Metallogenic Response of Ailaoshan Gold Belt in Indosinian Orogeny

Tibet Plateau, the Ailaoshan gold belt is one of the economically most significant gold production bases in China. Since 1980s, Mojiang gold deposit (large scale), Zhenyuan (super large scale and was called as “Laowangzhai gold deposit” in the early literatures) gold deposit, Daping gold deposit (super large scale) and Chang’an gold deposit (large scale), and a series of smallto medium-sized gold deposits and mineralization points have been discovered. Among them, Zhenyuan gold ore field, located in the northern section of the Ailaoshan gold belt, is the most representative and the largest one. For a long time,the metallogenic age of Ailaoshan gold belt is controversial , most scholars thinked that is the products of the Himalayan period. However, the latest research results show that Re-Os isochron age is 229± 38Ma,which from metamorphic quartz graywack type gold ore of Donggualin ore body of Zhenyuan gold field. Moreover, the Re-Os isochron age of carbonate rock type gold ore from Laowangzhai gold body is 268±34Ma(Shi guiyong et al, unpublished data). They may get support evidences from previous Pb isotopes studies. 51samples (27 ore sample) from the different sections in Zhenyuan gold ore field were analyzed for Pb isotope By Hu et al (1995), and model ages of 200 ~ 270Ma were obtained from the pyrite. Accordingly, a point of view for the Ailaoshan gold belt taking shape in the Late HercynianIndosinain was proposed [1]. In addition, Pb isotopic model age of gold ores from the nearby Mojiang gold deposit is 114 to 304Ma, with a peak value of 250Ma [39]. Chen et al (2002) also thought that there existed a close genetic relationship between Mojiang gold deposit and the ultrabasic rocks from ophiolite melange, and the magmatism and gold mineralization in the Mojiang area last a longer time. Field geological survey show that V orebody of Chang'an gold deposit was cut through by granitic potash feldspar porphyry, whic isotope age is 30~35Ma. Therefore, it is obviously not appropriate for V orebody's metallogenic age was determine used by zircon's U-Pb age. In addition, the geological phenomenon has not caused the attention of geologists that some breccia type gold ore was hosted in siltstone type ore and their reserves also occupies certain proportion in V orebody. It is worth noting that some diabase and gabbro,while scattered in Chang'an golddeposit, but a part of them itself is gold ore. Moreover, Baimazhai nickel-copper sulfide magmatic deposit's Re-Os dating is 259±18Ma nearby. In addition, the same evidence also exist in Daping gold deposit,which the lamprophyre of Himalayan period cut through dold orebody. The tectono-magmatic evolution historyof Ailaoshan orogenic belts show that it is an intracontinental orogenic belt, which developed based on rift environment of passive continental margin.Ailaoshan collision orogenic process has experienced the three stages of Indosinian epoch, Yanshan epoch and Himalayan epoch. Among of them, indosinian orogeny is the strongest, and occured superposition of a wide range regional metamorphism. During the orogenesis, three deep faults (Red River, Ailaoshan and Jiujia-Mojiang) formed. In particular, the formation of Jiujia-Mojiang deep fault derived a series of nearly parallel secondary shear faults in its two sides. Formation of the different scale brittle-ductile shear zones played an important role in the generation, migration and precipitation of the ore-forming fluid. During the intensive tectonic-magmatic activity, the gold was enriched and mineralized, which should be one of the most intensive gold mineralization in this area. Syn-collisional granites widely distributed in Ailaoshan orogenic belt. Zircon's UPb isotopic age from 2 granites (sampling location is Daliangzi) and a quartz porphyry (sampling location is Donggualin) were obtained in Zhenyuan gold field, they are 248±4.5Ma、252.4±2.2Ma and 245.8±2.8Ma (unpublished data). The isotope age from other magmatic rock in Ailaoshan orogenic belt are shown in table 1. As compared with Indosinian orogeny, heat flux SHI Guiyong, SUN Xiaoming, PAN Weijian, LI Xin and LI Xiaojie, 2014. Metallogenic Response of Ailaoshan Gold Belt in Indosinian