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Changming Wang, York University (Canada)
Qiuming Cheng, York University (Canada)
Jun Deng, China University of Geosciences (China)
Shuyun Xie, York University (Canada)
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Situated in the East Qinling metallogenic belt on the southern margin of North China Carton, the Nannihu ore-forming field is the second largest super-large skarn-porphyry Mo-W area in the world, including three large deposits: such as the Nannihu porphyry Mo-W deposit, the Sandaozhuang skarn Mo-W deposit and Shangfanggou porphyry-skarn Mo deposit as well. The total metal reserves of Mo and W associated amount to 1,982,900 tons and 737,900 tons, respectively. Outside the ore field, there are some hydrothermal Pb-Zn-Ag deposits found in recent years. According to the study of geochemical characteristics, spatial distribution patterns of belts and the stable isotopes and source of ore-forming materials, the following results have been obtained:
(1) 1:50000 stream sediment survey shows that there exist some obvious and complete anomalous belts with element associations of Mo-W-Bi-Cu-Zn-Pb-Ag-As-B-Ge around the Nannihu magmatic rock, indicating a geochemical evolutionary series from Mo-W element association to Zn-Pb-Ag element association from the center to outside, corresponding to the metallogenic systems of the porphyry-skarn type Mo-W deposits and medium-low temperature hydrothermal type Pb-Zn-Ag deposits.
(2) The ore-forming fluids show that these deposits are in the same metallogenic system and the hydrothermal solution is mainly derived from primary magmatic water in the early stage and from the mixture of the primary magmatic water and meteoric water in the later stage, with an obvious decreasing tendency respectively in temperature, salinity and gas-liquid ratio of the ore-forming fluids around the Nannihu magmatic rock. Sulfur and lead isotope components show that the metallogenic substances and related porphyries are mainly derived from the lower crust.
(3) Re-Os isotopic dating data have displayed that the large-scale minerogenetic epoch occurred at about 140 Ma. Mesozoic tectonic evolution study shows that the large-scale mineralization was the consequence of lithospheric extension during the late stage of the tectonic regime changing its main stress from NS-trending to EW-trending.
(4) Hidden magmatic chamber in the earth crust supply huge amounts of metallogenic substances for Mo-W- Pb-Zn-Ag metallogenic systems, which can be proved by aeromagnetic anomaly and drill hole data.
The study of metallogenic system has implications for exploration, particularly in large magmatic hydrothermal districts where various types of base and precious metal deposits may occur.
Key words: Endogentic metallogenic systems, Superlarge deposit, Re-Os isotope, Ore-forming fluids, Nannihu ore field
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