The orogenic gold deposits in Southeast Guizhou are an important component of the Xuefeng polymetallic ore belt and have significant exploration potential, but geochronology research on these gold deposits is scarce. Therefore, the ore genetic models are poorly constrained and remain unclear. In the present study, two important deposits (Pingqiu and Jinjing) are investigated, including combined Re–Os dating and the He–Ar isotope study of auriferous arsenopyrites. It is found that the arsenopyrites from the Pingqiu gold deposit yielded an isochron age of 400 ± 24 Ma, with an initial 187Os/188Os ratio of 1.24 ± 0.57 (MSWD = 0.96). An identical isochron age of 400 ± 11 Ma with an initial 187Os/188Os ratio of 1.55 ± 0.14 (MSWD = 0.34) was obtained from the Jinjing deposit. These ages correspond to the regional Caledonian orogeny and are interpreted to represent the age of the main stage ore. Both initial 187Os ratios suggest that the Os was derived from crustal rocks. Combined with previous rare earth element (REE), trace elements, Nd–Sr–S–Pb isotope studies on scheelite, inclusion fluids with other residues of gangue quartz, and sulfides from other gold deposits in the region, it is suggested that the ore metals from Pingqiu and Jinjing were sourced from the Xiajiang Group. The He and Ar isotopes of arsenopyrites are characterized by 3He/4He ratios ranging from 5.3 × 10−4 Ra to 2.5 × 10−2 Ra (Ra = 1.4 × 10−6, the 3He/4He ratio of air), 40Ar*/4He ratios from 0.64 × 10−2 to 15.39 × 10−2, and 40Ar/36Ar ratios from 633.2 to 6582.0. Those noble gas isotopic compositions of fluid inclusions also support a crustal source origin, evidenced by the Os isotope. Meanwhile, recent noble gas studies suggest that the amount of in situ radiogenic 4He generated should not be ignored, even when Th and U are present at levels of only a few ppm in host minerals.
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) δ 34S 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 H2S 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 H2S 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, H2S 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 H2S and Au may be derived from a magmatic source. Additional geochronology and isotopic tracer studies are needed to resolve this uncertainty.
The causes of low-recovery for cadmium(Cd) separated and purified by the ion exchange resin one-column procedures were investigated.Based on the ion exchange resin one-column procedures,a proved method was proposed and the washing procedure and the volume of washing solution was adjusted.The results indicated that the recovery of Cd was up to 99.82%,the potential interferences on Cd isotopes(including Sn,In,Zn,Pb and other interfering matrix elements) were present at negligible levels.The improved method could meet the needs of the measurement of Cd isotopic ratio.
Abstract The Lanping basin, Yunnan province, SW China, is located at the juncture of the Eurasian and Indian Plates in the eastern part of the Tibetan Plateau. The Lanping basin, in the Sanjiang Tethyan metallogenic province, is a significant Cu–Ag–Zn–Pb mineralized belt in China that includes the largest sandstone‐hosted Zn–Pb deposit in the world, the Jinding deposit, as well as several Ag–Cu deposits (the Baiyangping and Jinman deposits). These deposits, with total reserves of over 16.0 Mt Pb + Zn, 0.6 Mt Cu, and 7,000 t Ag, are mainly hosted in Meso‐Cenozoic clastic rocks and are dominantly controlled by two Cenozoic thrust systems developed in the western and eastern segments of the basin. The Baiyangping, Babaoshan, and Hetaoqing ore deposits are representative of the epithermal base metal deposits in the Lanping basin. The microthermometric data show that the ore‐forming fluids for these deposits were low temperature (110–180 °C) and had bimodal distribution of salinity at moderate and mid to high salinities (approximately 2–8 wt.% and 18–26 wt.% NaCl equivalent). The C and O isotope data indicate that the ore‐forming fluids were related to hot basin brines. We present new He and Ar isotope data on volatiles released from fluid inclusions contained in sulfides and in barite in these three deposits. 3 He/ 4 He ratios of the ore‐forming fluids are 0.01 to 0.14 R/Ra with a mean of 0.07 Ra (where R is the 3 He/ 4 He ratio and Ra is the ratio for atmospheric helium). This mean value is intermediate to typical 3 He/ 4 He ratios for the crust (R/Ra = 0.01 to 0.05) and the ratio for air‐saturated water (R/Ra = 1). The mean ratio is also significantly lower than the ratios found for mantle‐derived fluids (R/Ra = 6 to 9). The 40 Ar/ 36 Ar ratios of the ore‐forming fluids range from 298 to 382 with a mean of 323. This value is slightly higher than that for the air‐saturated water (295.5). The 3 He/ 4 He ratios of fluids from the fluid inclusions imply that the ore‐forming fluid for the Baiyangping, Babaoshan, and Hetaoqing deposits was derived from the crust and that any mantle‐derived He was negligible. The content of the radiogenic Ar ranges between 0.2 to 20.4%, and the proportion of air‐derived 40 Ar averages 94.1%. This indicates that atmospheric Ar was important in the formation of these deposits but that some radiogenic 40 Ar was derived from crustal rocks. Based on these observations coupled with other geochemical evidence, we suggest that the ore‐forming fluids responsible for the formation of the Ag–Cu–Pb–Zn polymetallic ore deposits in the Baiyangping area of the Lanping basin were mainly derived from crustal fluids. The fluids may have mixed with some amount of air‐saturated water, but there was no significant involvement of mantle‐derived fluids.
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