| 其他摘要 | The Gejiu Sn-polymetallic deposit (Yunnan, China) is renowned at home and abroad for being one of the super large Sn-polymetallic deposits in the world. Detailed studies on the regional geology and the local geology of the deposit have been carried out in this study. Systematic microscopic observation of the ores and the quantitative and qualitative electron-probe analysis of the ores and minerals have been done. Meanwhile, the contents of trace elements, rare earth elements and the Pb, S, He, Ar isotopic compositions of the ores and rocks have been analyzed. Based on these data, the sources of the ore-forming metals and fluids have been discussed. The results showed that the Gejiu Sn-polymetallic deposit mainly experienced extensive and important Mid-Triassic submarine exhalative hydrothermal mineralization and Yanshanian magmatic hydrothermal mineralization. Finally, a genetic model was proposed. During Middle Triassic, the Gejiu area was in a special sedimental environment, i.e. in a restricted marginal sea basin within the,continental rift system. The basalt, erupted in Mid-Triassic, is closely related to the Gejiu tin polymetallic mineralization both in time and space. It might be derived from the upper mantle with enriched ore-forming elements through the metasomatism which played an important role on the formation of the Gejiu super large Sn-polymetallic deposit. The occurrences of the "interbedded oxide orebodies" and their ore types have shown some syngenetic sedimentary information. The systematic and detailed microscopic observation has revealed that the Gejiu tin deposit has initially undergone submarine exhalative mineralization. Some orebodies in the contact zone were developed through the strong rework of the initial submarine exhalative orebodies by the Yanshanian granitic hydrothermal fluids. Some of the interbeded orebodies have been partly reworked by the Yanshanian granite magmatic fluids. The abundant Sn enriched in oolitic pyrite and colloidal pyrite show that the exhalative hydrothermal activity brought large amount of Sn and other ore-forming elements for deposition. The existence of cassiterite (SnO2) inclusions in pyrite imply that the submarine exhalative mineralization has potential to form tin deposit. The characteristics of the rare earth elements of ores and cassiterite separates, which were selected from tourmaline vein type ores and interbeded hematite ores, indicate that the tourmaline vein type ores were formed through the mineralization of granite magmatic fluids, whereas the interbeded hematite ores were initially formed by submarine exhalative mineralization. The rare earth elements of the sulfide separates selected from the interbeded massive sulfide ores indicate that both submarine exhalative mineralization and the granite magmatic hydrothermal mineralization have played role in the formation of some interbeded orebodies. It is accordant with the microscopic observations. 7. The Pb and S isotopic compositions of the ores indicate that the Pb and S were derived partly from submarine exhalative mineralization and partly from granite magmatic fluids. The He isotopic values indicate that mantle-derived fluids were involved in the ore-forming fluids at early stage through the submarine exhalative deposition. On the other hand, the input of large amount of radiogenic He suggests the Yanshanian granite magmatic fluids were involved in the mineralization at late stage. The dense earthy hematite ores and part of the limonite ores distributed in the "interbeded oxide orebodies", were formed directly by the submarine exhalative hydrothermal mineralization, not by the oxidation of sulfides. Based on the major elements of the granite in the Gejiu West, an analogical study has been given for the granite in the Gejiu East. It is shown that a large amount of iron, especially the Fe3+, was enriched in the granite in the Gejiu East. This is interpreted that the reworking of the submarine exhalative orebodies by the intrusion of the Yanshanian granitic magma resulted the iron enrichment in the granite in the Gejiu East. Meanwhile, the special rare earth elements (REE) distribution patterns of the Laochang granite indicate that the Laochang granite could strongly interacted with the "interbeded oxide orebodies". 10. Based on the results above, a genetic model for the mineralization of the superlarge Gejiu Sn polymetallic deposit is proposed: This deposit was formed through several geological processes in a relatively long time period with characteristics of multi-sources of ore-forming materials and two mineralization processes. It is believed that the interbeded orebodies of mainly amorphous iron-oxides, which is located relatively far away from the granite body, were formed by the extensive and important Mid-Triassic submarine exhalative hydrothermal mineralization, the interbeded orebodies relatively close to the granite were reworked by the Yanshanian magmatic hydrothermal mineralization, whereas the orebodies in the contact zone were mainly formed by the Yanshanian magmatic hydrothermal mineralization. Therefore, it is concluded that both of the submarine exhalative hydrothermal mineralization and magmatic hydrothermal mineralization have made contributio n for the formation of the Gejiu tin deposit. |
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