Geologic, fluid inclusion and isotopic characteristics of the Jinding Zn-Pb deposit, western Yunnan, South China: A review

摘要

With a reserve of approx 200 Mt ore grading 6.08 percent Zn and 1.29 percent Pb (i.e., a metal reserve of approx 15 Mt) hosted in Cretaceous and Tertiary terrestrial rocks, the Jinding deposit is the largest Zn-Pb deposit in China, and also the youngest sediment-hosted super giant Zn-Pb deposit in the world. The deposit mainly occurs in the Jinding dome structure as tabular orebodies within breccia-bearing sandstones of the Palaeocene Yunlong Formation (autochthonous) and in the overlying sandstones of the Early Cretaceous Jingxing Formation (allochthonous). The deposit is not stratiform and no exhalative sedimentary rocks have been observed. The occurrence of the orebodies, presence of hangingwall alteration, and replacement and open-space filling textures all indicate an epigenetic origin. Formation of the Jinding Zn-Pb deposit is related to a period of major continental crust movement during the collision of the Indian and Eurasian Plates. The westward thrusts and dome structure were successively developed in the Palaeocene sedimentary rocks in the ore district, and Zn-Pb mineralisation appears to have taken place in the early stage of the doming processes. The study of fluid inclusions in sphalerite and associated gangue minerals (quartz, celestine, calcite and gypsum) shows that homogenisation temperatures ranged from 54 to 309 deg C and cluster around 110 to 150 deg C, with salinities of 1.6 to 18.0 wt. percent NaCl equiv. Inert gas isotope studies from inclusions in ore- and gangue-minerals reveal 2.0 to 15.6 percent mantle He, 53 percent mantle Ne and a considerable amount of mantle Xe in the ore-forming fluids. The Pb-isotope composition of ores shows that the metal is mainly of mantle origin, mixed with a lesser amount of crustal lead. The widely variable and negative delta~(34)S values of Jinding sulphides suggest that thermo-chemical or bacterial sulphate reduction produced reduced sulphur for deposition of the Zn-Pb sulphides. The mixing of a mantle-sourced fluid enriched in metals and CO_2 with reduced sulphide-bearing saline formation water in a structural-lithologic trap may have been the key mechanism for the formation of the Jinding deposit. The Jinding deposit differs from known major types of sediment-hosted Zn-Pb deposits in the world, including sandstone-type (SST), Mississippi Valley type (MVT) and sedimentary-exhalative (SEDEX). Although the fine-grained ore texture and high Zn/Pb ratios are similar to those in SEDEX deposits, the Jinding deposit lacks any exhalative sedimentary rocks. Like MVT deposits, Jinding is characterised by simple mineralogy, epigenetic features and involvement of basinal brines in mineralisation, but its host rocks are mainly sandstones and breccia-bearing sandstones. The Jinding deposit is also different from SST deposits with its high Zn/Pb ratios, among other characteristics. Most importantly, the Jinding deposit was formed in an intracontinental terrestrial basin with an active tectonic history in relation to plate collision, and mantle-sourced fluids and metals played a major role in ore formation, which is not the case for SEDEX, MVT, and SST. We propose that Jinding represents a new type of sediment-hosted Zn-Pb deposit, named the 'Jinding type'.