Mineralization Of Mesozoic-Cenozoic Sandstone-hosted Zn-Pb Deposits In The Southwstern Tianshan,xinjiang,NW China

A large number of Mesozoic-Cenozoic sandstone-hosted lead-zinc deposits have been found in the Southwestern Tianshan,Xinjiang,NW China.These deposits are highly clustered in the sedimentary foreland basins,indicating the favourabale geological conditions and great ore prospecting potential.The Uragen Zn-Pb deposit,located in the northern segment of the Kashi sag,is the only large sandstone-hosted Zn-Pb deposit in this region.It is characterized by the typical mineralization styles and perfectly records the ore-forming processes without intensive post-reconstruction.It therefore provides an excellent example to study the mineralization of these Mesozoic-Cenozoic sandstone-hosted lead-zinc deposits.In this contribution,based on detailed field investigation and microscopical observation,the Uragen deposit was selected to decipher the mineralization age,source of ore-forming material,ore-forming fluid and precipitation mechanism.Through comparative analysis with other sandstone-hosted Zn-Pb deposits,the regional metallogenetic regularity and key ore-controlling factors were also summaried to predict the prospecting orientation for rapidly expanding Zn-Pb resources in the Southwestern Tianshan.The main achievements in this study are as follows:?1?By the far-field effect of the collision between Lhasa and Qiangtang blocks and the combined effect of a strong aridification event throughout the entire Tianshan area,alluvial and fluvial red clastic rocks were widely deposited in the Early Cretaceous along the Southwestern Tianshan.These rocks were probably derived from the Proterozoic metamorphic basement and the Paleozoic meta-and/or sedimentary rocks with high Zn-Pb contents in the Southwestern Tianshan,and they formed the most important host rocks in this region.?2?Pyrite Re-Os dating results reveal that the Uragen Zn-Pb deposit may has undergone three major episodes of mineralization,including Late Eocene?45³5Ma?,Late Oligeocene-Miocene?30-18Ma?and Late Miocene?6.3¹0.7Ma?.The multi-stage mineralization has the good corresponding relationships with the regional oil-gas injection and intensive uplifting and exhumation events in the West Tianshan.These episodes may be genetically linked to the far-field effects of the collision between the Eurasia block and the composited block of India block and Kohistan-Ladakh arc,the activity of the Main Pamir Thrust?MPT?,and the activity of the Pamir Front Thrust?PFT?,respectively.?3?The fluid inclusion studies of sphalerite and the association of hydrothermal calcite reveal that the homogenization temperatures range from 92?to 193?,with a dominant range from 100?to 150?,and the salinities are estimated to be0.53-19.63 wt.%NaCl equivalent,with a dominant range from 4%to 14%.We therefore proposed the ore-forming fluid is characterized by low temperature and medium-low salinity.C-O isotopic compositions of hydrothermal calcite reveal that the source of carbon may be related to dehydroxylation of organic matter,indicating the possible addition of oil-gas and/or oil basin brine.According to the result of the mixed massing of the?³⁴SV-CDT values for galena from the Uragen deposit,we proposed that the reduced sulfur was mainly sourced from marine sulfate reduction including thermal sulfate reduction?TSR?and bacteria sulfate reduction?BSR?.?4?Systematic Pb isotopes and REE studies of the sulfides and possible source rocks were conducted for the Uragen Zn-Pb deposit,revealing that the ore metals may be mainly derived from the red clastic rocks of the fifth member of the Lower Cretaceous Kezilesu Group.Petrographic observations indicate that the host clastic rocks have undergone diagenetic reddening and bleaching processes.The reddening is related to the in situ oxidation of mafic minerals such as biotite to produce iron oxide and hydroxide?IOH?that accompanyed the liberation of ore metals from the silicate phase.During subsequent crystallization of IOH to hematite,preferential adsorption could have been the main mechanism for separating Zn-Pb from Cu.The bleaching results from the dissolution of IOH during the oil-gas infilling stage,and then substantial amounts of leachable Fe-Zn-Pb were extracted from the red clastic rocks by the ore-forming fluid based on mass balance calculations.?5?The formation of the Uragen Zn-Pb deposit may be related the reduced ore-forming fluid due to the injection of oil-gas or oil field brine.The reduced fluid probably moved through the coarse-grained red clastic rocks of the fifth member of the Lower Cretaceous Kezilesu Group,and at the same time the ore metals were leached during the bleaching process.These metals were transported to the ore site and precipitated when the fluid interacted with the overlying gypsum bed of the Paleocene Aertashi Formation and gypsum cement in the sandstone/conglomerate of the fifth member of the Lower Cretaceous Kezilesu Group.?6?In combination with the above discussion,the mineralization of the Mesozoic-Cenozoic sandstone-hosted lead-zinc deposits in Southwestern Tianshan is genetically linked to the far-field geodynamic processes of collisions between the southern continental blocks in the Neo-Tethys domain and the southern margin of Eurasia block.The zinc-lead mineralization may be controlled multiple factors including the basin structure,bleaching related to oil-gas reduction,paleo-uplift and paleoclimate of hot and dry.