Metallogenesis Of MVT Lead-zinc Deposits In Huili-huidong,Sichuan Province

Huili-Huidong area in Sichuan province is the typical and significant lead-zinc polymetallic ore-concentrated area in the southwestern margin of Yangtze platform.There are numerous large-middle scale lead-zinc deposits being exploited in the Sinian Dengying Formation dolomite in the area,with the total lead-zinc resources of 5 million tons.These deposits are characterized by high Pb-Zn grade and large scale,which have high economic and research values.In this paper,Daliangzi and Tianbaoshan lead-zinc deposits are selected as the representative in the Huili-Huidong area.Based on the researches of predecessors,and under the guidance of the theories of ore prediction in exploration areas,the studies on the geological setting of ore-forming processes,geologic features as well as geochemical characteristics of typical ore deposits are conducted.As a consequence,the mineralization processes of MVT Pb-Zn deposits in the Huili-Huidong are systematically collated and summarized.In addition,regional geological prospecting model is constructed,which deepen the understanding of the metallogenic theory of MVT Pb-Zn deposits in the Huili-Huidong and provide references for ore prediction of similar ore deposits.According to the field geological investigation,Daliangzi and Tianbaoshan Pb-Zn deposits are hosted in the Sinian Dengying Formation dolomite,and there is generally sandstone or clasolite with high permeability under the orebodies and argillaceous clasolite with low permeability at the top of the orebodies.The orebodies are respectively controlled by the NWW-trend and nearly EW-trend fault structure in space.The metallic mineral are mainly Galena,sphalerite and a small amount of pyrite,copper,and gangue minerals are consist of dolostone,calcite and square.Structure of ores is mainly massive structure and brecciated structure,and texture of ores focuses on granular texture and metasomatic relict texture.The wall rock alteration types are simply,consist of dolomitization,calcilization,silicification and pyritization.Based on the researches on fluid inclusion of minerals,including sphalerite,calcite,quartz and dolomite,the metallogenic temperature of Daliangzi and Tianbaoshan Pb-Zn deposits are respectively 117.5??320.3?and 104??256?,and salinity are respectively 5.41 wt%_{Na Cleq}?16.24wt%_{Na Cleq} and 6.01wt%_{Na Cleq}?19.13wt%_{Na Cleq},manifesting that the ore-forming fluid is characterized by the medium-low temperature and medium-low salinity.The ionic composition is the type of Ca²⁺-Na⁺-Cl^--SO₄^2-,which is similar to basin brine according to the component analysis.It also could observed that there are some organic constituent,CH₄,in some fluid inclusions,which show the participation of organic matter.H and O isotopic research indicated that the ore-forming fluid is composed of a mixture of basin brine and organic fluids.C isotopic research indicated that the wall rock have the features of marine carbonatite and the carbon in the fluid mainly derived from the dissolution of carbonatite.Moreover,there is genetic relationship between hydrothermal dolomite,calcite and surrounding rocks according to their REE geochemical characteristics,and the ore-forming fluid flew through the REE-rich layers.Sr isotopic research also indicated that the ore-forming fluid flew through the basement strata with high Sr ratio,and the water-rock reaction and isotopic exchange take place.Based on the S isotopic study,sulfur of both deposits mainly derives from thermochemical sulfate reduction of marine sulfate with the participation of organic matter as the reductant.and sulfur in the Tianbaoshan deposit slightly derives from the basement strata.According to the lead isotopic geochemistry characteristics,metallogenic material derived from wall rock,and partly from basement strata.Based on the geological and geochemical characteristics above,the regularity of ore formation in the area is concluded.Dalingzi and Tianbaoshan lead-zinc deposit belong to MVT-type deposits,and the ore-forming processes is as follows.we propose that the predominant ore-forming epoch is late Indosinian in response to the orogenic activity,and then gravity and extrusion stress drove the large scale hydrothermal fluids,the mixed fluid of interlayer water,intergranular water and atmospheric precipitation,migrated along angular unconformity and regional faults.During migration,hydrothermal fluids leached lots of metal cationic ions and halogen,such as Pb²⁺,Zn²⁺and Cl^-and so on from the basement rocks and Dengying Formation dolomite to form ore-bearing brine.When ore-bearing brine moved into extension faulted and fractured zone,water rock reaction occurred and formed a large number of dolomitization.After that ore-bearing brine mixed with organic matters in the stratum or reduced sulfur-bearing fluids,and thermochemical sulfate reduction took place.With the changing of physical and chemical conditions of ore-forming fluid,metal sulfide formed and therefore formed lead-zinc orebodies of MVT deposit in the area.In conclusion,there is close relation between the MVT Pb-Zn deposits in Huili-Huidong area and the Dengying Formation dolomite containing gypsolyte,fracture structure and hydrocarbon source rocks.And the orebodies are controlled by the Si-Ca contact surface and NWW-trend and nearly EW-trend fault structure.In the paper,mineralized geological bodies,metallotectonics and structural surface,as well as metallogenic characteristic indication are recognized,and then the"Trinity",i.e.a possible geological prospecting model is initially established in the Huli-Hudong,Sichuan Province.