Geology, fluid inclusions, and oxygen isotope geochemistry of the Baiyinchang pipe-style volcanic-hosted massive sulfide Cu deposit in Gansu Province, northwestern China

The Baiyinchang massive sulfide Cu deposit (Zheyaoshan and Huoyanshan mines) is hosted by an early Cambrian, submarine, felsic volcanic succession within an extrusive cryptodome associated with an overlying basaltic flow, in a Late Proterozoic-early Paleozoic submarine volcanic belt in the north Qilian orogen, north-western China. The deposit is comprised of two mineralized zones: a 30-cm-thick, strata-bound Zn-rich sulfide lens associated with hematitic Fe-Mn cherts, and an underlying, discordant massive ore-dominated sulfide zone enveloped by a hydrothermal alteration pipe that is zoned from chlorite in the center to quartz-sericite at the margin. The discordant sulfide zone accounts for 90 percent of the Cu reserves of the Zheyaoshan mine. It consists of four main ore types: (1) pipelike pyrrhotite-pyrite ± chalcopyrite ore, (2) massive sulfide ore, (3) a disseminated ore halo, and (4) footwall stringer ore. The pyrrhotite-pyrite ± chalcopyrite pipe has an elliptical shape in plan and is 30 × 50 m across. The pipe partially replaces the overlying massive pyrite lens and extends downward at least 150 m, to be gradually replaced by chalcopyrite-rich stringer veins and chalcopyrite-bearing quartz veins surrounded by a discordant hydrothermal alteration envelope. Massive chalcopyrite-pyrite lenses discordant to volcanic bedding, containing relict patches of felsic volcanic host rocks, are commonly enveloped by a disseminated sulfide halo within a chloritized volcanic unit. These features suggest that Zheyaoshan is a pipe-style deposit that formed mainly by subsea-floor replacement of volcanic host rocks. Studies of fluid inclusions indicate that there are four types: (1) type I two-phase, aqueous fluid inclusions, (2) type II daughter mineral-bearing, multiphase fluid inclusions, (3) type III CO 2-rich fluid inclusions, and (4) type IV CH 4-rich fluid inclusions. Type II inclusions have high homogenization temperatures (T h) ranging from 320° to 430°C, contain high salinity fluids (31-38 wt % NaCl equiv), and coexist with CO 2-rich fluids found in vapor-rich, high-T h (up to 487°C), moderate salinity (10-16 wt % NaCl equiv) inclusions in the discordant sulfide zone and associated altered rocks, suggesting a possible contribution of a magmatic fluid to the hydrothermal system. The coexistence of vapor-rich, high-T h (>300°C) and aqueous, low-T h (<220°C) type I fluid inclusions in the stringer zone suggests that heated seawater mixed with magmatic fluid (gas) in the feeder zone. Most type I fluid inclusions in the massive chalcopyrite-pyrite body and in the strongly chloritized pipe have a low T h (62°-225°C) and high salinities (15.0-23.0 wt % NaCl equiv), suggesting that a dense brine zone developed in fractures in the subsea floor where sulfides accumulated by open-space filling and replacement of host volcanic rocks. Eleven quartz samples from the overlying discordant sulfide zone yielded a restricted range of δ 18O values between 8.8 and 11.1 per mil, from which we calculate that the corresponding hydrothermal fluids had δ 18O values ranging from -5.3 to +3.1 per mil over a temperature range of 160° to 278°C. Whole-rock δ 18O values for the altered volcanic rocks in the pipe yielded a much wider range, from 1.6 per mil in the chlorite core to 8.7 per mil in the outer sericite-chlorite zone, suggesting that a low-δ 18O seawater-dominated hydrothermal fluid interacted with the footwall volcanic rocks. Oxygen isotope data for quartz from both the stringer zone and the altered host volcanic rocks also record a contribution of the magmatic fluid to the Zheyaoshan submarine hydrothermal system. Assuming that the analyzed quartz precipitated from a hot (300°-430°C) hydrothermal fluid, as suggested by the high-temperature and high-salinity fluid inclusion data, the δ 18O values of the hydrothermal fluid in equilibrium with quartz (δ 18O values of 9.0-11.1‰) range from 2.0 to 8.0 per mil, which corresponds to the δ 18O range between magmatic fluid and seawater. The ore-forming fluids responsible for the Cu-Zn mineralization at Baiyinchang belonged to the H 2O-NaCl- CO 2-CH 4 system. A felsic magma chamber is thought to have been situated 1 to 1.5 km below the sea floor, and this likely supplied the necessary heat to drive seawater convection and introduced an H 2O + CO 2-dominated, high-temperature, high-salinity (magmatic) brine to the Baiyinchang hydrothermal system. A narrow, steeply dipping, funnel-shaped brine zone was present in the margin of the fracture zone above the magma chamber, and this was trapped within the porous felsic volcaniclastic rocks. This brine zone is considered to have been the key factor for the formation of the Zheyaoshan pipe-shaped sulfide orebodies. The Cu-bearing, high-temperature (>300°C) fluids are considered to be the product of mixing of magmatic brine with seawater and the replacement of the surrounding volcanic rocks resulted in the formation of the discordant, massive ore-dominated sulfide orebodies at Zheyaoshan. © 2008 Society of Economic Geologists, Inc.