Study On Genesis Of The Mazhala Au-Sb Deposit In South Tibet

The Mazhala Au-Sb deposit is located in the Tethys Himalayan region of south Tibet.As a typical Au-Sb deposit in Southern Tibet Au-Sb Polymetallic Belt,the research degree is relatively low,and the genesis is still controversial.The main viewpoints involve orogenic deposit and magmatic-hydrothermal.In this article,based on the research of regional geologic survey and Previous studies,by means of integrated petrography,mineralogy,element and stable isotope geochemistry,and fluid inclusion analyses,this study aims to provide more comprehensive and credible evidence for ore genesis.The results of the Mazhala Au-Sb deposit and their geological significances are summarized as follows:1.By the regional geologic survey and Previous results analysis,the Mazhala Au-Sb deposit occurs in the Early-Middle Jurassic Lure Formation(J_1-2l)and the ore body is controlled by E-W fracture.The silicification,carbonatization,pyritization,arsenopyritization and sericitization are the main factors related to Au and Sb mineralization.Based on petrographic observations and mineral assemblage,ore formation of Mazhala Au-Sb deposits can be divided into sedimentary-metamorphism stage and hydrothermal sulfide stage,among which the metallogenic stage can be further divided into pyrite-arsenopyrite stage,quartz-stibnite stage and carbonate stage.2.Based on different textural pattern of pyrites revealed by microscope and secondary electronic image,they are dicided into four types:framboidal pyrite?Pyrite0?,core-margin structures Pyrite?Pyrite 1,core-Pyrite 1a and margin-Pyrite 1b?,girdle structures Pyrite?Pyrite 2?,and fine-grained idiomorphic pyrite?Pyrite 3?.According to petrography and trace element characteristics,Py0 is a typical sedimentary pyrite.Pyrite in hydrothermal sulfide stage?py1-3?is a sedimentary pyrite formed by hydrothermal transformation.3.The pyrite 0 in wall rocks contains highest Au,Sb contents and other trace elements,which indicates that the Au,Sb sourced from wall rocks.The study of the He-Ar isotope of sulfide indicates that there is a small amount of mantle-derived material in the ore-forming materials,which may be related to the intermediate-basic volcanic rocks which are widely distributed in the region.Pyrite 1a contains high W,Sn,Be,Ti,Zn and other high-temperature elements,which are related to the regional leucogranite magmatic fluid.4.Pyrite 1a?³⁴S from 0.34‰to 3.19‰,close to the values of magmatic fluid,indicating that the early ore-forming fluid has a dominant magmatic fluid.Results show that the?³⁴S values of pyrite in hydrothermal sulfide Stage?pyrite 1-3?vary over a narrower interval and are mainly between 0 and 7‰,and it is observed that the?³⁴S values first increases and then decreases,indicating that the fluid has a multi-source mixing feature.5.The metallogenic fluid characterized of Mazhala deposit by medium temperature?277??291??,low salinity?wt%Na CI equiv:1.05?5.01%?and enrichment in CO₂.Petrographic observation of FIA suggests that precipitation of ore-forming materials were mainly caused by fluid immiscibility.6.Through comprehensive discussion,it is concluded that the regional N-S extensional tectonic activity,the dome tectonic uplift and the leucogranite magmatic activity are the main controlling factors leading to the mineralization of Mazhala deposit.In the early stage,magmatic fluid mainly migrated on a large scale and mixed with metamorphic fluid to form ore-forming fluid.Comigration of ore-forming materials from activated regional strata.Under the influence of tectonic pressure reduction,the fluid is obviously immiscibility,which leads to the phase separation of the fluid,which is the main mechanism of metal precipitation in Mazhala deposit.At the same time,the shallow meteoric water moves along the structure and interacts with the rapidly rising ore-forming fluid,which may change the chemical composition of the ore-forming fluid,reduce the temperature to some extent and accelerate the precipitation of ore-forming material.