Magmatism And Mineralization Of Compound Massif In Xiuwacu Tungsten-Molybdenum Deposit,northwest Yunnan Province

The Southwestern Sanjiang Tethys is one of the most complex orogenic belts in the world and the most important nonferrous-noble metal metallogenic zones in China.However,it is still lack of theoretical evidence to interpret the composite metallization of large and super large deposits in this area.This situation is more serious in Shangri-La arc which is located in the south of Yidun Arc.At present,two porphyry metallogenetic systems of Indosinian and Yanshanian have been found and assessed with numbers of large and super large porphyry Cu and Mo deposits distributing inside both of them.But the research on the composite metallization is still very poor in the Shangri-La Arc.Not only compound massif which consists of two intrusions formed in Indosinian and Yanshanian,but also W-Mo ore bodies are distributed in the Xiuwacu deposit.So this deposit offers an important window to research the composite metallization and the genesis relationship between those two magmatic activities in the Shangri-La arc.We obtained the following results by studying the petrological,geochemical,isotope geochemical characteristics of the compound massif,fluid inclusions,stable isotope of metal sulfide,the change and conservation of Xiuwacu deposit.(1)We can distinguish the temporal and spatial distribution of the Xiuwacu compound massif and the contact relationship with each other by the LA-ICP-MS zircons U-Pb dating results.That is,the Indosinian intrusion which was formed between 200?215 Ma is located on the east side of F4 while the Yanshanian intrusion which was formed between 75?88 Ma is located on the west side of F4.In a special zircon of the sample XWWR-1,the age of the core is 219 Ma while the edge is 79 Ma.So there should be an inherited relationship between those two magmatic activities.The network aplite is distributed inside the intrusion of Late Triassic with the age of 78.7 Ma,and the ore forming age is 80?88 Ma by the molybdenite Re-Os dating technology.Both of them are similar to the forming time of Yanshanian intrusion,which may indicate that the magma-hydrothermal fluid of Late Yanshanian migrated along the fault-crack system in the Xiuwacu deposit,then the aplite veinlets and the W-Mo ore bodies were formed at the favorable position.(2)Two intrusions have the similar geochemical characteristics,belonging to the shoshonitic series with metaluminous to slight peraluminous,depleted in HFSE of Nb,Ta,Sr,P,Ti and enrichment of LREE and LILE of U,Rb,Th,K.However,the biotite granite has less intense Eu negative anomaly,lower saturated temperature,higher oxygen fugacity,positive ?Hf(t)value,younger two-stage model age,which indicates that it comes from the partial melting of the mantle material(about 84-92%)and oceanic crust(8-16%).While the porphyry monzogranite has negative ?Hf(t)value and it comes from partial melting of ancient thickened lower crust and it contains about 10?30% mantle material and 70?90% lower crust material.(3)The fluid inclusions in scheelite show that they are medium-low temperature(153?293 ?),medium-low salinity(1.4?11.7%)H2O-Na Cl solution.A positive correlation between temperature and salinity indicates that there may be a mixture of fluids from different sources.It shows that ore-forming material is mainly derived from the granitic magma by studying the isotopic composition of C-H-O in fluid inclusions,the sulfur isotope of metal sulfides and the content of Re in molybdenite.(4)The central ages of zircon fission track are distributed between 62.8±1.3 Ma and 47±2Ma,while the central ages of apatite fission track are distributed between 14.9±0.6Ma and 22.0±1.0Ma and an inversion simulation result of apatite fission track was obtained accordingly.Combining the 40Ar/39 Ar age of biotite and its sealing temperature,the zircon U-Pb age and its sealing temperature,we obtained a heat history evolution model diagram of the Xiuwacu deposit.The calculation results show that this deposit has an uplifting amount of 6426?6915m(average 6650m),a denuding amount of 1982?2471m(average 2200m),with an average denuding rate of 0.129 mm/a.Based on these,we judge that there is still great potential to prospect W-Mo ore bodies underground in the Xiuwacu deposit.