Research On The Geological, Geochemicai Characteirstics And Genesis Of Awanda Gold Deposit In The Southwestern Tianshan Area, Xinjiang

The Awanda gold deposit which is located in the Southwestern Tianshanorogenic belt, is a recently found medium-size mineral deposit in Baicheng County,Akesu City, Xinjiang. The rocks in the Awanda gold deposit area containcoarse-grained massive marble of the Ordovician Yinanlike Group and metamorphicrocks which were ascribed to the Mesoproterozoic Aksu Group by Eighth GeologicalTeam, Geology Bureau in Xinjiang Uygur Autonomous Region. Gold mineralizedzones can be distinguished within shear zones among subsidiary faults in the micaquartz schist host rocks between the two EW-striking regional faults, Keerawandaregional fault and Aertengkesi regional deep fault.The author applied petrological geochemistry and LA-ICP-MS detrital zirconU-Pb dating on the siliceous schist wall rocks from the So-called Akesu Group, whichis closely related to the mineralization of Awanda gold deposit, with the aim to discusssedimentary environment, provenance and age of these rocks. The result shows thatthe protoliths of the ore-hosting strata are possibly composed of mudstones andsandstones with low maturity. Most of the provenances are quartzose sediments andthe sedimentary environment is likely related to depositional basin nearby the activecontinent margin. LA-ICP-MS detrital zircon U-Pb datings indicate that the zirconages have peaks with weighted age of405or406Ma. The youngest peak age of theseconcordant age data is405Ma which could be regarded as the maximum sedimentaryage of the schist. Therefore it was considered that the sedimentary age of the so-called Akesu group ore-hosting stratum is not older than the Early Devonian, rather than theChangcheng period as previously reported.Three gold mineralized zones have been delineated in the Awanda area up to2012. Six lenticular ore bodies have been found. Ore styles include sheared-fracturedquartz-sulfide schist, there is no apparent difference in the characteristics between orebody and ore-hosting rock. The ore minerals consist mainly of arsenopyrite, pyrrhotite,pyrite, with minor loellingite. Wall rock alteration consists of silicification,sericitization and with rare carbonatization.It was shown that fluid inclusions in mineralized quartz consist of majortwo-phase aqueous inclusions and minor three-phase CO2-bearing ones. Thetwo-phase aqueous inclusions have a bimodal-distributed homogenizationtemperatures of188~380℃and salinities of6.90~20.78%, while the CO2-bearinginclusions have the homogenization temperatures of238~347.1℃and salinities of2.8~7.0%. The result of arsenopyrite geothermometer shows that the evolution ofore-forming fluids experienced from the high-temperature epoch to thelow-intermediate-temperature one. In the high-temperature epoch, ore-forming fluidsare characterized by the high homogenization temperatures of270~380℃, thetrapping temperatures of345~420℃and the calculated trapping pressures of74~142MPa, belonging to a low-intermediate-salinity H2O-CO2-NaCl system. In thelow-intermediate-temperature epoch, ore-forming fluids are characterized by the highhomogenization temperatures of188~270℃, the trapping temperatures lower than304℃but higher than270℃and the calculated trapping pressures lower than104MPa but higher than52MPa, belonging to a low-intermediate-salinity H2O-NaClsystem.The measured δ18O values for the gold-bearing quartz range from15.9to20.3‰,and the estimated δ18O values for fluids range from7.8to12.2‰. The δD values forfluid inclusions of the gold-bearing quartz range from116.3to87.6‰. The resultssuggest that the ore-forming fluid was of metamorphic origin, derived by dehydrationof deeply buried carbonaceous black shales.The δ34S values which have association with hydrothermal sulfide minerals change from3.6to+6.2‰, with most of the values between2‰and+1‰. Thecalculated δ34SH2Svalues for H2S in hydrothermal fluids range from4.8to+5.8‰,with an average of0.4‰, suggesting that the sulfur was sourced from host rockswith an average crustal sulfur composition. Lead isotopic compositions of sulfidesfrom gold-bearing ores range from17.4625to18.5356for206Pb/204Pb ratios,15.4943to15.6468for207Pb/204Pb and37.8825to38.8238for208Pb/204Pb, respectively. Weinterpret these data to reflect a mixed source for Pb, involving host rocks andmetamorphic fluids or deep origin.The Awanda gold deposit can be classified as orogenic gold deposit and sharemany features similar to those of the Muruntau-type gold deposits. Followingmetamorphism and deformation associated with the final collision in STAC,large-scale thrusting produced regional thrust faults at different scales and subsequentore-controlling subsidiary faults. The ore-forming metamorphic fluids were channeledalong these resulting in gold mineralization within the shear zones.