Geological And Geochemical Characteristics And Ore Genesis Of The Caixiashan Pb-Zn Deposit,Eastern Tianshan,NM China

The Eastern Tianshan belt,located in the southern part of the Central Asian Orogenic Belt,is an important polymetallic belt,and in recent years,the Caixiashan Pb-Zn deposit has been idtenified within the Central Tianshan Terrane in the southern segment of the Tianshan belt.This study reported a detailed ore geology of the Caixiashan Pb-Zn deposit,and fluid inclusions,and C-H-O-S stable and Pb radiogenic isotopes geochemistry.The main achievements made in this study are as follows:?1?The Caixiashan Pb-Zn deposit is hosted by the clastic rocks+carbinates assemblage of the first lithologic section of the Mesoproterozoic Kawabulake Group,with the Pb-Zn orebodies occurring as stratiform-like,veins,and lenticular mainly in the dolomite marble,to some extent,showing stratigraphic control.Sulfide minerals are dominated by sphalerite,pyrrhotite,galena,and pyrite,with minor chalcopyrite and arsenopyrite,and gangue minerals mainly include quartz,calcite,dolomite,tremolite,and chlorite.?2?Ore formation at Caixiashan consists mainly of a hydrothermal mineralization period and experienced a epigenetic oxidation period,and hydrothermal activity have been further subdivided into four stages?I to IV?,i.e.,calcite+dolomite+quartz+pyrite stage I,calcite+dolomite+quartz+spahlerite+pyrrhotite±arsenopyrite stage II,calcite+dolomite+quartz+galena+pyrite±chalcopyrite stage III,and late quartz+calcite stage IV.Lead and zinc mineralization mainly occurs in the vein ores of stage II and III.?3?Six types of fluid inclusions are distinguished in the calcite-and quartz-bearing veins,i.e.,liquid-rich two-phase?L-type?,pure-liquid phase?PL-type?,vapor-rich two-phase?V-type?,pure-vapor phase?PV-type?,halite-bearing?H-type?inclusions,and daugther mineral-bearing inclusions?S-type?.Fluid inclusions of stage I to IV yiealed homogenization temperatures of 336-494°C,240-357°C,140-297°C,and 71-156°C,corrsponding salinities of 4.0-18.0 wt.%NaCl equiv.,4.2-17.3 wt.%NaCl equiv.,0.2-13.5 wt.%NaCl equiv.,and 1.2-7.6 wt.%NaCl equiv.,respectively.The ore-forming fluids at the Caixiashan deposit are characterized by high-to moderate temperatures,moderate salinities,and low densities,belonging to the H₂O-NaCl system.?4?Hydrogen and oxygen isotope compositions of quartz indicate that the ore-forming fluids at Caixiashan have a dominantly metamorphic signature and were diluted by meteoric water.Carbon and oxygen isotope compositions of calcite,dolomite,and marble demonstrate that the primary carbon sources from the dissolution and low-temperature alteration of carbonates.Sulfur isotope compositions of sphalerite,galena,pyrrhotite,and pyrite samples indicate the sulfur was predominantly inherited from the Precambrian marine sulfates by thermochemical sulfate reduction.The Pb ratios of sulfides,granitoid rocks,and country wall rocks suggest that ore-forming metals may have been primarily derived from the Precambrian basement.?5?Combined with the fluid inclusions studies and characteristics of the isotopes geochemistry,we suggest that significant temperature decrease,fluid mixing,and pervasive dolomitization are the key factors resulting in large-scale ore precipitation.