Typical Minerals Chemical Characteristics And Discussion On Genesis Of The Mengxing Zn-Pb Deposit,Western Yunnan (SW China)

The Mengxing Zn-Pb deposit,located in the middle-western parts of Baoshan block of Sanjiang metallogenic belt,western Yunnan?SW China?,is the large-scale deposit with the highest level of exploration.Due to the low degree of research,however,there is a great dispute in the ore genesis.In order to reveal the origin of ore-forming fluid and its physical and chemical conditions,and to discuss the metallogenic mechanism and ore genesis,on the basis of detailed study on geological characteristics of deposit,we apply the ICP-MS and LA-ICPMS methods to the calcite,sphalerite and pyrite from different stages for studying its trace elements geochemistry in this paper.The geological features of ore deposit indicate that the deposit is jointly controlled by the nearly NS-trending Mengxing syncline and the interlayer fracture zone in the Lower member of the Shangrenheqiao formation,the Middle Silurian.The orebodies,occurring as layered,stratiform-like,lenticular and pod-like,are hosted in laminated limestone and clastic limestone.The ore mainly have veined,brecciated and disseminated structures,which shows the features of epigenetic mineralization.The deposit is characterized by simple mineral assemblage?mainly sphalerite,galena and calcite?and weak wall rock alteration?mainly carbonation?.These features are consistent with the typical MVT-type Zn-Pb deposits.Trace element geochemistry shows that the REE contents,characteristic parameters and distribution patterns of stage?calcite are more similar to those of wall rock strata,while different from stage?calcite.But the Y/Ho-La/Ho discriminant diagram demonstrates that two stages of calcite are produced from the same source in different stages.Stage?sphalerite is enriched in Cu,Cd and Sb,while stage?sphalerite is mainly enriched in Cd,but two stages of sphalerite are all depleted in Fe,Mn,In,Sn and Co.The contents of each trace element in stage I and stage II pyrite are proximate and have a consistent change trend,with enrichment in Ni,As,Sb and depletion in Sn,Bi.These features are also consistent with the MVT-type Zn-Pb deposits.The Y/Ho ratios of calcite and sphalerite are close to those of wall rock strata,which indicate the ore-forming fluid of the Mengxing Zn-Pb deposit mainly originates from the basin brine which is stored in the strata.The?Eu and?Ce characteristic parameters of calcite and trace element enrichment characteristics and geological thermometers of sphalerite indicate that the early-ore stage is characterized by low temperature,enriched in Cd and reducibility.In this stage,the large number of S^2-is formed by thermochemical reduction and combined with metal ions Zn²⁺,Pb²⁺and so on,which results in a large amount of sulfide precipitation.The late-ore stage may be affected by the large-scale magmatic activity in the block,which provides some heat for the migration of the ore-forming fluid,resulting in the increase of the metallogenic temperature.The ore-forming fluid in this stage is still characterized by enriched in Cd and reducibility.Comprehensive studies suggest that the geological characteristics and the geochemical features of trace elements of calcite,sphalerite and pyrite from the Mengxing deposit are similar to those of typical Mississippi Valley type Zn-Pb deposit?MVT-type?,indicating that the deposit belongs to MVT-type.Combining with the regional metallogenic geological background,we believe that the deposit was formed under the background of continental collision and extrusion between the Tengchong block and Baoshan block after the closure of the Meso-Tethys?Nujiang Ocean?.