Mineralogical Characteristics And Significance Of Garnet From Jiama Porphyry-Skarn Copper Polymetallic Deposit,Tibet

The Jiama porphyry-skarn Cu-polymetallic deposit is located in Maizhokunggar County,approximately 68 km east of Lhasa,the capital city of Tibet,Tibet.The Jiama deposit is composed of four types of mineralization,including the deep porphyry Cu-(Mo)orebody,middle skarn Cu polymetallic orebody,upper Cu-Mo orebody in hornfels and independent Au orebody in peripheral structural zone.The skarn orebody occurs between limestone,marbles of the Duodigou Formation and hornfels and sandstone of the Linbuzong Formation.The underlying Duodigou Formation provides most Ca contents for garnet,while medium-acid magma and Linbuzong Formation provide the main Si,Fe and Al.As contents for the main mineral of skarn belt in Jiama deposit.Garnet is a useful proxy for investigating the evolution and source of the magmatic-hydrothermal fluid and ore-forming mechanism.Garnets in Jiama occur as different colors,ranging from light green,yellow green,brown,red and black.The garnet geochemical compositions determined by electron microprobe showed that it contains 35.61% SiO2,2.27% Al2O3,33.00% CaO and 25.31% Fe2O3 on average,respectively.All components belong to Andradite-Grossular series,while Andradite occupy most of the proportion.Horizontally,the central part of mining area are mainly Andradite series,and the series increase whille moving towards to the direction of Niumatang and Tongshan on the edge of mining area.Most of the Jiama garnet has typically zonal structre,and the fluctuation of Al2O3 and Fe2O3 in those zones reflects that the magma has different sources and the dissolution of hydrothermal fluid is probably discontinuous.Hence the garnet formed by hydrothermal fluid and limestone under contact metasomatic reaction has the zone where Al2O3 and Fe2O3 alternates.Meanwhile,single garnet zone records the overall increasing of Fe2O3 from core to rim,which shows the increase of oxidation and pH of fluid corresponding to early to the late ore-forming stage of Jiama.The fluids came from different sources and was mixed with rain on the later stage.The rare earth element distribution patterns of garnet are listric-shaped REE,while the proportion of the heavy rare earth element and the light rare earth are almost the same,which all have positive Eu anomalies.The trace elements of garnet are rich in Pb,U,Ti and deplete in Th,Nb.According to the field work,the formation of garnet was earlier than sulfide.The volume of the contact zone of skarn and wall rock would decrease because garnet has higher density than carbonate rocks,which promoting the migration of ore-forming fluids and providing a large amount of ore-bearing space for later sulfide deposit.Hence,the skarn orebody is rich in metal sulfide,making skarn mineralization becoming the dominant mineralization type of Jiama copper deposit.According to the in-situ microanalysis of garnet in Jiama,we found that the spatial variation of principal trace element in garnet could effectively establish the temporal and spatial structure of Jiama magmatic rock-skarnmineralization,and further indicate the ore-forming mechanism of Jiama deposit,which would enrich the ore-forming theory of Jiama deposit.