Study On The Metallogenic Mechanism Of Changpo-Tongkeng Tin-polymetallic Deposit, Dachang, Guangxi

Changpo-Tongkeng tin-polymetallic deposit located in Dachang, Guangxi Province is in the Southwestern margin of the Jiangnan old land, and the middle of Danchi metallogenic belt. As an important nonferrous metal mining industry base of China and a famous ultra-large tin-polymetallic deposit in the world ,with the proven tin reserve over one million ton and associated with Cu, Zn, Pb, Sb, Ge, In, Cd and Se etc, it has the great significance both in academic and economic. So it has attracted the attention of geologists and has been researched and exploited for nearly 60 years and has accumulated abundant data on the regional geology, stratum, tectonics, petrology, isotope and physical-chemical conditions of mineralization etc. Nevertheless, there has been debated on the metallogenesis. The difference focused on three aspects: metallogenic epoch, the origin of ore-forming materials and the source of ore-forming fluids.Based on the previous research achievements, the thesis studied the debatable problem by detailed investigation on mineralogy, petrology, mineral deposits, deposit geochemistry, isotope geochemistry and isotopic chronology, and achieved following progresses:1.The field investigation and the indoor observation and research showed that the formation of stratiform ore bodies is the replacement of the favourable stratum by ore-forming fluid. The stratiform ore-bodies and vein-type ore-bodies interpenetrated each other, which suggesting they were formed at the same mineralization processes. These results provide solid essential geological data for correct understanding of the deposit genesis.2. Discussed the change and migration characteristics of ore-forming elements among minerals with different occurrence, ore-bodies of different types from the aspects of mineralogy, ore petrology, trace elements, rare earth elements and isotopes of lead and sulfur. The results indicated that the ore-forming material of different ore-bodies is homogenous, which related with magma activity and come from the crust and maybe combined material from mantle. The migration of ore-forming material is from the bottom to up and from near to far from rock mass. Rare earth element standard distribution pattern in ore and rocks that formed in different ore forming stages are similar. TheδEu abnormality values from negative at the early stage to positive at the late stage, suggest the changes of ore-forming environment from a relative reduction to the relative oxidation.3. Fluid inclusions in ore bodies of different types are CO₂-H₂O type and with same features, suggesting their formation in the same mineralization process. He and Ar isotope data shows that the ore-forming fluids mainly derived from crust, but partly derived from mantle. The mantle-derived helium reduced from the early to late stages, crust- derived helium changed in opposite way, in which precipitated water involved in the late stage.4. As curtained the diagenetic and mineralizatic epochs of the mine by using suitable dating methods for different object, which suggest that the Dachang tin-polymetallic deposit is not formed in Devonian, but in the Yanshan period, particularly centralized in the mid-Cretaceous. Proposing that the deposit formation is not connected with single magma activity, but with multistage of magamatism. The rock and ore formation epochs should be unanimous in which perhaps ore-forming process would last for short time after the rock formation. The results provided sufficient chronological basis for building metallogenic model.5. Integrating information of Changpo-Tongkeng No. 91, No. 92 and Gaofeng No.100 ore bodies to analysis the ore forming condition, movement and enrichment pattern of ore-forming material, the thesis suggest that the ore formation closely related with granite of Yanshan period, in which magma movements provided ore-forming materials and energy; favourable stratum and structure conditions supplied ore-forming transport channels, space and force. The rational miracle combination of magma movement, stratum and structure led to the formation of the ultra-large deposit and the alteration of mineralization environment resulted in ore zoning, in which the skarn-type copper-zinc sulfide ore formed in the lower part of the deposit, tin- polymetallic sulfide ore formed in the upper part. The No.100 ore body is formed by the ore-bearing fluid filling"collapsed space"in the reef limestone, which resulted from fault activities or detachment of different stratums. Comparing No.100 to No.91 and No.92 ore-bodies, although their occurrence is different, the genesis of ore-forming materials and sources of ore-forming fluid are consistent.In summary, all evidence confirmed that the Changpo-Tongkeng tin-polymetalltic deposit is epigenetic and related to Yanshan period magmatic hydrothermal movement. This research replenished and deepened previous achievements and has a great significance for guiding prospecting around Dachang.