Metallization Of Ajialongwa Carlin-Like Gold Deposit In Ganzi-Litang Suture Zone, West Sichuan Province

In order to realize origins of ore-forming fluids and materials of Ajialongwa gold deposit, based on regional geology settings and characteristics of the deoposit, this thesis made research on: geochemistry of fluid inclusions; major and trace elements of host/ore rocks and pyrites; stable and Pb isotope compositions. In addition, comparing it with Qiuluo gold belt, which deposited in suture zone as well, this article summa- rized the metallogenic mechanism of Carlin-like gold deposits located in suture zones. There were only H2O-NaCl aqueous ore-forming fluid inclusions, which were low in homogenous temperature (Th=120～215℃) and salinity (0.18～5.26 wNaCl%); the gas phase composition of fluid inclusions gave priority to H2O (91.80%～97.63%), and CO2 (2.015%～7.297%) secondaryly, while Na+, K+ and SO42- as the main liquid phase composition; theδ18O values of ore-forming fluid ranged from -0.36‰to 1.91‰, and theδD from -124.243‰to -114.968‰, which variation ranges were narrow. It showed that ore-forming fluids were meteoric water, having taken circulation at shallow depth. Except pyrites'ΣREEs were lower, chondrite normalized rare earth element patterns for host/ore rocks and pyrites of two types (pyrites of mineralization period and diagenetic pyrites) were consistent, which were right deviation slightly and had weakly negative anomaly for Eu; there were consistant trace element composition,δ34S and Pb isotope composition for pyrites of the two types, that theδ34S values ranged from -13.249‰to -8.091‰and the composi- tion of Pb isotope was near to the growth curve of upper crust in Zartman graphic. It proved that ore materials were sourced from host rocks, while the sulfur derived from diagenetic sulfides which formed in an open volcanic-sedimentary system and the reduction of organic compounds. That theδ14C values were -4.693‰～8.644‰and the existence of olefin gas for ore fluid inclusions, indicated the carbon derived mainly from marine carbonate dissolution and partly organic matter. Fluids density (ρ) was 0.859～0.968 g/cm3, and trapping pressure of inclusions (p) 2.21～19.62 bar. The study holded that oceanic crust subduction at late Indosinian epoch produced gold rich ore source bed, then arc-continent collision at early Yanshan stage and inner continent conversion at Himalayan stage provided the tectonic thermal dynamic condition for the deposit, which precipitated meteoric water to leach ore materials from the surrounding rocks and exchange isotopes with them to form ore-forming fluids at shallow depth, then fluids raised through Rize-Samalongwa basement fault, and via interact with diagenetic sulfides or other ferroan minerals, deposited in secondary fracture zone of Ajialongwa that developed at Himalayan stage and had high degree of porosity and permeability, hence Au, As, Sb, W, B, Hg assembled, low temperature and salinity, disseminated Carlin-like gold deposit formed. In common with Qiuluo gold belt, diagenetic sulfides were not generated in exhalative sedimentary condition, low sulfide content was not enough to form syngenetic deposit, and also very limited H2S in ore fluids of the later hydrothermal system. As a result, it's sulfur-arsenic complexes that transported gold rather than sulfhydryl complexes, which differred from Carlin-type/like gold deposits located in basins.