Metallogenic Mechanism And Model Of The Paleogene Fe-Cu-Pb-Zn Skarn Deposits In Nyainqentanglha,Tibet

This contribution investigates the Yaguila skarn Pb-Zn-Ag-Mo,Mengya'a skarn Pb-Zn-Ag-(Mo),Jiaduobule skarn Fe-Cu and Qiagong skarn Fe-(Pb)deposits from the Nyainqentanglha metallogenic belt in the Tibet.Based on the field work and detailed lab research,this paper summarized the detailed geology and metallogenic conditions of the Nyainqentanglha metallogenic belt.On the base of zircon U-Pb geochronology,this paper build the evolution of magma in the research area,including the early Cretaceous(110-125 Ma),the late Cretaceous(80-90 Ma),the Palaeocene(50-65 Ma)and the Miocene(13 to 20 Ma),and the Paleocene(50-65 Ma)is the major period.Combine with the mica Ar-Ar geochronology,molybdenite and pyrrhotite Re-Os geochronology,this paper constraint the major metallogenic time of skarn Fe-Cu-Pb-Zn deposits in the area is 50-65 Ma.Therefore,the skarn Fe-Cu-Pb-Zn mineralization mainly formed during the early stage of continental collision,and is controlled by the roll-back of the Neo-tethys plate after steep subduction.Combined with the intrusion geochemical and Sr-Nd-Pb-Hf isotopic characteristics,the Pb-Zn skarn mineralization in area mainly comes from the upper-middle crust,whereas the magma sources of Fe and Cu skarn mineralization are mainly from the lower crust,with a small amount of mantle source material.Compared the magmatic evolution between Fe-Cu and Pb-Zn deposits,ore-forming rock with higher differentiation characteristics in Pb-Zn mineralization.The detailed mineralogical studies reveal that the Jiaduobule Fe-Cu skarn deposit displays early proximal high temperature characteristics and overlap by the late Cu minerals with low temperature.The euhedral garnet was contained in the sulphide(mainly are bornite)in the garnet skarn.Solid solution of bornite and chalcopyrite is close to 1:1 and contains a large amount of Bi content(0.41 wt.%).Moreover,the replacement of bornite formed later independent Bi minerals(wittichenite)and chalcopyrite with lower Bi content.Tin contents in garnets decrease from early Gr70-50(15-364 ppm)though Fe skarn Gr50-30(20-202 ppm)to late Cu skarn Gr10-0(0.14-34 ppm),suggesting that Sn probably separated into Sn-rich Fe-fluid and Sn-depleted Cu-fluid during deposit evolution.This supported by the different Sn contents in magnetite between magmatic magnetite(2.7-7.3 ppm),magnetite in massive Fe-ore(13-71 ppm),and magnetite associated with Cu-sulfides(3.3-8.3 ppm).This suggests that the Sn contents in Jiaduobule garnets demonstrate the usefulness of trace elements in common minerals such as magnetite and garnet as an indicator for separation of iron and copper from magmatic-hydrothermal fluid.Based on ore deposit geology,and combined with the mineralogy,S-Pb-H-O isotopes,fluid inclusions,we established the genetic models of Fe-Cu-Pb-Zn skarn deposits in the the Nyainqentanglha metallogenic belt.The Pb-Zn-Ag mineralization and Mo mineralization in the Yaguila deposit belong to the same ore-forming process,which is related to magma formed by the partial melting of upper crust during the Paleocene collision setting.The ore-forming rock is quartz porphyry,the ore-forming fluid is mainly from magmatic hydrothermal,and the fluid boiling is the main mechanism for the Pb-Zn-Ag mineralization and Mo mineralization.Mengya'a Pb-Zn-Ag skarn deposit formed in 53 Ma ago.At present,the ore-forming intrusion has not been found in the mining area,which maybe in the depth or periphery.Hydrothermal is controlled by the common development fractures of the mining area,and the mixing with meteoric water later probably is the main mechanism for the Pb-Zn-Ag precipitation and mineralization.The Jiaduobule skarn Fe-Cu mineralization is associated with biotite monzogranite.The Fe-Cu mineralization displays zonation from south to north.The temperature cooling is the key factor of Fe-mineralization in the southern of deposit.The early Cu mineralization has the characteristics of high temperature(> 400 ?),and the later Cu mineralization formed by the mixing between magmatic-hydrothermal and meteoric water.The Qiagong Fe skarn and vein type Pb controlled by fractures belong the same products of same magmatism.The pressure decrease of ore-forming fluid in the early stage lead to fluid immiscible and precipitation of magnetite.The fluid boiling in the later stage controlled by fracture is the main factor of the distal Pb mineralization.The Eu-anomalies in different generations of garnets from Jiaduobule and Qiagong reflect the changes of fluid salinities.