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, northwestern 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 ‚àöv= 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 chalcopyritebearing 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 CO2-rich fluid inclusions, and (4) type IV CH4-rich fluid inclusions. Type II inclusions have high homogenization temperatures (Th) ranging from 320¬¨‚àû to 430¬¨‚àûC, contain high salinity fluids (31‚Äö-38 wt % NaCl equiv), and coexist with CO2-rich fluids found in vapor-rich, high-Th (up to 487¬¨‚àûC), moderate salinity (10‚Äö-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-Th (>300¬¨‚àûC) and aqueous, low-Th (<220¬¨‚àûC) 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 Th (62¬¨‚àû‚Äö-225¬¨‚àûC) and high salinities (15.0‚Äö-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 ˜í¬•18O values between 8.8 and 11.1 per mil, from which we calculate that the corresponding hydrothermal fluids had ˜í¬•18O values ranging from ‚Äö-5.3 to +3.1 per mil over a temperature range of 160¬¨‚àû to 278¬¨‚àûC. Whole-rock ˜í¬•18O values MOST VOLCANIC-HOSTED massive sulfide (VHMS) deposits are comprised of massive sulfide lenses or sheets and an underlying discordant stringer zone, hosted by volcanic rocks (e.g., Franklin et al., 1981; Large et al., 2001; Gifkins et al., 2005). Economically significant stringer zones and pyritechalcopyrite pipes are present only in some deposits, such as the Mount Lyell deposit in the Cambrian Mount Read Volcanics of western Tasmania (Cox, 1981), the Ordovician Highway and Reward deposits in the Mount Windsor subprovince, Queensland (Doyle and Huston, 1999), and the Devonian Mount Morgan deposit, Queensland (Taube, 1986; Ulrich et al., 2002). Pipelike polymetallic sulfides are also present in the early Paleozoic Bawdwin deposit in northeastern Myanmar (Khin Zaw, 2003, 2004). The Baiyinchang Cu deposit in the Late Proterozoic-early Paleozoic submarine volcanic belt of northwestern China may be another example of a pipe-style VHMS deposit. Bian (1989) argued that it is a porphyry-type deposit, based on the lack of exhalative-sedimentary massive sulfides and the close relationship of the sulfide orebody with felsic porphyries in the district. The deposit was discovered in the 1950s and is being exploited from the Zheyaoshan and Huoyanshan mines (Song, 1955, 1982). The Zheyaoshan mine, with current copper reserves of ~0.89 Mt remaining after exploitation of 51 Mt of ore, produced ~0.6 Mt of Cu, whereas 15 Mt of ore was exploited from the Huoyanshan deposit to produce ~0.26 Mt of Cu (Baiyin Company, unpub. data). The copper grade of the Baiyinchang deposits ranges from 0.4 to 4.6 percent, with an average of 1.17 percent Cu. Three significant observations in the district indicate that the Baiyinchang deposit is atypical in comparison to many other submarine hydrothermal systems.1. The hydrothermal alteration pipe and the associated discordant sulfide zone dips steeply and extends 500 to 700 m downward into the host felsic volcanic rocks below the paleosea floor (Song, 1982; Peng et al., 1995). This provides a unique opportunity to study the deep structure of the submarine hydrothermal system and the sulfide deposit. 2. The sulfide orebody, hosted in felsic volcanic rocks, is spatially associated with mineralized, shallow-level felsic porphyry bodies (Cheng, 1980; Song, 1982; Bian, 1989; Peng et al., 1995) and this provides an opportunity to evaluate the role of synvolcanic intrusions in the genesis of massive sulfides. (3) The stringer ores and the chalcopyrite-bearing quartz veins are well developed in the root of the discordant sulfide pipe, and preliminary fluid inclusion studies from this zone found high-temperature (>350¬¨‚àûC), high-salinity hydrothermal fluids(Liu, 1982). Herein we describe the geology and the deep structure of the Baiyinchang deposit, and discuss its genesis,based on detailed fluid inclusion and oxygen isotope studies.Regional Geology The Baiyinchang Cu deposit is the largest VHMS deposit in the Late Proterozoic to early Paleozoic marine volcanic belt of the north Qilian orogen, northwestern China (Fig. 1A). This belt is 1,200 km long and more than 50 km wide, strikes west-northwest, and extends from Gansu, Qinghai, and Shaanxi to Henan provinces in northwestern China. The western segment of the belt is bounded by the large-scale Althy strike-slip fault, whereas the eastern segment is connected with the north Qinling early Paleozoic orogenic belt. The marine volcanic belt underwent a complex tectonic history during the Qilian orogeny, beginning with rifting and continental break-up in the Late Proterozoic-Early Cambrian, and followed by the development of a trench-arc-basin system in the Ordovician, an arc-continent collision in the late Ordovician to Devonian, and finally, accretion to the southern margin of the China-Korea platform (Fig. 1A; Xiang and Dai, 1985; Xia et al., 1991, 1996; Wu et al., 1994). The marine volcanic rocks of the belt host 54 known VHMS deposits (Fig. 1A, B) with a total metal content of 5.8 Mt of combined Cu, Pb, and Zn (Wu et al., 1994; Hou et al., 1999). Three major VHMS-bearing volcanic belts have been recognized in the Qilian orogen based on their spatial-temporal distribution and the tectonomagmatic associations of these volcanic rocks (Fig. 1A). The Late Proterozoic to Early Cambrian belt (III2) is the most important and hosts a bimodal volcanic sequence with a range of Sm-Nd ages from 522 to 606 Ma (Xia et al., 1996); it is unconformably overlain by arc tholeiitic basalt during the Late Cambrian and Ordovician (Fig. 1B; Xia et al., 1998). The bimodal volcanism, associated with rifting of the Proterozoic basement, formed at least four volcanic domes, termed the Baiyin and Heishishan domes in 270 ZENGQIAN ET AL. 0361-0128/98/000/000-00 $6.00 270 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-˜í¬•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¬¨‚àû‚Äö-430¬¨‚àûC) hydrothermal fluid, as suggested by the high-temperature and high-salinity fluid inclusion data, the ˜í¬•18O values of the hydrothermal fluid in equilibrium with quartz (˜í¬•18O values of 9.0‚Äö-11.1‚ÄövÑ‚àû) range from 2.0 to 8.0 per mil, which corresponds to the ˜í¬•18O range between magmatic fluid and seawater. The ore-forming fluids responsible for the Cu-Zn mineralization at Baiyinchang belonged to the H2O-NaCl- CO2-CH4 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 H2O + CO2-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¬¨‚àûC) 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 oredominated sulfide orebodies at Zheyaoshan.