FLUID INCLUSION EVIDENCE FOR MAGMATIC BRINE BELOW THE SUMMITVILLE HIGH-SULFIDATION CU-AU DEPOSIT, CO AND ITS POTENTIAL RELATION TO THE BRITTLE-DUCTILE TRANSITION AND MINERALIZATION

Secondary hypersaline (H) fluid inclusions in igneous quartz from a 1.4 km drill hole at the Summitville high-sulfidation Au deposit document the presence of a magmatic brine 500 m below the base of the ore zone or 1.1 km below the paleosurface at the time of mineralization. Although vapor (V)- and liquid (L)-rich inclusions are abundant throughout the drill hole, H inclusions(halite±sylvite±unknowns) are ubiquitous only in the bottom 900 m and rarely observed above this level. As this boundary is approached from below, H inclusions become larger, more irregular in shape, and packed with daughter phases, yet average Th_v-l and NaCl_eq values remain relatively constant (365±20 °C, 39±3 wt. %). Through the same interval average Th_v-l values of L-V inclusions increase upward from 340 to 400 °C but decrease from 15 to 6 wt. % NaCl_eq. Immediately above the boundary average Th_v-l and NaCl_eq values of L-V inclusions are 285 °C and 7 wt. % and at the deposit level 240 °C and 5 wt. %. Moreover, unusual inclusions containing a clear, immiscible sulfur-carbon liquid that reversibly turns yellow upon heating are found near the boundary. Single inclusion LA-ICP-MS analyses reveal relative elemental enrichments among inclusion types when compared on an equal NaCleq basis: 2-5x for Cu and Au in V inclusions and As in V and some L-V inclusions, and 2-6x for K, Ba, Sr and Fe in large H inclusions packed with multiple daughter phases, implying a fluid buffered by alteration of K-feldspar bearing rocks.

The upper limit of the hypersaline inclusions is interpreted as an interface between pervasive, dense magmatic brine at depth and fluids of moderately salinity above that are most likely mixtures of vapor condensate and meteoric groundwater. The coincidence of the interface with Th_v-l values of 365-400 °C implies a control of the interface position by the brittle-ductile transition. Co-planar V and H or L-V inclusions indicate that phase separation was common throughout the drilled section with the brine representing a residual liquid of this process. Collectively, the data suggest the potential for vapor generated during phase separation below the fluid interface to transport ore metals along structures across the brittle-ductile transition to the Summitville deposit where metal deposition was triggered by cooling and mixing with meteoric groundwater.