Geological Geochemical Characteristics And Genesis Of The Tiantangshan Tin Polymetallic Deposit, Longchuan County, Guangdong Province

Tiantangshan tin polymetallic deposit, located in Longchuan county, Guangdong province, is a medium-sized polymetallic deposit found in recent years. Its geotectonic position is in the east of the Nanling metallogenic belt, the juncture of the Heyuan deep fault zone and the Dadongshan-Liyuan-Luofu east-west extending deep fault zone, south-central margin of the Mesozoic Dachangsha volcanic basin. At the same time of the exploration work, a series of detailed studies of petrography, petrogeochemistry, chronology, microscopic temperature measuring of fluid inclusions and stable isotope analysis, etc, have been carried out on the Tiantangshan tin polymetallic deposit. The diagenetic and metallogenic epoch, mineralization and formation mechanism are discussed in this paper. And it provides scientific basis for guiding the exploration work. The main results are as follows:1) Detailed field geological survey and indoor research show that Tiantangshan tin polymetallic deposit is hosted in volcanic-sub volcanic rocks. Ore bodies occur mainly in the top and outer contact zones of the greisenization quartz porphyry and altered fracture zones of the volcanic rocks. The wall rock alterations are largely strong silicification, biotitization, topazization, greisenization, chloritization and sericitization.2) The petrogeochemistry study results on the volcanics, alkali-feldspar granite, quartz porphyry and greisenization quartz porphyry, which are closely related to the mineralization, show that, generally, the rocks have the characteristics of high silicon, rich aluminum, poor magnesium, concentrated Rb, Th, U, Ta, Nb, Zr, Hf, strong Ba, Sr, K, P, Ti loss and strong negative Eu anomaly. The studies on trace and rare earth elements show that the alkali-feldspar granite, greisenization quartz porphyry have the same material source, and are probably the products of the same magmatic evolution in different stages. While, the greisenization quartz porphyry is the altered rock mass which is formed by the hydrothermal modification.3) We carried out SHRIMP method U-Pb chronology study on the ore-forming geological bodies and drew a conclusion that the zircon 206U/238 U weighted average ages of the volcanics, alkali-feldspar granite, quartz porphyry of the mining area are 136.2±1.2Ma、138.4±1.2Ma and 137.9±2.2Ma, respectively. Combined with the tectonic dynamics background, we believe that Tiantangshan granitic rocks formed in the extensional environment caused by plate retreating of the subduction of the Pacific plate.4) We carried out biotite Ar-Ar method chronology on the biotite alteration of the main metallogenic stage and drew a conclusion that the deposit metallogenic age is between 129.7±1.1Ma~133.5±0.75 Ma which is consistent with the diagenetic age within the error range, namely mineralization is consistent with magmatism activity. We believe that the Tiantangshan tin polymetallic deposit and magmatic rocks are products of the early Cretaceous and there is a close genetic relationship between them.5) Through field work and the microscopic identification, we divided the mineralization into alkali-metasomatism stage, topaz-greisenization stage, cassiterite-wolframite oxide stage and chalcopyrite-polymetallic sulfide stage. According to the fluid inclusions and stable isotopes(H-O-S) study, we discussed the metallogenic mechanism. In different stages, the temperature and salinity characteristic parameters of ore-forming fluid show a continuous evolution which is a typical feature of hydrothermal deposits. The source of ore-forming materials are mainly from the granitic rocks in the early Cretaceous. The early ore-forming fluid are largely from molten magma water of the granitic rocks, and eventually become a mixture fluid with atmospheric precipitation or surface water. The phase separation caused by immiscible role of ore-forming fluid and the decrease of fluid temperature and salinity caused by the involvement of a small number of low temperature and salinity atmospheric precipitation break the relatively stable fluid system, and break the balance of the ore-forming fluid physical and chemical system, which is the mechanism of the precipitation of cassiterite and oxide.The injection of amounts of meteoric water and fluid boiling is the mechanism of chalcopyrite and other sulfide precipitation.6) On the basis of comprehensive analysis, we think that Tiantangshan tin polymetallic deposit has a close genetic relation with the sub-volcanic activity after volcanic magmatism, and the genetic type is subvolcanic hydrothermal fluid-greisen type tin polymetallic deposit.