The Metallogeny Of The Jiama Copper Polymetallic Deposit In Tibet

The Jiama Cu polymetallic deposit in the Mozhugongka County of Tibet is located in the central part of the Gangdese metallogenic belt. There are three types of orebodies:skarn-type, hornfels-type and porphyry-type. Metal resources of Cu, Mo, Au, Ag and Pb+Zn are large scales that Cu is >6,000,000t, Mo is >600,000t, Pb+Zn is >500,000t, associated Au is >140t and associated Ag is >8000t. Jiama has been a Cu polymetallic deposit at the super-large scale with complicated ore body types, multiple metals, high grades and huge economic profits in China and even in the world. This study has discussed on the metallogeny of the deposit based on the basic geological mapping, core logging, thin section description and microanalyses and also combined with analyses on major, trace and rare earth elements, S-H-O isotopes, Re-Os dating of molybdenite and U-Pb dating of zircon. The metallogenic model has been established so as to provide a theoretical guide for regional exploration.The skarn-type orebody is stratiform-like occurring in the interlaminar structural belt between the marble (limestone) of Duodigou Formation in the Upper Jurassic(J3d) and the hornfels and slate of Linbuzong Formation in the Lower Cretaceous(K1l) This thick orebody contains disseminated, mssive and veinlet mineralizations of Cu, Mo, Pb, Zn, Au and Ag. The hornfels-type orebody is at the top of the skarn-type and porphyry-type orebodies with large thickness as tubular shape. It contains the typical veinlet-disseminated Cu and Mo (Au, Ag) mineralizations. The porphyry-type orebody as tubular shape contains the Mo (Cu) mineralization in the granite porphyry, monzogranite porphyry and granodiorite porphyry. The study on ore mineralogy shows that the major ore minerals are chalcopyrite, bornite, molybdenite, sphalerite, galena, tratehedrite and chalcocite, and Au-Ag minerals include native Au, native Ag, electrum and hessite. Alterations include the skarnization and typical alteration combinations in the porphyry Cu system that are potassium silicification (biotite-quartz-anhydrite), beresitization (pyrite-sericite), propylitization (epidote-chlorite-carbonate) and argillation.Re-Os isotopic dating of molybdenite indicated that the model ages ranged from 19.19±0.3Ma to 14.2±0.2Ma. The isochron age of molybdenite from porphyry is 14.66±0.27Ma, from hornfels is 14.67±0.37Ma, and from skarn is 15.32±0.2Ma. The isochron age of all 65 samples from these three types of orebodies is 15.36±0.16Ma and the average age is 15.01±0.16Ma in the Miocene. The ages of intrusions ranged from 17.0±0.5Ma to 13.9±0.1Ma with three magmatic events as least. Pre-ore magmatic event is earlier than~20Ma, syn-ore magmatic event is between 17.0±0.5Ma to 14.81±0.16Ma and the last stage of magmatism is 14.2±0.2Ma~13.9±0.1Ma. Mafic intrusions include gabbro and lamprophyre intruded at pre-ore, intermediate-felsic magmatic rocks were formed during syn-ore as granite (monzogranite) porphyry and granodiorite porphyry and few Au-bearing diorite and intermediate-felsic dykes occurred at the end of magmatism. During the hydrothermal mineralization period, there are four stages due to the different metals:Mo stage, Cu stage, Pb-Zn stage and Au-Ag stage.Metallogenic material was mainly derived from magma. Cu is closely related to granodiorite porphyry, whereas Mo is closely related to granite and monzogranite porphyry. The zonation of metals is controlled by the porphyry intrusion. The metal zonation is from Mo to Cu(-Au-Ag) vertically lower to upper, and from Mo(-Cu) to Cu(-Mo-Au-Ag) to Cu-Pb-Zn(-Mo-Au-Ag) laterally from the porphyry center to outer contact zone. The δ34S values of sulfides and porphyry are close to zero and similar to magma source. The characteristics of lead isotope of sulfides and porphyry are also similar. The study of fluid inclusions and hydrogen-oxygen isotopes indicated the source of fluid was magma.The skarn-type orebody was controlled by the interlaminar structural belt. The carbonate rocks of Duodigou Formation in the Upper Jurassic had provided the favorable country rock for skarn formation. The hornfels caused by heating derived from sandstone and shale of Linbuzong Formation in the Lower Cretaceous had played a role of a natural barrier. The intrusions of granite and granodiorite porphyries provided the huge heat and fluids for the hydrothermal process which was favorable for fluid convection and provided space for metal precipitation in the cracks on the top.Above all, the Jiama Cu polymetallic deposit is a typical skarn-porphyry type ore deposit rather than a marine exhalative sedimentary deposit. The skarn-type Cu, Mo, Pb, Zn, Au and Ag orebody, hornfels-type Cu and Mo (Au, Ag) orebody and porphyry-type Mo(Cu) orebody have consisted the model with three types of orebodies in one deposit. The skarn-type deposit can be super-large scale in China as we see in Jiama. The metallogeny study on Jiama has replenished the theory of skarn ore deposits and also has been the theory base for further exploration.