The Gacun polymetallic, Ag-rich, volcanic-hosted massive sulfide deposit occurs in a Triassic submarine calc-alkaline volcanic belt that forms part of the Yidun collisional orogenic zone of southwestern China. The deposit is hosted in felsic volcanic rocks associated with an underlying mafic unit (a bimodal suite), which formed in an intra-arc rift basin at about 1,000 m water depth. The volcanic rocks underwent regional lower greenschist facies metamorphism and related deformation during the Yanshan-Himalayan orogeny, resulting in folding and shearing of the ore lenses. The deposit is made up of three mineralized zones: a sheet-like upper massive sulfide zone with exhalite (barite, chert, and jasper), a middle stringer-stockwork strata-bound zone hosted in rhyolitic volcanic rocks, and an underlying lower stringer strata-bound zone in dacitic volcanic rocks.

Fluid inclusion studies indicate that two-phase primary fluid inclusions in quartz in the lower stringer ore zone homogenized between 299° and 319°C (average temperature 308°C), whereas those in sphalerite yielded a temperature range from 185° to 260°C (average temperature 241°C). Homogenization temperatures of fluid inclusions in quartz from the middle stringer-stockwork zone ranged from 150°to 350°C and averaged 243°C. The homogenization temperature of fluid inclusions in quartz and sphalerite from the massive sulfide zone show a bimodal distribution corresponding to temperatures from 250° to 150°C for quartz and from 200° to 140°C for sphalerite. Fluid inclusions in barite fragments from an interpreted submarine vent recorded higher homogenization temperatures ranging from 208° to 358°C, whereas those in fined-grained barite from the upper gray-white massive baritic ore gave lower homogenization temperatures ranging from 98° to 125°C.

Salinities of fluid inclusions show a wide range from 4.2 to 21.3 wt percent NaCl equiv in the three mineralized zones. Fluid inclusions in quartz from the lower stringer zone have the highest salinities (17.1–21.3 wt % NaCl equiv), whereas those in barite from gray-white massive baritic ores have the lowest salinity (average 4.0 wt % NaCl equiv), close to that of normal seawater. Salinities of fluid inclusions in sphalerite ranged from 5.1 to 14.5 wt percent NaCl equiv, and the high-salinity end member was found mainly in the massive sulfide zone. Preliminary laser Raman spectroscopic analysis of fluid inclusions from the three mineralized zones identified CO2, CH4, and H2S.

Oxygen isotope compositions of eleven quartz samples separated from the stringer and stockwork ores and of two silica breccias in the basal sulfide massive ores yielded a limited range from 13.7 to 16.4 per mil. Fourteen host felsic volcanic rock samples and seven quartz samples separated from several alteration zones display a wide range in bulk rock δ18O values from 8.0 to 17.1 per mil. For the volcanic rocks in various alteration zones, a significant increase in δ18O value was recorded in the deposit outward from the orebodies and downward from upper to lower volcanic units. The most strongly altered quartz-hyalophane zone gave a bulk rock δ18O value of 11.3 to 14.4 per mil. The moderately altered sericite-quartz zone yielded slightly higher bulk rock δ18O values of 12.5 to 15.4 per mil. The weakly altered lower volcanic unit has much higher bulk rock δ18O values ranging from 15.1 to 17.1 per mil; these values are comparable to those measured for the zeolite zone in the Kuroko deposits of Japan.

The hydrothermal system that formed the Gacun deposit was relatively high temperature (up to 350°C); it had a relatively high gas content and high salinity and was enriched in 18O. These data suggest that the dominant 18O-rich hydrothermal fluid was derived from a felsic magma chamber at depth. The high-salinity fluid inclusions in the massive sulfide zone suggest that ore formed in a sea-floor brine pool filled by episodic venting of mixed subsea-floor fluids. Fluid inclusions with salinities close to that of normal seawater and homogenization temperatures around 250°C in sphalerite and barite within the ore zone suggest that seawater was heated to that temperature by felsic intrusions. Near the mineralized center and main path of hydrothermal fluid discharge at Gacun, fluid inclusions in the strata-bound zone recorded that a minor amount of cold seawater mixed with magmatic fluid. In comparison, away from the mineralized center, a large amount of cold seawater mixed with magmatic-derived fluid and heated seawater in the strata-bound zone.

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