Geological Characteristics And Mineralization Of The Shanghu REE Deposit In Korla, Xinjiang

There are many energy resources,including coal,oil and natural gas and metal resources,such as copper,gold,lead and zinc and iron in Xinjiang province,China.In recent years,various types of REE deposits have been found in Xinjiang province,such as the Bachu and Qieganbulake REE deposits associated with the alkaline-carbonate complexes and the Boziguoer REE-Nb-Zr deposit associated with alkaline granite,but the REE deposits associated with pegmatite are rarely reported.According to the previous works and recent field investigations,a REE deposit associated with pegmatite was found in the Shanghu area,Korla.The Shanghu REE deposit,with REE reserves about 7871 tons,is located in the western segment of the Kuluketage block,which is a northern part of the Tarim plate and has been underwent multiple magmatism and deformation events.About 21 REE-rich pegmatite dykes,with most of them being northwest–southeast-trending and few north–south-trending,are distributed in the Shanghu area(about 2 km2)and intrude into the migmatite.The ore minerals in the pegmatite from the Shanghu REE deposit include allanite(the main ore mineral phase and generally disseminated in the pegmatite),zircon and ilmenite.The pegmatite dykes are generally 80 to 1000 meters long and 0.2 to 20 meters wide,and the longest one is 1149 meters but the shortest one is only 20 meters.The migmatite in the Shanghu area was dated at 1930 Ma(zircon U-Pb)and has negative and varying ?(Hf)t values(-15-3.1).Zircons from the migmatite are of metamorphic origin and some of them contain inherited(detrital)magmatic zircons.Geochemical features,e.g.,negligible heavy REE fractionation,and textures e.g.,isometric with internal growth structures consisting of rounded shells,of the zircons from the Shanghu migmatite indicate that they are syn-crystallization with garnet and their ages can reflect the time of peak metamorphic conditions.Based on these data,we propose that the Shanghu migmatite formed by melting of crust under high pressure(granulite condition)and was genetically linked to the continental collision during Columbia supercontinent assembly.The zircons(U-Pb age: 1810 Ma)from the Shanghu pegmatite are typical magmatic zircon due to they have well-developed oscillatory zoning and high Th/U ratios,which imply that the pegmatite is of magmatic origin.However,some zircons in the pegmatite have been affected by late stage hydrothermal alteration,implying at the late stage of magmatic evolution,fluid saturation and exsolution occurred.Thezircons from the pegmatite share similar ?(Hf)t values(-9.7--5.7)with these of the migmatite,indicating the pegmatite have similar source with the migmatite or formed by partial melting of these migmatite.Consequently,we propose that the pegmatite was formed by partial melting of crustal source in a post-collision setting.Based on the systematic petrological,geochemical and zircon U-Pb geochronological studies,we constraint the metallogenic model of the Shanghu REE deposit: At ~1930 Ma,continental collision induced the involvement of some sediments into the deep crust(granulite condition)and formed the Shanghu migmatite.At ~1810 Ma,decompression melting of the migmatite during exhumation of subducted continental crust generated a fluid-rich magma,which is the pristine magma of the Shanghu pegmatite.Accompanying with magmatic evolution and fractional crystallization,fluid is gradually aggregated in the residual melts.Therefore,at the late stage of the magamic evolution,a magmatic-hydrothermal system was formed and the REE mineralization was constrained by the fluid derived from this stage due to the hydrothermal origin of main ore minerals,e.g.,allanite.Besides,there are many diabase dykes are distributed in the region.Based on our geochemical studies,we propose that the Shanghu diabase may have formed in a continental margin extensional setting.Consequently,the Shanghu REE deposit in Koerle is associated with pegmatite,which formed at ~1810 Ma.The Paleoproterozoic tectonothermal event in the Tarim plate may have played an important role in the formation of the REE deposit.