The Ore-forming Fluid And Metallogenesis Of Nanyangtian Tungsten Deposit In Malipo, Yunnan Province, China

Laojunshan is an important Sn-W-related metallogenic province in Southeastern Yunnan where more than 26 Sn (W) deposits have been explored, locating at the ore concentration areas named Nanyangtian-Chayeshan, Dazhushan-Xinzhai, Manjiazhai-Tongjie, respectively. The main genetic type of ore deposit is skarn type. Based on the pervious researchers and detail field investigation, we selected the Nanyangtian deposit to conduct the detail study. Focusing on the geological characteristics, fluid inclusions, stable isotope, chronology and synthetically comparative study etc., in order to clarify the source and evolution of ore fluids, deposit genesis and geodynamic settings, and compare Nanyangtian with Xinzhai and Nanyangtian with Dulong, respectively. The main advance achievements from this study are as follo wings:1.The Laojunshan area occurred in Southeastern Yunnan-Western Guangxi tin-tungsten polymetallic ore belt. Its geotectonic position locates at Peri-Pacific tectonic domain, and the northern margin of North Vietnam in South China fold system. The host rocks are Cambrian metamorphic rocks, comprising of schist, two-mica monzonitic gneiss, plagioclase gneiss, leptynite. The major types of magmatic rocks are the felsic-basic volcanic rocks and felsic intrusive rocks. The main regional structure are the NW-trending, NS-trending and EW-trending faults.2. In this paper we report zircon U-Pb ages, geochemical and petrological characteristics for the Laojunshan granites located in the western part of Nanyangtian tungsten deposit. Three samples from three phases of the granitic rocks in Laojunshan intrusion have been analyzed by the LA-ICPMS zircon U-Pb techniques, yielding ages of 87.2±0.6Ma, 86.8±0.4Ma and 85.9±0.4Ma, respectively. Bulk analysis reveals that three phases are strongly-peraluminous, silicon-rich and alkali-rich granites and their ACNK values fall mainly into a small range of 1.10-1.38. Moreover, Harker diagrams show that these granitoids experienced strong fractional crystallization during magmatic evolution. All granites show enriched Rb, La and Zr and depleted Ba, Sr and Ti, as well as a uniformly flat REE-pattern with a marked negative Eu anomaly. The granites were possibly derived from partial melting of continental crust.3. The Nanyangtian tungsten deposit occurs in metamorphic rocks of the Lower Cambrian Chongzhuang Formation. Ore bodies, with the simple shape, are largely concordant with the bedding of the strata and the distribution of the ore bodies is spatially related to skarn. The main ore type is skarn-scheelite. The formation of tungsten deposit experienced a period composed of skarn stage, retrogressive alteration satge, quartz-sulfide stage and calcite-sulfide stage. Tungsten mineralization is probably in the retrogressive alteration satge.4. There are three dominant types of fluid inclusions, as evidenced by petrography characteristic of fluid inclusions of garnet, epidote, quartz, fluorite and calcite formed at different ore-forming stages in the Nanyangtian tungsten deposit; they are liquid-rich inclusions, gas-rich two-phase inclusions and daughter-minerals bearing polyphase inclusions. Systemic measurements of homogeneous and freezing temperatures, salinities and laser Raman spectroscopic analysis of fluid inclusions were conducted. The gas composition of ore-forming fluids in the Nanyangtian tungsten deposit mostly are H2O, CH4 and N2, with a small amount of CO2, whereas liquid composition primarily is H2O. Fluid inclusions in early stage show a range of homogenization temperatures and salinity from 200℃to 340℃and from 6.45% to 53.26 wt% NaCleqv respectively, with the densities ranging from 0.83 to 1.15g/cm3. Homogenization temperatures and salinity of fluid inclusions in late stage mainly vary from 160℃to 300℃and from 4% to 8 wt% NaCleqv respectively, and the density of the fluids is 0.67-0.96g/cm3, which are obviously lower than the fluid inclusions in early stage.5. Theδ18OSMOW values of garnet, quartz and calcite range from 3.78‰to 16.23‰, with the correspondingδ18OH2O values between 4.52‰and 10.28‰, and 8D values of fluid inclusions between-83‰and-59‰. The combined isotopic data imply that the ore-forming fluids in the Nanyangtian tungsten deposit were mainly derived from magmatic fluids, with a minor part from the metamorphic water. Theδ34S values of pyrite from the Nanyangtian tungsten deposit define a narrow range of 4.2%o-8.1‰, reflecting the characteristics of the sulfur from mantle source, with the affect of crustal contamination.6. We present Re-Os dating of three molybdenite samples separated from the Nanyangtian tungsten deposit and new 40Ar-39Ar phlogopite closely coexisting with cassiterite ages for the Xinzhai tin deposit to constrain timing and duration of mineralization. The Re-Os model ages were obtained ranging from 209.1±3.3Ma to 214.1±4.3Ma; Phlogopite from the Xinzhai tin deposit yielded a 40Ar-39Ar pleatu age of 209.5±1.1Ma and isochronal age of 209.0±2.2Ma. The ages above provided the mineralization information during the Indosinian in southeastern Yunnan.7. The mineralization in Nanyangtian tungsten deposit may be related to hydrothermal superposition. Laojunshan granite is the later formed part of the rock, there may be the earlier undetected "body in body". The Sn and W mineralization were formed in late post-orogenic or post-collisional stage during the Indosinian in Southern China and controlled by a geodynamic setting of the extension-cracking.8. There are some similarities between the Nanyangtian and Xinzhai in the following aspects:the formation in mining area, ore body characteristics, ore-forming fluid and metallogenic epoch. (1) The main exposed strata are the Cambrian metamorphic rocks; (2) The tungsten-tin ore bodies are basically concordant with the bedding of the strata, with the same shape of the rock; (3) Fluid inclusions generally have widely varying homogenization temperatures and low salinity; (4) The mineralization of the Nanyangtian and Xinzhai both occurred in the Late Triassic during the Indosinian; There are differences between Nanyangtian and Dulong in the following areas:the strata, ore-controlling rock, ore-controlling structure, mineralization age.9. The main ore-controlling structures in region are NW-trending faults, NS-EW-trending cross-type structure, Laojunshan fold, magmatic intrusive contact structure and interlayer detachment structure.