The Yulong ore body is the largest Cu deposit (6.22 million metric tons [Mt] at 0.99% Cu) in the 300 km long Himalayan porphyry copper belt, and is controlled by major Cenozoic strike-slip faults in the eastern Indo-Asian collision zone. It is associated with a steeply dipping, pipe-like multiphase (42-35 Ma) monzogranitic stock. The host rocks are potassic calc-alkaline or shoshonitic, and show geochemical affinities with adakites. They appear to have been derived from a thickened lower crustal source in East Tibet.The Yulong deposit consists of a ring-shaped, high-grade Cu-Au zone overlying and/or surrounding a porphyry-type Cu-Mo ore body. Cu-Mo mineralization produced a steeply dipping, pipe-like, veinlet-disseminated ore body within the stock. Associated hydrothermal alteration produced K-silicate and quartz-sericite assemblages within the stock, and contemporaneous propylitic alteration in the Upper Triassic sandy-slate wall rock. Fluid inclusion and δ 18 O-δD data indicate that the ore-forming fluid was supercritical, and exsolved from a high-level magma chamber at >620°C; it then separated into a hypersaline aqueous liquid and a coexisting low-salinity vapor at 340°-600°C.The high-grade Cu-Au zone (3 Mt at 4.74% Cu, and 4.5 g/t Au) is dominated by a supergene chalcocite-malachite blanket resting on an underlying supergene/hypogene sulfide transition unit and a hypogene pyrite-chalcopyrite sulfide unit. The Cu-Au zone was controlled by a subhorizontal or gently outward dipping breccia horizon developed along the marginal fracture zone near the roof of the stock, produced by hydrothermal brecciation due to regional uplift and/or fluid boiling. Alteration associated with the hypogene Cu-Au mineralization was texture-destructive advanced argillic alteration, characterized by associations of quartz, kaolinite, dickite, endellite, montmorillonite, hydromica, and minor alunite. It mainly developed within the breccia horizons, and partially over-1 printed the early-formed K-silicate zone and the quartz-sericite zone. Associated mineralization was of the high-sulfidation epithermal-type, characterized by chalcocite, tennantite, covellite, bornite, and minor pyrite, which formed the main ore body in the high-grade Cu-Au zone. Epithermal fluids also caused the dissolution of early-formed sulfides and remobilization of Cu-Mo, the latter transported into the intense advanced argillic alteration halo within the mineralized stock. This late-stage alteration and mineralization is attributed to a CO 2 -rich, low-temperature (<350°C), low-salinity (<12 wt% NaCl equiv.) meteoric fluid, involving input of magmatic fluid.Based on alteration, mineralization, fluid inclusion and stable isotopic data, a two-stage genetic history has been reconstructed for the Yulong deposit. It spans (1) a magmatic hydrothermal environment reflecting the emplacement of the monzogranite stock and Cu-Mo introduction through (2) hydrothermal fluid infiltration of breccia zones to epithermal overprinting.
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 × 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°to 430°C, contain high salinity fluids (31-38 wt % NaCl equiv), and coexist with CO2-rich fluids found in vapor-rich, high-Th (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-Th (>300°C) and aqueous, low-Th (<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 Th (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 18 O values between 8.8 and 11.1 per mil, from which we calculate that the corresponding hydrothermal fluids ha...
The Dongping gold deposit (>100 t Au) occurs about 200 km inboard of the northern margin of the North China craton. The deposit is mainly hosted by syenite of a middle Paleozoic alkalic intrusive complex that was emplaced into Late Archean basement rocks. Both groups of rocks are intruded by Late Jurassic to Early Cretaceous crustal-melt granite dikes and stocks, some within a few kilometers of the deposit. The gold ores were deposited during this latter magmatic period at about 150 Ma, a time that was characterized by widespread regional north-south compression that formed the east-west-trending Yanshan deformational belt. The ores include both the telluride mineral-bearing, low sulfide quartz veins and the highly K-feldspar-altered syenite, with most of the resource concentrated in two orebodies (1 and 70). Fluid inclusion microthermometry indicates heterogeneous trapping of low-salinity (e.g., 5-7 wt % NaCl equiv) fluids that varied from a few to 60 mole percent nonaqueous volatile species. Laser Raman spectroscopy confirms that the vapor phase in these inclusions is dominated by CO 2 but may be comprised of as much as 9 mole percent H 2 S and 20 mole percent N 2 ; methane concentrations in the vapor phase are consistently <1 mole percent. The variable phase ratios are consistent with fluid immiscibility during ore formation. Fluid inclusion trapping conditions are estimated to be 250°to 375°C and 0.6 to 1.0 kbar. Helium isotope studies of fluid inclusions in ore-stage pyrites indicate 3 He/ 4 He ratios of 2.1 to 5.2 Ra (Ra = 1.4 × 10-6 for air) for orebody 1 and 0.3 to 0.8 Ra for orebody 70. The former data suggest that at least 26 to 65 percent mantle helium occurs in the fluids that deposited the veins in orebody 1. The lower values for orebody 70, which is characterized by a more disseminated style of gold mineralization, are interpreted to reflect an increased interaction of ore fluids with surrounding crustal rocks, which may have contributed additional 4 He to the fluids. A mantle source for at least some of the components of the gold-forming fluid is consistent with upwelling of hot asthenosphere and erosion of as much as 100 to 150 km of cool Archean lithosphere beneath the craton during this time. The Dongping deposit is located along the 100-km-wide north-south gravity lineament, which marks the western border of the thinned crust. As both regional metamorphism of Mesoproterozoic and younger cover rocks, and widespread granite magmatism, also occurred at ca. 150 Ma, it is unclear as to whether one or both of these also contributed fluid and/or metals to the hydrothermal system. Importantly, these new data suggest that economically significant gold deposits of similar mineral style and fluid composition, which are scattered along the margins of the craton, may all be products of a fluid originally partly sourced within the mantle.
The middle‐lower Yangtze area underwent a series of complex tectonic evolution, such as Hercynian extensional rifting, Indosinian foreland basining, and Yanshanian transpression‐transtension, resulting in a large distinctive Cu‐Fe‐Au metallogenic belt. In the tectonic evolution, large‐scale migration and convergence of fluids toward foreland basins induced during the collisional orogeny of the Yangtze and North China continental blocks were of vital importance for the formation of the metallogenic belt. Through geological surveys of the middle‐lower Yangtze area, three lines of evidence of large‐scale fluid migration are proposed: (1) The extensive dolomitic and silicic alteration penetrating Cambrian‐Triassic strata generally occurs in a region sandwiched between the metallogenic belt along the Yangtze River and the Dabie orogenic belt, and in the alteration domain alternately strong and weak alteration zones extend in a NW direction and are controlled by the fault system of the Dabie orogenic belt; it might record the locus of the activities of long‐distance migrating fluids. (2) The textures and structures of very thick Middle‐Lower Triassic anhydrock sequences in restricted basins along the river reveal the important contribution of the convergence of regional hot brine in restricted basins and the chemical deposition or their formation. (3) Early‐Middle Triassic syndepositional iron carbonate sequences and Fe‐Cu‐Pb‐Zn massive sulfide deposits alternate with anhydrock sequences or are separated from the latter, but all of them occur in the same stratigraphic horizon and are intimately associated with each other, being the product of syndeposition of high‐salinity hot brine. According to the geological surveys, combined with previous data, the authors propose a conceptual model of fluid migration‐convergence and mineralization during the Dabie collisional orogeny.
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