Study On Metallogensis Of Wulagen Lead-zinc Deposit In Wuqia, Xinjiang

The Wulagen lead-zinc deposit, located at Kashi Sag in northwestern margin of the Tarim basin, western Xinjiang, is hosted in Mesozoic-Cenozoic sandstones and conglomerates, where lots of organic matters are widely spread, such as residues of oil & gas and dry asphalt. Recent studies and prospecting have confirmed that the geologic condition of this deposit is favorable for mineralization, and the mineral potential should be very large. However, its genesis and metallogenic epoch remain controversial for long time, which limited further regional mineral prospecting.In this paper, the author mainly summarized the research progress in lead-zine deposit metallogenic and organic mineralization. And many methods, such as the isotope tracer, fluid inclusion, organic geochemistry, fission track dating of apatite, and comprehensive comparison, were used in order to better understand regional metallogenic geological settings and trace the ore-forming elements. The nature of the ore-forming fluid and its relation to the lead-zinc mineralization, the mineralized epoch of the deposit were discussed in order to establish the origin model of the Wulagen lead-zinc deposit. The main achievements obtained by this paper were as following:(1) Continued uplifting of the southwestern Tianshan range and West Kunlun Mountains during the India-Asia collision and their convergence afterward, was accompanied by the dextral strike-slip of the Talas-Fergana fault in Mesozoic to Cenozoic. And regional lead-zinc hosting rocks were composed of sedimentary material, such as sandstones, conglomerates with mudstonesof braided river-littoral facies, deposited in Kashi sag during this time period in the Late Cretaceous.(2) The value of sulfur isotopic compositions (δ34S)from ore-bodies changed from-27.9% to+15‰, showing a wide distribution and two-terminal element. This indicated that the sulfur might have been derived from the marine sulfate thermal and chemical reduction.The lead isotopic ratios are 17.771～18.721(206Pb/204Pb),15.402～15.748(207Pb/204Pb), and 37.92～39.103(208Pb/204Pb), respectively, with very narrow variation. Lead isotopes are near the orogenic belt evolution curve on the 206Pb/204Pb-207Pb/204Pb diagram and 208Pb/204Pb-206Pb/204Pb diagram, suggesting that the metal were from the Proterozoic strata.(3) Lots of organic matters, such as residues of oil & gas and dry asphalt, are widely spread in the Wulagendeposit and its adjacent areas. Petroleum and gas inclusions in the ore-bearing sandstones under microscope are also found, including mostly liquid hydrocarbon inclusions and few hydrocarbon containing brine inclusions. Measurements of mineralization fluid yielded the homogenization temperature varying from 78 to 369 ℃, with concentrating on 100～220℃, implying that the ore-forming fluids is not a single basin cryogenic fluid, may be experienced for many times the thermal activity of transformation of the mixed fluid. The salinity incelestite, barite and calcite highly related with the lead-zinc mineralization was mostly lower than 8 wt% NaCl, indicating that the ore-forming fluids inWulagendeposit was characterized by low temperature and low salinity, with a higher trend of the fluid salinity.(4)The content of organic carbonin ore-bearingrocks andsamples with oil&gasremainder varied from 0.02 to 12.43%, thecontent weremore higher. But that in no ore-bearing rocks or no organic remainder bearing samples was generally low, ranging from 0.02 to 0.1%. The average content of in ore-bearing samples is 3.56%. The total content of organic carbon and ∑S is positively correlated with the lead-zinc mineralization intensity, suggesting that the lead and zinc mineralization process was accompanied with the transformation from original oxidation to reduction in the ore-hosted rocks, and that the oil and gas should be as the reducing agent. The Fe2+/Fe3+ value in ore rocks, ore-bearing and oil&gas remainder-bearing samples, and gray or black sandstone reducedby oil & gas was close to 1, indicating that the environment was tend to be neutral or reduction environment as response to the reduction of oil and gas.(5) The content of chloroform bitumen "A" in organic matter changed from 0.004 to 0.025 wt%. The ration of total hydrocarbons was 46.22% in average, and that of "non-hydrocarbon+ asphaltenes" was 53.77% in average, and the average of saturated hydrocarbons to aromatic hydrocarbons was 9.59, showing the organic matter was similar to Ⅰ-type parent material. These characteristics suggested that the organic matter should be sourced from the marine algae. The ration of Pr/Ph in ore-rocks varied from 0.41 to 1.84 with 1.04 in average, indicating a deoxidization environment for the organic matter. Odd-even predominance index (OEP) was from 0.75 to 1.07 and the average was 0.92, suggesting that the maturity of organic matter was relatively high. Carbon preference index (CPI) was from 1.03 to 1.30 with 1.16 in average, indicating a higher degree in thermal evolution.(6) Apatitesfrom ore-bearing sandstones, oil&gas reduced sandstones and original red sandstones were chosen out for fission track dating. The measurement results suggested that the metallogenic epoch is probably the Eocene to early Oligocene in age (49.5-35.2Ma).(7)Most mineralization characteristics, especially in ore-hosted rocks,oil and gas reduced remainders in the Wulagen deposit could be compared with those in the Jingdinglead-zinc deposit, in Yunan Province. Combined with the oil and gas reduction found in Bashibulake uranium deposit, a new metallogenic model of the Wulagen deposit, oil and gas reduction mineralization model, was proposed, suggesting thatore-forming elements deposited and accumulated accompanied with the reduction of oil and gas well permeability, original oxidation sandstones and conglomerates when the lead-zinc-bearing fluids flowed through Mesozoic to Cenozoic strata.