The genesis of the Xitieshan SEDEX lead-zinc deposit in Qinghai Province: lead isotope evidence

Abstract：The Xitieshan superlarge Pb-Zn deposit is located in the north margin of Qaidam basin, Qinghai Province, and originally contained about 50 million tons of ore with the grade of 8% Zn and 10% Pb. It was formed in a rift basin in the Ordovician period. It has been recognized that there is a complete exhalation sedimentary system that includes breccias pipe-stockwork, non-bedded and bedded ores and marble from exhalation sediments. Pb isotope compositions of sulfides can be divided into two groups, one consists of most of the samples and includes most of the bedded ore and all of the non-bedded ore, pipe breccias and veins in marble under the non-bedded ore. They assume a linear array characterized by (207Pb/204Pb)= 1.254(206Pb/204Pb）-7.296 with the regression coefficient of 0.977 (n=19). The other group includes carbonaceous schist occurring above the marble and non-bedded ore and pyrite from bedded ore, and assumes a linear array of （207Pb/204Pb）=0.123（206Pb/204Pb）+13.347 with the regression coefficient of 0.902 (n=5). The Pb isotope composition of the intersection point of the two arrays is 206Pb/204Pb=18.253，207Pb/204Pb=15.590，and 208Pb/204Pb=38.370. A study of Pb isotope geochemistry suggests that the first array, consisting of most of the ore samples, represents a two-component mixing line in which the less radiogenic component can be identified as orogenic lead reservoir probably coming from Tianjianshan Group through the exhalation brine. The radiogenic end-member can be identified as upper crust lead reservoir probably derived from seawater. The lead and the ore-forming metals mainly came from the exhalative brine during the sedimentary-exhalative metallization, with a small amount derived from the seawater. Precambrian metamorphic rocks might also have contributed a little material in the distal vents of the exhalative system or during the intermission of exhalation. The bedded ore body and the non-bedded ore body, which endured the same geological evolution, were all formed during the exhalative sedimentation. Instead of being generated from the late reformation, the non-bedded ore body was probably hydrothermal ore body without experiencing submarine eruption. The mineralization in Duancenggou was relocated along the schistosity in the late reformation, and the lead isotope was further homogenized.