| In the Cu-Au-Pb-Zn-Ag polymetallic mineralization belt of the conjuncture of Sichuan,Yunnan and Guizhou,occurred the Taiping Pb-Zn deposit in strata of Proterozoic metamorphic rocks accompanied by late Proterozoic volcanic rocks.Previous studies on this type of Pb-Zn deposits are limited and therefore a comprehensive study,including work on the mineralogy,petrology and ore deposits of the Taiping Pb-Zn deposit and regional mafic-intermediate volcanic rocks,are conducted.The main purpose of this work is to investigate the geodynamic of petrogenesis and mechanisms controlling metallogenesis,and to establish a reliable oreforming model incorporating ore-controlling factors which may help future prospection.The trachyandesite in 4th member of the Mesoproterozoic Tong'an Formation has an age of ~1470 Ma,which puts the lower limit of Tong'an Formation.The element geochemical characteristics suggest that the trachyandesite was produced via magmatic crystallization during intracontinental rift,a process similar to the Zuosuo(~1596Ma)and Hewei(~1688Ma)basic rocks.In the Taiping Pb-Zn deposit,ore bodies occurred in the Mesoproterozoic Tongan Formation and are controlled by the NE-trending faults.Wall-rock alterations are dominantly silicification and carbonation.Three paragenetic stages of mineralization are identified,including an early deposition of arsenopyrite and pyrite,subsequent precipitation of polymetallic sulfides which are mainly composed by sphalerite,pyrite and chalcopyrite,and to a lesser extent,galena and chalcopyrite.Genetic mineralogical work reveals that the compoisition of sphalerites is Fe= 0.86~4.42(wt%)and Mn=44.3~51.6(ppm),Zn/Cd ratios varying from 305.77 to 338.47,and the Ga/In ratios higher than 1.Pyrites are depleted in S and Fe,and arsenopyrits are enriched in S but depleted in As.All the above features suggest that the Taiping Pb-Zn deposit may belong to the strata-bound or sedimentationreformation type deposit.In addition,the pyrite-sphalerite-chalcopyrite-gersdorffitegalena sequence,which is similar to the Viburnum Trend MVT lead-zinc deposit,and the typomorphic characteristics of these minerals imply that the deposit formed in intermediate-lower temperature and intermediate shallow-seated environments.Ore-forming fluid is characterized by medium-low temperature(112.6~188.7?),medium salinity(10.61%~19.68%NaCl),medium-low pressure(32.909~49.889MPa)with Na+,Ca2+,Mg2+,Cl-and H2 O being dominated in chemical composition.The oreforming fluid was mainly derived from formation water and was mixed with meteoric water.Carbon was mainly from the dissolution of marine carbonate rocks,and sulphur(?34S=10.4‰~12.3‰)was dominantly derived from the reduction of marine sulfate in the strata.Lead isotope showed normal lead characteristic indicating that lead came from the basement strata and was affected by metamorphism.87Sr/86 Sr values of trachyandesite are 0.77 to 0.87,reflecting it was intermingled with crust source.For sphalerite,(87Sr/86Sr)i(t=714Ma)=0.72064~0.72135,indicating that metallogenetic materials mainly came from the basement strata.The enrichment of Zn,Pb,Cu,Ag,As,Co and Ni was associated with silicification and carbonation.A new mineralization age was obtained in Sichuan-Yunnan-Guizhou lead-zinc metallogenic belt by using Rb-Sr dating of sphalerite(714±11Ma),which indicates that the lead-zinc deposits in the basement strata formed in Neoproterozoic,and further shows that the Chengjiang Period in this area is an important mineralization stage for Cu,Zn and Pb.The SN trend and NE faults,lithology of Mesoproterozoic strata,geochemical anomaly symbol and wall rock alteration can be used as criteria for ore prospecting in the same kinds of deposits. |
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