Study On Ore-controlling Structure Characteristics And Ore Genesis Of Chuduoqu Lead-zinc Mine In Tuotuohe Region,Qinghai Province

Chuduoqu lead-zinc deposit is one of the new discovered large lead-zinc deposits in Tuotuohe region in recent years, with 40.25 million tons of(333+334) Pb metal quantity, 11.27 million tons of Zn metal quantity, 0.92 million tons of Cu metal quantity, which is believed to have a great prospecting potential. But uncertain genetic type of ore deposit and fuzzy ore-controlling structure pattern as well as other factors seriously restrict ore-search breakthrough, so it is necessary to do research about the deposit comprehensively and deeply, to provide the basic reference for ore-search breakthrough in Tuotuohe region.This paper is based on field investigation, regional geological background, geological characteristics and geological features of ore deposit of Chuduoqu lead-zinc mine are found out fully. This paper ascertains ore-forming fluids and ore-forming fluid evolutionary process by doing research on fluid inclusion, isotope geochemistry and other aspects about the deposit, makes ore genesis clear.The ore bodies of Chuduoqu lead-zinc deposit occur in Xiali Formation(J2x), next came Suowa Formation(J3s), mainly exist in cataclastic micritic limestone, cataclastic powder crysta limestone and cataclastic feldspar-quartz sandstone, mineralization in limestone is much better than that in sandstone, limestone provides good metallogenic environment and ore bearing space. The ore bodies are mainly bedded and veinlike in the NWW faulted and shattered zone. The first-order NWW-trending fault is ore-transmitting structure in the Tuotuohe Basin. The second-order NWW-trending tense-shearing fault, sub-fractures derived from the first-order NWW-trending fault, is ore-hosting structure. Ore-controlling structure was formed in an extensional environment of collision post-orogenesis during the Eocene, not the opinion hold by the former researchers that mineralization is controlled by nappe structure. Mineralization is closely related to the large-scale Neozoic potassic-ultrapotassic magmatism in Tuotuohe region, as well as potassic-ultrapotassic hypabyssal instrusive rocks.The ore minerals of Chuduoqu lead-zinc deposit consist mainly of pyrite, chalcopyrite, galena, sphalerite, limonite, azurite, pearceite and specularite; The gangue minerals consist mainly of quartz, calcite, barite, gypsum and less sericite. Ore textures mainly have automorphic-hypautomorphic granular texture, allotriomorphic granular texture, poikilitic texture, cataclastic texture, residual texture and pseudomorph texture. Ore structures mainly have massive structure, brecciated structure, vein structure, mesh-vein structure, disseminated structure and banded structure. Wall rock alteration mainly have carbonatization, baratization, epidotization, sericitization, silicification and clayzation. With the existence of significant mineralization zoning in Chuduoqu lead-zinc mining area, quartz-specularite mineralization zone of higher temperature in the first mineralization stage exists in the southern part, calcite-galena-sphalerite-chalcopyrite mineralization zone of medium and low temperature in the second and the third mineralization stages exists in the northern part.The ore-forming process includes three stages which are marked by the mineral assemblages ofquartz-specularite(stage I), quartz-barite-polymetallic sulfide(stage II), and carbonate(stage III). The fluid inclusions are composed of gas and liquid facies, the metallogenic pressure(the major metallogenic stage) is about 15.66~29.42 MPa(average of 21.63 MPa), the mineralization depth(the major metallogenic stage) is about 1.57~2.94 km(average of 2.16 km), the ore-forming fluid of the major metallogenic stage(stage II) is of the middle-low temperature(182℃~273℃), low-middle salinity(4.01%~14.94%) and low density(0.82 g/cm3~0.98 g/cm3). Temperature, pressure and salinity were gradually decreased from earlier to later phase. Temperature of the ore-forming fluid in the first mineralization stage was higher, minerals such as specularite were formed in the oxidation environment, pyrite and chalcopyrite began to crystallize in the later stage. In the second mineralization stage, the ore-forming fluid became oxidative which can be interpreted as being resulted from large-scale precipitation of sulfides and the mixing of shallow meteoric water in the initial stage, minerals such as baryte and quartz began to crystallize largely. Crystallization of minerals such as baryte and quartz finished with the temperature decreased, the ore-forming fluid was transformed into neutral-reductive fluid, minerals such as pyrite, chalcopyrite, sphalerite and galena began to crystallize. In the third mineralization stage, net vein calcite and acicular gypsum were formed with the ore-forming fluid changed from neutral-reductive fluid into oxidative fluid.Carbon isotope, hydrogen isotope and oxygen isotope characteristics indicate the ore-forming fluid mainly belonged to magmatic hydrothermal fluid. Along with mineralization carrying on, the ore-forming fluid shows some characteristics of mixture fluid with participation of meteoric water in late stage of mineralization. The ore-forming fluid is different from that of MVT Pb-Zn deposits which came from basin brine. Sulfur isotope characteristic indicates sulfur in the metallogenic materials mainly come from relatively unitary magma source. The results of lead isotope indicate a mixed feature of lead from Chuduoqu lead-zinc deposit, the ore lead is mainly crust-derived and mixed with minor mantle-derived lead, the subduction-orogenesis orogeny led to the mixture of lead. The ore lead mainly came from Cenozoic volcanic rocks which have close relation with magmatism, it is in accordance with the ore-forming material source which the results of sulfur isotope show.Basied on the results, Chuduoqu lead-zinc deposit belongs to mesothermal hydrothermal vein deposit. Mineralization is closely related to the large-scale Neozoic potassic-ultrapotassic volcanic rocks in Tuotuohe region. The vein ore bodies are controlled by the NWW-trending fracture tectonics. Ore-controlling structure was formed in an extensional environment of collision post-orogenesis during the Eocene.