ALBERT

Description: The Hongniu-Hongshan Cu skarn deposit (77.8 Mt at 1.8% Cu) is located in the central part of the Zhongdian porphyry and skarn Cu belt in southwestern China. Skarn and orebodies occur mainly between the different units of the Upper Triassic Qugasi Formation or within altered limestone adjacent to Late Cretaceous intrusions (78–76 Ma). Three main paragenetic stages of skarn formation and ore deposition have been recognized on the basis of petrographic observations: (1) pre-ore-stage hornfels with diopside (Di 87–72 Hd 12–7 ), small-scale endoskarn with reddish grossular (Adr 22–57 Gr 78–43 ), diopside (Di 83–92 Hd 7–15 ), vesuvianite, and abundant exoskarn with red-brown andradite (Adr 75–98 Gr 2–22 ), sahlite (Di 28–41 Hd 58–71 ), and wollastonite; (2) syn-ore-stage retrograde minerals, sulfides (pyrite, chalcopyrite, pyrrhotite, molybdenite, galena, and sphalerite), quartz, and calcite; and (3) post-ore-stage calcite veins. Sulfur isotope values of sulfides are relatively high, with an average 34 S = 4.9 (n = 40), suggesting that the ore-forming fluid was magmatic and that the sulfides precipitated from a relatively reducing ore fluid. The coexistence of silicate melt and primary fluid inclusions in quartz phenocrysts of the mineralization-related quartz monzonite porphyry indicates the simultaneous entrapment of fluid and melt, and records the process of the aqueous fluid exsolving from the crystallizing melt. The initial single-phase fluid has a salinity of 8.8 to 12.7 wt % NaCl equiv and homogenization temperatures of 566° to 650°C, corresponding to pressures of 680 to 940 bar and lithostatic depth of 2.5 to 3.5 km. The primary fluid inclusions in the pre-ore-stage garnet and pyroxene composed of coeval vapor-rich (V type) and halite-bearing (S-I and S-II types containing sylvite) inclusions (32–〉79 total wt % salts) share similar homogenization temperatures (450°–550°C), indicative of the occurrence of fluid unmixing under lithostatic pressures of ~550 to 780 bar (〉2.0-km depth). Primary fluid inclusions trapped in syn-ore quartz, calcite, and epidote show the common development of S-type inclusions (~37.3 wt % NaCl equiv) with coexisting V-type, liquid-rich (L type), and CO 2 -bearing (C-I type) inclusions, all of which have homogenization temperatures of 300° to 400°C and trapping pressures of 100 to 400 bar (~1.5-km depth). Brine inclusions homogenized by halite dissolution after vapor disappearance in both the pre- and syn-ore stages are interpreted to have been trapped under overpressured conditions (〉1,520 bar). Oxygen isotope analyses were conducted on garnet, wollastonite, epidote, quartz, and calcite. The pre-ore-stage garnet and wollastonite have 18 O fluid values of 5.6 to 8.1, whereas the syn-ore-stage epidote, quartz, and calcite have more variable 18 O fluid values in the range of 3.9 to 17.5. The 18 O fluid values of the post-ore-stage vein calcite (15.2–21.3) are much higher than both the pre- and syn-ore stages. The vapor phase of inclusions contains H 2 S, CH 4 , and C 2 H 6 in the syn-ore stages. All these observations reveal that (1) the formation of the Cu skarn deposit was dominated by a magmatic hydrothermal system, (2) multiple fluid pulses contributed to the formation of the pre- and syn-ore-stage skarn minerals and sulfides, and (3) the increase in pH due to the neutralization of the acidic fluid could be the main factor controlling the large-scale ore deposition in Hongniu-Hongshan.

Description: Many Carlin-like Au deposits occur within the late Paleozoic and Triassic Youjiang basin of southwest China. The Huijiabao trend in Guizhou Province contains over 300 metric tons (t; 10.6 Moz) of Au at an average grade of 7 to 18 g/t in a narrow corridor that is about 20 km long and 5 km wide. Petrographic and SEM studies of pyrite in barren host rocks and high-grade orebodies led to the recognition of four stages of pyrite. Py1 consists of fine-grained framboidal crystals in black mudstone. Py2 is comprised of coarser grained euhedralsubhedral clusters that are spatially related to organic matter. Py3 is coarse grained, euhedral, and occurs as overgrowths on Py1 and Py2. Py3’s porous texture, inclusion of randomly oriented detrital minerals, and association with quartz recrystallization suggest it was deformed during Late Triassic orogenesis with Py1 and Py2. Py4 generally occurs as rims on Py1 to Py3 and is intergrown with arsenopyrite. Sensitive high-resolution ion microprobe (SHRIMP) 34 S analyses of each pyrite type and arsenopyrite show that Py1 is related to Py2 and that Py3 is related to Py4 and arsenopyrite. The S isotope compositions of Py1 (–7.5 to +5.9) and Py2 (–5.3 to +7.9) are bimodal, which suggests that H 2 S was generated by biogenic sulfate reduction in open marine and sulfate limited systems during sedimentation and/or diagenesis. The compositions of Py3 (–2.6 to +1.5), Py4 (–1.2 to +1.5), and arsenopyrite (–0.8 to +0.9) are homogeneous and have an intermediate range of values near 0 that suggest that H 2 S was derived either from average pyrite (0.2) in sedimentary rocks or from a concealed magmatic source. Laser ablation-inductively coupled plasma-mass spectrometer (LA–ICP–MS) trace element analyses (As, Ni, Co, Cu, Ag, Se, V) support different origins and show that Py3 and Py4 are ore related. The lower w (Co)/ w (Ni) and w (S)/ w (Se) ratios of Py1 and Py2 are consistent with formation during sedimentation or diagenesis, whereas the higher ratios of Py3, Py4, and arsenopyrite are consistent with a hydrothermal origin. The lower concentrations of Au in Py1 (0.23–2.5 ppm) and Py2 (0.06–12 ppm) show that little Au was added during sedimentation or diagenesis. The higher concentrations of Au in hydrothermal Py3 (1.1–110 ppm) and Py4 (0.34–810 ppm) indicate that most of the Au was introduced during subsequent hydrothermal fluid flow. The low Au contents of arsenopyrite (0.09–0.52 ppm) suggests they formed from Au-depleted fluids. The Au/As ratios of Py1 and Py2 are typical of diagenetic pyrite whereas Py3 and Py4 have ratios that approach those of ore-stage pyrite in Nevada Carlin-type deposits. The fracturing of Py3 and its cementation by Py4 suggests that ore fluid movement was associated with deformation. Published isochron ages on arsenopyrite (Re-Os ~200 Ma) and late calcite-realgar veinlets (Sm-Nd ~135 Ma) in the Huijiabao trend are older than mafic dikes (84 Ma) exposed ~20 km to the east. If the 200 and 135 Ma ages are valid, H 2 S and Au may be derived from a sedimentary source because igneous intrusions of this age have not been found. If these ages are not valid and the gold deposits are actually Late Cretaceous in age, then H 2 S and Au may be derived from a magmatic source. Additional geochronology and isotopic tracer studies are needed to resolve this uncertainty.