Geology, Geochemistry And Origin Of Ore-forming Substances Of Mengya'a Skarn Lead And Zinc Ore Deposit, Tibet, China

As one of the most important types of ore deposit in Gangdese area of Tibetan, skarn-related ore deposits are currently the main source of iron, copper, lead, zinc and silver production in Tibetan. The Mengya'a lead and zinc deposit in Tibet, which is located in Nyainqentanglha metallogenic sub belt of south Gangdise-Nyainqentanglha metallogenic belt(retro-arc depressions and uplifts) , is a significant and typical skarn ore deposit in this area.Based on ore deposit geology, by studying geochemistry of the ore deposit comprehensively and discussing the origin of ore-forming substances, this thesis suggests that Mengya'a lead and zinc deposit is of stratabound skarn type. Finally, the genesis of the ore deposit has been specified.Geochemical studies on major and minor elements in strata demonstrate that Laigu Formation and Luobadui Formation have been influenced differently by hydrothermal sedimentation during their sedimentation and diagenesis. Meanwhile, geochemical studies on minor elements in strata suggest rocks from Laigu Formation were deposited under aerobic conditions, In sharp contrast, rocks from Luobadui Formation were deposited under anaerobic conditions. Diversity in contents of minor elements in rocks from Laigu Formation and Luobadui Formation were resulted from different tectonics and origins of sediments. Conclusions made from element geochemical researches correspond with idea that sedimentary environments of Laigu and Luobadui Formations are rifted continental margin and rifted arc respectively. Studies on ore-forming elements in strata manifest that Cu> Pb and Zn contents of Laigu Formation generally higher than those of Luobadui Formation and obviously surpass the abundance of continental crust, which makes Laigu Formation possible to be the potential source bed of this area.Geochemical studies on minor elements and REE (Rare Earth Elements) show that, characteristics of minor elements and REE of quartz porphyry in mining area closely resemble those of rock masses 4 kilometers southwest to the mining area, from which it could be inferred that the 2 rock masses share a same source and have a relationship considering the evolution process. The source rocks possibly are clay-depleted metasandstones from upper crust, which has obvious island arc -syn-collisional natures. Additionally, comparative research on quartz porphyry in mining area and porphyry masses in Gangdise areas demonstrates the geochemistry of the 2 sorts of porphyry are similar to that of porphyries of main collision stage but different to that of post collision extending stage. So it could be inferred that quartz porphyries in this area were resulted from partial melting of materials from upper crust induced by increasing temperature and pressure that caused by shrinking crust.Study on chemical composition of altered wall rocks in mining area suggests that substances like Al₂O₃ K₂O SiO₂ emigrated from sandstones and limestones during hydrothermal process, meanwhile CaO -, Fe₂C>3 and MgO migrated in . However, considering plates, Al₂O₃ Fe₂O₃> MgO and Na₂O emigrated out and CaCK SiC>2 migrated in , which is consistent with the geology that carbonatization and skarnization are present in sandstones and limestones adjacent to ore bodies, in addition, carbonatization and silicification usually exist in plates close to ore bodies. According to comparative study on altered wall rocks adjacent to ore bodies and wall rocks away, hydrothermal fluids leached ore-forming elements as Cu> Pb and Zn out of wall rocks. Above has demonstrated that the ore deposit is strata bound and entwined with hydrothermal metasomatism.Researches on REE in metal sulphides manifest that, REE distribution patterns of ores and rocks from Laigu Formation are quite similar, which represents being rich in light REE and depleted in heavy REE. The curve is gently right-dec lined. Furthermore, obvious Eu negative anomalies and Ce invisible or weak positive anomalies, which suggest that with respect to the origin of ore-forming substances, ores and rocks from Laigu Formation have a genetic relationship with each other somewhat.Study on lead, sulphur, carbon and oxygen isotopes shows that, strata provided a certain amount of ore-forming substances for Mengya'a ore deposit. The source of ore-forming substances has undergone dual effects by magma and strata, and has characteristics of the both. According to the facts that no correlations exist between Pb and Zn in ores, ore-bearing wall rocks are relatively more depleted in Zn during hydrothermal metasomatism. However, comparing to continental crust, rock masses are more enriched in Pb, from which it could be inferred that Pb in ore-forming substances is probably sourced from rock masses. As for zn, it is possibly derived from strata.Considering geology and mineralization as a whole, the ore deposit is one of strata bound skarn lead and zinc deposits. Beginning with hydrothermal sedimentation, in such environment with extensive magmatism resulted from melting of substances of crust caused by collision of Indian plate and Asian Plate in Himalayan, the mineralization was formed in certain lithologic associations by ore- bearing fluid that resulted from long term metasomatism along the beds and leachinj ore-forming substances by magma hydrothermal fluids with certain ore-forminj substances.