EVIDENCE FOR THE BRITTLE-DUCTILE TRANSITION AT THE SUMMITVILLE HIGH-SULFIDATION AU DEPOSIT, CO BASED ON PETROGRAPHIC, THERMOMETRIC AND LA-ICP-MS ANALYSIS OF FLUID INCLUSIONS

Core from a 1.4 km drill hole at the Summitville high-sulfidation Au deposit records the transition from the epithermal acid-sulfate environment to the quartz-sericite-pyrite alteration envelope of a possible porphyry Cu-Au deposit at depth. Secondary liquid-vapor inclusions in igneous quartz through this transition are of four basic types: 1) hypersaline (H) packed with halite+sylvite +unknown minerals, 2) saline (S) with halite crystals, 3) liquid+vapor only (L-V), and 4) vapor-rich (V). The distribution, size, shape, thermometry and composition of the inclusions vary with depth. H and S inclusions are ubiquitous below 3100 m in elevation but rare or absent above this level, whereas V and L-V inclusions are abundant throughout the drilled interval. Vapor to liquid homogenization temperatures (Th_v-l) and salinities (NaCl_eq) of H and S inclusions are relatively consistent at 380±20 ºC and 39±2 wt. %. Th_v-l values for L-V inclusions are also 380±20 ºC below 3100 m but abruptly decrease above this level to ~220 ºC in the Summitville ore body; L-V salinities more gradually decrease from 15-17 to 2-5 wt. % over this same vertical range.

Single inclusion LA-ICP-MS analysis of deep samples indicates that the “packed” sylvite+halite bearing H inclusions are enriched in K, Mn, Ba, Sr, Pb and Zn, but depleted in Na, Fe and Cu relative to the halite-only S inclusions. These differences imply the H brines may have been involved in the alteration of K-feldspar to quartz+sericite+pyrite in the porphyry environment. Na-normalized element averages for each inclusion type show a preferential enrichment of As and Au in the vapor phase.

Collectively, these data define a fluid interface at ~3100 m in elevation between hot, dense magmatic brines at depth and overlying cooler, lower density fluids. The abrupt decrease from maximum Th_v-l values near 400 ºC suggests the fluid interface was coincident with and controlled by the brittle-ductile (B-D) transition. Brine and vapor generation likely resulted from phase separation of intermediate salinity magmatic fluids resulting in considerable metal partitioning including enrichment of Au and As in the vapor phase. Transport of fluids, vapor and metals across the B-D transition to the epithermal environment largely occurred along structural and volcanic conduits.