Metallogenic Model Of Rich Iron And Copper(Gold) Deposits In Western Part Of Bangong Co-Nujiang Metallogenic Belt, Tibet

Bangong Co-Nujiang suture in Tibet, is one of China’s19major metallogenic areas (or belts). But it also has the lowest grade of research. Multiple skarn-type and porphyry-type iron-copper metal deposits are distributed in the western part of the suture zone. And the typical deposits in the area are Caima medium skarn-type iron deposit, Gaerqiong medium skarn-type copper-gold deposit, Galale small skarn-type gold (copper) deposit and Duobuza super-large porphyry-type copper-gold deposit. On the basis collecting previous data systematically and field geological investigation, this thesis has studied the petrography and mineralography feature, geochemical characteristics and isotope geochemistry of the four typical deposits, established four mineralization model, further established a regional metallogenic model of the western section of the Bangong Co-Nujiang suture zone. Then we research the tectonic evolution in the western part of the suture zone, and provide a theoretical basis for further prospecting.(1) In Caima skarn-type iron ore district, there are at least three stages of magmatic activity. The first stage is between the early period of Early Jurassic and the early period of Middle Jurassic. The second stage is between the late period of Middle Jurassic and the early period of Late Jurassic. The third stage occurred in the early Cretaceous. And the second stage of magmatic activity is most affinitive to the mineralization. Ore-forming fluid salinity ranged from4%to4.5%,and the ore-forming temperature is divided into120～180℃,200～300℃,360～380℃,400～750℃in four phases.(2) In Gaerqiong skarn-type copper-gold ore district, the mineralization epochs conclude endogenesis(skarn-sulfide-gold)and surface(malachite, azurite, limonite)stages. The mineralization and diagenesis almost occured simultaneously, between85.9Ma and95.3Ma. The homogenization temperature of V-L inclusions is between in87～356℃,and it suggested that the deposit may have experienced two stages of mineralization. The salinity is mostly between in7%-7.5%,and it belong to mesothermal and medium-salinity ore fluid.(3) Galale skarn-type copper-gold deposit formed in Late Cretaceous, and there is no rock and mineral jet lag. There are two stages of magmatic activity in ore district. The first stage is the magmatic intrusion of the east side in the early period of Early Cretaceous, in which there was not ore-forming geological conditions, resulting in formation of non-ore-forming rock. The second stage is intrusion of ore rock in late Cretaceous in west side. In deeper place in Galale and Gaerqiong deposits, it maybe the same batholith. (4)In Duobuza ore concentrated area, the mineralization and diagenesis almost occured simultaneously, between115.54Ma and122.45Ma. A large number of tourmaline was found in the surrounding rock(the Jurassic)sampled in the drilling of Bolong ore district. Some exist in the late quartz veins, and some exist in the spots of spot slate in the form of fine particles. They belong to dravite-iron tourmaline series, and tourmaline iron reduction increase with depth until it turn to dravite. Therefore it is speculated that the mineralization in Bolong ore district may be subject to the impact of the boron-rich gaseous hydrothermal alteration, and the existence of the impact of of gaseous hydrothermal fluids in other ore districts is to be further studied.(5) The mineralization of four typical deposits occurred above the mature Proterozoic basement. And the mineralization of Caima, Gaerqiong and Galale occurred in subduction zone island arc setting. In Gaerqiong and Galale ore districts, mantle source material made greater contribution to the formation of ore-forming rock. The rock mass in Caima and Duobuza ore districts belong to crustal source type. In Duobuza ore district to the east of Suture zone, ocean basin have been closed in the late period of Early Cretaceous, and turn to the continent-continent collision orogenic process. On the basis of mineralization and metallogenic background of deposits, it can be concluded that the western part of Bangong Co Nujiang oceanic basin subduced to Qiangtang block in northern in Late Jurassic, subducted to Lhasa block in southern in the early period of Late Cretaceous, and closed later than the early period of Late Cretaceous closure.