Geochemistry And Ore-Formation Of The Gejiu Super-large Tin-polymetallic Deposit

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 isotapic 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.1. 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.2. 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.3. The occurrences of the "interbedded oxide orebodies" and their ore types have shown some syngenetic sedimentary information.4. 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.5. 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.6. 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...