Geological Characteristics And Genesis Of The Cuonadong Beryllium Polymetallic Deposit In Zhaxikang Ore-concentration District,Tibet

The Cuonadong beryllium polymetallic deposit,located in the southern part of Zhaxikang ore-concentration district in southern Tibet,controlled by the Cuonadong gneiss dome and its geographical position closes to the Southern Tibet Detachment Systen,is a new type of mineral and mineralization discovered recently in Tethys Himalayan metallogenic belt.The discovery of the deposit reveals that the Himalayan metallogenic belt has great potential for rare metal mineralization and prospecting.it opens a new window for the exploration of rare metal in the Himalayan metallogenic belt and it is a typical example for the study of Cenozoic leucogranite and rare metal mineralization in Himalayan.The Cuonadong beryllium polymetallic deposit is located in the Cuonadong gneiss dome which has obvious three structural layers by two lower detachment faults around the dome geometry from inner to outer,it is divided into lower unit(core),middle unit(mantle)and upper unit(margin).The core rock assemblages primarily composed of granite gneiss,leucogranite,and a little migmatite,with a large amount of pegmatite veins intruding into;leucogranite and a large amount of pegmatite veins penetrating the lower unit.The mantle rock assemblage is a set of strongly metamorphic and deformable schist with interlayer carbonate.The margin is primarily comprised of slightly sedimentary rocks of metamorphic Triassic Nieru formation and Jurassic Ridang formation,which is constituted of sericite chlorite sand Slate and a small amount of phyllite.Among them,the carbonate rock in the mantle rock is the main ore-bearing rock layer in the mining area.The ore bodies of the Cuonadong beryllium polymetallic deposit are located in the skarn of the contact zone between Cenozoic leucogranite(or pegmatite)and carbonate rock and are distributed in a ring shape around the dome.The ore-forming elements are mainly beryllium,symbiotic tin and tungsten.Cuonadong mining area is dividided into 4 sections,namely Zabulu,Yunaza,Rina and Qugaerbei ore section.A total of 15 beryllium polymetallic ore-bodies are delineated in the mining area with a length of 175~ 4200 m,a thickness variation range of 0.8~ 48.23 m and a dip angle of 10~ 25°.The shape of orebody is mainly layered,banded and lenticular.Ore minerals are mainly beryllite,bertrandite,cassiterite and scheelite,etc.The macroscopic geological phenomena in the field and the comprehensive study in the laboratory show that the skarns in the Cuonadong deposit are typical calcareous skarns with obvious zonation.the skarns are garnet skarns(red)? garnet,diopside,magnetite(specularite)skarns(dark red,dark green)? diopside,hornblende,specularite skarns(dark green,dark black)? idocrase,epidote skarns(dark green)? white striate.Among them,the skarn belt with specularite is the most closely related to beryllium,tin and tungsten mineralization.At present,two stages of mineralization have been discovered in the Cuonadong mining area,the first stage of mineralization is weakly oriented two-mica granite with a crystallization age of 18.25± 0.16 Ma.The second stage of mineralization is garnetmuscovite granite with a crystallization age of 15.89± 0.35 Ma.Combined with the regional tectonic activity background,it is shown that the formation of the Cuonadong deposit is closely related to the N-S extension in the early stage of the region and the E-W extension in the later stage.During Miocene detachment of early STDS,massive weakly oriented two-mica granite were subjected to large-scale bottom splitting in the Cuonadong area,and the magma is intruded into the mantle rock in a layered manner and then came into contact with the carbonate rocks for metasomatism,thus forming the W-Sn-Be skarn-type polymetallic ore body.A series of N-S or nearly N-S fault were formed in the E-W extension stage of the late period.garnet-muscovite granite magma(~16 Ma)is intruded along these fault structures and formed Be-W-Sn skarn-type skarn-type polymetallic ore bodies by contact metasomatism with carbonate rocks.To sum up,the Cuonadong deposit is a typical magmatic hydrothermal type(skarn type)deposit and controlled by both N-S extension and E-W extension in Himalayan.