Biography
Prof. Wenbo Li
Prof. Wenbo Li
Key Laboratory of Orogenic Belt and Crustal Evolution, School of Earth and Space Sciences, Peking University, China
Title: Hydrothermal graphite leading to the precipitation of gold in Haoyaoerhudong deposit, Northern China
Abstract: 

Haoyaoerhudong is the largest carbonaceous metasediments hosted gold deposit on the north margin of North China Craton This deposit has reserved more than 7 Moz gold with average Au grade of 0.59 g/t by 2012. Gold mineralization is characterized by lenticular orebodies hosted in the Mesoproterozoic carbonaceous schist, phyllite and slate sequences. Representative hydrothermal alterations are silicification, sulfidation, biotitization and graphitization. Based on the cross-cutting relationship of hydrothermal veins and microscopic observations, four mineralization stages have been identified from Haoyaoerhudong, including the pyrite- pyrrhotite- nickel sulfides stage (I), quartz-biotite-sulfide stage (II-1), quartz-sulfide stage (II-2), and quartz-carbonate stage (III). Gold was mainly precipitated in stage II-1/2 as free gold associated with hydrothermal pyrrhotite and biotite.

The compositions of fluid inclusions are variable in different stages of vein quartz. Type I fluid inclusions from quartz in stage II-1 are enriched in H2O-CO2-CH4±N2, whereas Type II fluid inclusions from quartz in stage II-2 are dominated by H2O-CH4±CO2±N2 or H2O-N2±CH4. Type III fluid inclusions are rare in the quartz from stage III, with their aqueous phases dominated by H2O. The homogenization temperature and salinity are progressively decreased from Type I fluid inclusions (315-510 ℃, 1.7-18.4 wt.% NaCl), through Type II fluid inclusions (234-334 ℃, 1.0-14.3 wt.% NaCl), to the Type III fluid inclusions (98-239 ℃, 0.2-11.8 wt.% NaCl). The decrease of CO2 content in fluid inclusion was accompanied by the precipitation of graphite in hydrothermal veins. Microscope observation and Raman spectra suggest that both the fine-grained graphite from altered schist (Gr-1/2) and coarse-sized graphite from gold-bearing veins (Gr-3/4/5) exhibit a hydrothermal characteristic of high crystallinity. Raman thermometer on graphite has then revealed a fluid cooling history from the quartz – biotite ± sulfide veins (546 ℃) and quartz – sulfide veins (495 ℃), to the graphite-bearing altered schist (423 ℃). The modeling of C-O-H hydrothermal system indicates that the graphite from Haoyaoerhudong was precipitated due to the adding of reductive components (e.g., CH4) and/or the cooling of the CO2-bearing fluid until it reaches 421 ℃. The formation of hydrothermal graphite would significantly consume CO2 and effectively destabilize the Au-bisulfide complexes. Gold then precipitated in the stage II-1/2 veins and altered schist together with sulfides, graphite, titanite and apatite. 

Hydrothermal titanite U-Pb analyses have suggested the gold mineralization in Haoyaoerhudong near 256 Ma, about ~ 13 Ma later than the regional magmatic activity (269 Ma). The δ13C values of graphite, varying from −27.1 to −26.0 ‰ VPDB, suggest that the carbon was of biogenetic origin. Apatite Sr isotopes (87Sr/86Sr: 0.708293 ~ 0.708842) and titanite Nd isotopes (εNd(t): -11.76 ~ -14.84) also point out the contribution of sedimentary rocks during mineralization. These geochronological and isotopic analyses demonstrated that the medium to high temperature, low salinity, carbonic fluids were mainly derived from the carbonaceous metasediments. The oreforming fluids may have been heated by Permian biotite granite, but no magmatic contamination was observed. The co-deposition of hydrothermal graphite and gold then features the mineralization style in Haoyaoerhudong deposit.

Biography: 
Dr. Wenbo Li is currently a professor in School of Earth and Space Sciences of Peking University. He is the member of Society of Economic Geologists (SEG), associate editors of Ore Geology Reviews and Journal of Asian Earth Sciences X, and life member, Chinese Society of Mineralogy, Petrology and Geochemistry. He earned his Ph.D. degree from the Institute of Geochemistry, Chinese Academy of Sciences in 2004, and then joined to Peking University. He worked at CODES ARC Centre of Excellence in Ore Deposits, University of Tasmania as a visiting scholar during 2013 to 2014. His research and teaching interests focus on economic geology and ore deposits exploration and he hosted 5 Chinese National Science Foundation projiects. He has published one book and over 100 articles in peer-reviewed journals and cited more than 2000 times. He also published two Monographs, which are about orogenic-type polymetallic mineralization associated with multistage orogenesis in northern North China Plate and zinc-lead mineralization in giant Huize Zn-Pb deposit in Yunnan Province, southwest China. He worked as consult geologist for Phelps Dodge (USA), Nine Gold (Mongolia), Baogang Group to evaluate mineral deposits in China, Mongolia, Australia, Burma and Indonisia.