Chalcophile elemental compositions and origin of the Tuwu porphyry Cu deposit,NW China |
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| Institution: | 1. State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing 210093, China;2. Department of Earth Sciences, the University of Hong Kong, Hong Kong, China;3. State Key Lab of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China;1. Key Laboratory of Orogen and Crustal Evolution, Peking University, Beijing, 100871, China;2. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China;3. State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, China;1. State Key Laboratory for Mineral Deposits Research, Institute of Geo-Fluids, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China;2. Geological Survey of Anhui Province, Hefei, 230001, China;3. No. 313 Geological Team, Bureau of Geology and Mineral Resources Exploration of Anhui Province, Lu''an 237010, China;1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China;2. Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong, China;3. State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing 210093, China;4. Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. Inner Mongolia Mining Company, China National Gold Group Corporation, Xinbaerhuyouqi 021400, China |
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| Abstract: | The Tuwu porphyry Cu deposit in the eastern Tianshan Orogenic Belt of southern Central Oceanic Orogen Belt contains 557 Mt ores at an average grade of 0.58 wt.% Cu and 0.2 g/t Au, being the largest porphyry Cu deposit in NW China. The deposit is genetically related to dioritic and plagiogranitic porphyries that intruded the Carboniferous Qieshan Group. Ore minerals are dominantly chalcopyrite, pyrite and enargite. Porphyric diorites have Sr/Y and La/YbN ratios lower but Y and Yb contents higher than plagiogranites. Diorites have highly variable Cu but nearly constant PGE contents (most Pd = 0.50–1.98 ppb) with Cu/Pd ratios ranging from 10,900 to 8,900,000. Plagiogranites have PGEs that are positively correlated with Cu and have nearly uniform Cu/Pd ratios (5,100,000 to 7,800,000). Diorites have concentrations of Re (0.73–15.18 ppb), and 187Re/188Os and 187Os/188Os ratios lower but common Os contents (0.006–0.097 ppb) higher than plagiogranites. However, both the diorites and plagiogranites have similar normalized patterns of rare earth elements (REE), trace element and platinum-group elements (PGEs). All the samples are characterized by the enrichments of LREE relative to HREE and display positive anomalies of Pb and Sr but negative anomalies of Nb and Ta in primitive-mantle normalized patterns. In the primitive mantle-normalized siderophile element diagrams, they are similarly depleted in all PGEs but slightly enriched in Au relative to Cu.Our new dataset suggests that both the diorite and plagiogranite porphyries were likely evolved from magmas derived from partial melting of a wet mantle wedge. Their parental magmas may have had different water contents and redox states, possibly due to different retaining time in staging magma chambers at the depth, and thus different histories of magma differentiation. Parental magmas of the diorite porphyries are relatively reduced with less water contents so that they have experienced sulfide saturation before fractional crystallization of silicate minerals, whereas the relatively more oxidized parental magmas with higher water contents of the plagiogranite porphyries did not reach sulfide saturation until the magmatic-hydrothermal stage. Our PGE data also indicates that the Cu mineralization in the Tuwu deposit involved an early stage with the enrichments of Au, Mo and Re and a late stage with the enrichment of As but depletion of Au–Mo. After the formation of the Cu mineralization, meteoric water heated by magmas penetrated into and interacted with porphyritic rocks at Tuwu, which was responsible for leaching Re from hosting rocks. |
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