflect thermal disturbance and/or cooling at some time after vein formation. Retention of a Re-Os age >1715 Ma by arsenopyrite demonstrates its robustness for yielding accurate mineralization ages, and exhibits the utility of Re-Os arsenopyrite geochronology for accurately placing ore-forming events within the known tectonic framework, as has previously been shown for the Meguma and Muruntau deposits. The results of this study suggest that the Re-Os geochronometer in coarse arsenopyrite has a closure temperature≥~500ºC.