Study On Ore-forming Fluid Characteristics Of Tongkeng Tin-polymetallic Deposit In Dachang, Guangxi

Tongkeng deposit, located in the Northwest Guangxi, is a superlarge deposit in Dachang tin-polymetallic ore-field, it is composed of shallow cassiterite sulfide type tin-polymetallic ore bodies and skarn type zinc-copper ore bodies in the deep region of the deposit, in which are newly discovered. By comparative studying on the characteristics of fluid inclusions, fluid components, hydrogen and oxygen isotopic characteristics in the Tin-polymetallic ore bodies and Zinc-copper ore bodies, the characteristics of ore-forming fluids and their temporal and spatial variation of the two kinds of ore bodies are discussed, further deepening the understanding of the genesis of the Tongkeng deposit. The main results and conclusions are as follows:(1) According to the field geological observation and the study of mineralography, the mineralization of different types of ore bodies in Tongkeng deposit can be divided in detail:Tin-polymetallic ore is divided into three stages:the first stage is cassiterite-arsenopyrite-pyrite-quartz stage; the second stage is cassiterite-sphalerite-jamesonite-quartz stage; the third stage is calcite-quartz stage, in which I and II stages are the main metallogenic stages in the Tin-polymerallic; Zinc-copper ore is divided into three stages:the first stage is arsenopyrite-quartz stage; the second stage is pyrrhotite-pyrite-chalcopyrite-arsenopyrite-marmatite-quartz stage; the third stage is stibnite-calcite-fluorite-quartz stage, in which II stage is the main metallogenic stage in the Zinc-copper ore bodies(2) The results of fluid inclusions show that the types and characteristics of fluid inclusions in the Tin-polymetallic ore bodies and Zinc-copper ore bodies are basically similar, there are CO2-H2O type, NaCl-H2O type and pure CO2 type in the main metallogenic stage, however, it is found that there are high salinity containing NaCl sub mineral fluid inclusions in the Zinc-copper ore bodies. From the main metallogenic stage (I and II) to late stage (III), the ore-forming fluid characteristics of the Tin-polymetallic ore is performanced for the varation characteristics of temperature decreased (from 270～390 to 110～210), salinity decreased (from 3～8 to 1～4), density increased (from 0.555～0.855 to 0.855～0.955); From the main metallogenic stage (II) to late stage (III), the ore-forming fluid characteristics of the Zin-copper ore is performanced for the varation characteristics of temperature decreased (from 285～435 to 195～255) and salinity decreased (from 4～12 to 0～6). Overall, the characteristics of ore-forming fluids in the Tongkeng deposit from deep to shallow are performanced for the temperature is decreased and the salinity is decreased.(3) The results of fluid components and sulfur fugacity calculation show that the two types of ore bodies have the same basic fluid characteristics, mainly for CO2 and H2O and part of gas phase CH4 and a small amount of gas phase H2, from the Zinc-copper ore to Tin-polymetallic ore, the content of CO2 is from low to high and the content of CH4 is from high to low, it is indicated that from Zinc-copper ore to Tin-polymetallic ore, the characterisitcs of fluids are from reducion to oxidation; in the main mineralization stage, the fs2 of the Tin-polymetallic ore is from 10-23.32 to 10-17.32, the fs2 of the Zinc-copper ore is from 10-23.23 to 10-22.37, the two types of ore bodies with low sulfur fugacity degree.(4) Through the CO2-H2O iterative calculation program, the capture pressure of the Tin-polymetallic ore bodies is from 44.46 to 109.9 MPa, and the mineralization depth is from 1.68 to 4.15km. The capture pressure of the Zinc-copper ore bodies is from 67.6 to 114.7MPa, and the mineralization depth is from 2.55 to 4.33km.(5) Hydrogen and oxygen isotopic characteristics show that the characteristics of the ore-forming fluids of the Zinc-copper ore bodies is performanced for the evolution of magmatic hydrothermal to atmospheric water; the characteristics of the ore-forming fluids of the Tin-polymetallic ore bodies are mainly magmatic water, which may be mixed with deep source fluid.(6) Comprehensive study of fluid characteristics, composition, hydrogen and oxygen isotopic characteristics indicate that the shallow Tin-polymetallic ore bodies and the deep Zin-copper ore bodies may be productd of different stages in the same metallogenic system. The genesis of the deposit is magmatic hydrothermal fluid metasomatic-filling.