Chronology Study And Discussion On Genetic Type Of Dachang Gold Deposit In Qinghai Province

Dachang gold deposit is located in the North Bayan Har orogenic belt. All the gold ore bodies are hosted in Middle Triassic sandstone interbedded with slate of the Bayan Har Mountain Group, the ore-bearing wall rocks reached greenschist facies. The gold ore bodies are controlled strictly by the fracture zone, all the ore bodies are in veined, veinlike and lenticular forms. The rock types were major alteration types with minor quartz types. The most major and common mineralizations consisted of those of pyrite and arsenopyrite, and alteration types were composed of major sericitization and carbonatization.The zircons from quartz veins in the Dachang gold deposit indicate that the zircons can be divided into magmatic and hydrothermal zircons. The magmatic zircons gave a206Pb/238U concordia age of265Ma-2121Ma before Triassic epoch, which is nothing to do with formation of the Dachang gold deposit. LA-ICP-MS U-Pb analyses of hydrothermal zircons gave the age from217to226Ma, with a weighted mean206Pb/238U age of (221±4) Ma, which is interpreted as the age of hydrothermal event. Gold mineralization of Dachang gold deposit is most likely related with this hydrothermal event, mineralization age is about (221±4) Ma, belonging to late Indosian. Indicate that the gold deposit be formed in the Late Indosinian subduction and collision.Zharijia granitiods directly intruded folded Triassic strata. The granites are composed of granodiorite and adamellite. Zircon LA-ICP-MS insitu U-Pb dating obtains the age of (199.7±2.4) Ma-(201.5±1.9) Ma should be the formation age of the granites, belonging to early Jurassic. However, U-Pb dating on hydrothermal zircons gave the age of (221±4) Ma, indicate that formatining of rocks is nothing to do with formating of the Dachang gold deposit.The ore-forming fluids belong to the H2O-CO2-NaCl type characterized by rich CO2, low-moderate temperature, low salinity, low density and strong reducibility, the ore-forming fluid evoled form metamorphic water and formation water in the early stage to the metamorphic water during mineralization, may be mixed with a small amount of mantle-source magmatic water.U-Pb dating on hydrothermal zircons obtains the timing of gold mineralization of (221±4) Ma, the ore deposit was formed during the late stage of Indosinian orogenic process,which is related genetically to late Indosian organic process. In addition, they also have some similar features with organic gold deposit as in other regions in ore-forming fluids and geological characteristics of the deposit. As a result, ethey should belong to a kind of orogenic gold deposits according to the proposed classification and can be divided into middle-low temperature hydrothrermal gold geposit.