The Metallogenesis Of The Chang'an Gold Deposit In Southern Ailaoshan Metallogenic Belt

The Chang'an deposit is one of the largest gold deposits in the Southern Ailaoshan metallogenic belt in Sanjiang region,and the occurrence of ore bodies is controlled by the brittle fractured zone of Ganhe fault.The distribution of ore bodies is controlled by the northwest F6 fault and its secondary fractures;The ore-bearing rocks is constituted predominantly by Lower Ordovician Xiangyang group,Upper Silurian Kanglang group and altered diabase in Himalayan period;the boundary between orebody and wall-rock is gradual,partial obvious.the Chang' an gold ore deposit is subdivided into oxide ores and sulfide ores at the changing depth at 1645m.Sulfides and gold as micro-disseminated tectonic in the ore,and the main wall rock alterations of the Chang'an goldfield are silication,sericitization,carbonicacidation and pyritization,etc.Which is characterized by low to moderate temperature alterations.The ore-forming fluid system is characterized by low to moderate temperature and salinity.The fluid compositions are dominated by H2O,CO2,Cl-,K+and Na+,and the CO2 content in main metallogenic stage varies between 21.162 mol%?32.832mol%.The ?13CCO2 values of the fluid system are-3.427‰?8.749‰,indicating the participation of marine carbonate and magmatism.The ?D value varies from-111‰ to-78.383‰,the 8180 values are 10.527‰?13.565‰ and the ?34S value is concentrated between +1‰ and +3‰,which suggesting the ore-forming fluid probably sourced from the magmatic-hydrothermal system,and the metamorphic water sourced from marine carbonate strata also played an important role in the main mineralization stage.The 208Pb/204Pb,207Pb/204Pb and 206Pb/204Pb values are 38.722?40.649,15.604?15.813,18.788?19.761 respectively.And the radioactive lead isotope(238U?232Th)are relatively enriched.These isotope data suggest that the ore-forming elements were mainly derived from strata and wall rock:The Devonian dolomite,fragmental rock,the Silurian dolomite and the Ordovician sandstone.In the late of Himalayan,Various types of igneous rocks intruded into the mining area,and the Ore-forming age(36?33Ma)is in coincidence with the igneous rocks or late.Integrated existing research the mineralization process analysis is as follows:In Cenozoic,the large scale upward alkali-rich magma caused the magmatic hydrothermal fluid to circulate and mix with the metamorphic water in the Jinping area of the Southern Ailaoshan belt.The mixed fluid extracted gold and other metallogenetic elements from the wall rock and other geological unites,then migrated them to the proper structural spaces,and rapidly precipitated under some favorable physical-chemical condition,finally formed the micro disseminated gold deposit.According to the comprehensive and contrastive study,the Chang'an gold deposit possesses the typical geological-geochemical characteristics of orogenic-type gold deposit and Carlin-type gold deposit,so should belong to the Carlin-like gold deposit as a transitional type between orogenic-type and Carlin-type gold deposit.