Origin of the Eskay Creek precious metal-rich volcanogenic massive sulfidedeposit: Fluid inclusion and stable isotope evidence

The Eskay Creek deposit is an unusual, polymetallic, Au-Ag-rich volcanogeni c sulfide-sulfosalt deposit located in the Iskut River area of northwestern British Columbia, Canada. Economic concentrations of precious and base met als are contained in the 21 zone, which is divided into a number of subzone s. As of December 31, 1998, total production and proven-probable reserves a re 1.9 million tons (Mt) at 60.2 g/t Au, 2,652 g/t Ag, 3.2 percent Pb, 5.2 percent Zn, and 0.7 percent Cu. The 21B zone, which contains the bulk of th e reserves, began production in 1995. The mineralization occurs mainly as w ell-preserved stratiform elastic beds of sulfide-sulfosalt debris and also as discordant footwall quartz sulfide veins. The hydrothermal system that formed the Eskay deposit was low temperature ( <200 degrees C) with a relatively high gas content. Fluid inclusion petrogr aphy and measured gas ratios are consistent with liquid-vapor phase separat ion occurring in the hydrothermal system. The calculated fluid pressures, f rom fluid inclusion data, are variable as a result of contributions from va por-rich fluid inclusions. Three samples gave calculated fluid pressures of similar to 150 bars that equate to a 1,500-m Mater depth. These analyses a re considered the best indicators of the boiling depth since they most like ly had the minimum contribution Er-om vapor-rich fluid inclusions. Oxygen i sotope ratios of quartz separates and whole-rock data suggest that the domi nant hydrothermal fluid was normal seawater at temperatures of around 200 d egrees C. Fluid inclusion leachates suggest mixing between a seawater-deriv ed fluid and a lower temperature (similar to 100 degrees C), more saline fl uid which has high K/Na and Cl/Br ratios compared to normal seawater. The h igh-salinity fluid has halogen and cation ratios that are consistent with a magmatic-derived fluid. The relationship of this fluid to mineralization i s uncertain. Sulfur isotope data suggest that the sulfide sulfur may have b een derived from either an igneous source or by reduction of seawater sulfa te. The dominant origin of the sulfate sulfur was seawater, although one ba rite sample may contain oxidized igneous sulfur Mineralization at Eskay Cre ek is inferred to have formed at, or near, the sea floor in a relatively sh allow-water setting, by fluid boiling which is an effective precipitation m echanism for gold and silver. The low solubility and ineffective precipitat ion mechanisms for base metals at these low temperatures resulted in minera lization with a high precious to base metal ratio. The best modern-day anal ogue of Eskay Creek mineralization is the JADE hydrothermal field in the Ok inawa trough.