Geological And Geochemical Studies On Bozshakol And Nurkazgan Cu-Au Porphyry Deposits In The Area Of Balkhash,Kazakhstan

Bozshakol and Nurkazgan are the most representative large-scale porphyry Cu-Au deposits in the Balkhash metallogenic belt.(Bozshakol: 4.1 million tons of Cu,100 tons of Au,the grade of Cu and Au is 0.31g/t and 0.28g/t;Nurkazgan: 3.93 million tons of Cu,229 tons of Au,the grade of Cu and Au is 0.78g/t and 0.45g/t).By the comparing study of them,We can discusses the reasons for the difference in reserves between the two and Provides supplementary information for the establishment of metallogenic theory of Porphyry Deposits in Metallogenic Area in Central Asia.The comparison of geochemical characteristics of rocks shows that the ore-bearing rocks in Bozshakol are mainly diorite and granodiorite.The mainly plagioclase in the diorite is albite,and the rocks are of calcium-alkaline series.The Geochemical characteristics are same to adakites,combined with the diagenetic age andegional geological structure from previous studies,it is inferred that the Bozshakol mineralization is related to the subduction of the Bozshakol intra-arc starting in Middle Cambrian.The Nurkazgan ore-bearing rock body is also diorite and granodiorite.The plagioclase in the granodiorite is mainly Medium feldspar and longer stone,and the hornblende is mainly a common magnesian hornblende.The main stone is the high-K calc-alkaline series.The characteristics of trace elements indicate that the ore-bearing rock masses have the characteristics of normal island-arc rocks.Combined with previous research results,it is inferred that Nurkazgan Cu-Au ore is formed with the Balkash-Junggar metallogenic belt is in the process of subduction in the northwest direction.The above results show that Bozshakol and Nurkazgan are formed with different tectonic settings.The oxygen fugacity of the ore-forming magma determined by the composition of hornblende in the granodiorite of the Nurkazgan mining area and the metallogenic process is Logf O2=-13.5~-12.1,(?FMQ=3.1~3.6,the average value is 3.3).The average water content was 0.75%,indicating that crystallization of hornblende crystallites occurred within a range of0.7km to 7.6km(average depth of 2.7 km).The magma environment with high oxygen fugacity and water content in the magma system at that time was very conducive to the formation of porphyry deposits.Combined with previous studies of magmatic oxygen fugacity(Logf O2=-11.7~-7.3,?NNO=1.8~7.4,mean value 4.3)in the Bozshakol mining area,it was confirmed that the oxygen fugacity of Nurkazgan magma is lower than that of Bozshakol.The geological characteristics of the deposits show that the orebodies of the Bozshakol and Nurkazgan deposits are as massive,fine veined,reticulate and disseminated forms in porphyry bodies.However,part of the ore body in Nurkazgan is in the volcanic rock surrounding rocks;the two types of major alterations are similar,but the main alteration types related to mineralization are different.The mineral composition of ore in the Bozshakol and Nurkazgan mines is similar,they is pyrite,chalcopyrite,molybdenite and a small amount of galena,sphalerite and natural gold.The Re-Os isotopic model age of the Nurkazgan deposit is 433±4 Ma.The comparison of the characteristics of fluid inclusions shows that the temperature and salinity of the ore-forming fluids in the early and middle stages of Nurkazgan mineralization are higher than that of Bozshakol,which is conducive to the enrichment of copper,while the temperature of the fluid reduces lead the mineralization into a low-temperature hydrothermal type at the late stage of Nurkazgan.Low temperature hydrothermal type is conducive to the enrichment of gold.Both ore-forming fluids have undergone boiling.Based on the comparison of sulfur isotope values between the main porphyry deposits in the core region of Central Asia and Nurkazgan,it was found that the ?34S values of the main porphyry deposits in the core region of Central Asia were concentrated between-3‰ and +2‰(Nurkazgan is 0.4‰~1.4‰),which is close to the range of meteoric sulfur and reveals that its source is mainly magmatic sulfur.The comparative study found that the differences between the Bozshakol and Nurkazgan Cu-Au deposits are as follows: The value of the magmatic oxygen fugacity of Bozshakol over Nurkazgan may be one cause of the difference in reserves between the two deposits;The higher salinity of Ore-forming fluid in Nurkazgan and the mineralization has transformed into a low-temperature hydrothermal type cuased dy addition of water in Nurkazgan deposits in the late time,which may be the causes of the difference in reserves between the two deposits;The Nurkazgan Cu-Au porphyry deposit was formed in the Ereme-ntau-Yili proliferative wedge formed after the Balkhash-Jungar metallogenic belt gently subducted in the northwest,and the Bozshakol Cu-Au porphyry deposit was formed in the subduction of the internal arc of the Bozshakol Ocean in the Middle Cambrian,the differences in the ore-forming periods and tectonic settings of the two deposits may be the third reason for the difference in deposit reserves.