| The Mengxing Pb-Zn deposit is located in the south-central part of Baoshan Block in southwest China.It is a representative large-scale Pb-Zn deposit in Baoshan Block.It has been exploited for66 years.Despite the high degree of exploration and development of this deposit,the geological scientific research work lags far behind,especially the understanding of ore-forming period,ore-forming material and ore-forming fluid source is still very vague.Therefore,on the basis of detailed field and literature research,this paper used LA-ICP-MS,laser Raman and other advanced techniques to conduct a comprehensive study on fluid inclusions in Mengxing Pb-Zn deposit for the first time,and new data of calcite C-O isotopes and published S isotope and H-O isotopic studies are also combined to discuss the source and evolution of ore-forming materials and ore-forming fluids,and determine the genesis of the deposit.The study will provide theoretical guidance for finding this type of Pb-Zn deposit.The Mengxing Pb-Zn deposit occured in the crystalline limestone,bioclastic limestone and stratified limestone of the lower member of the Shangrenheqiao Formation in the middle Silurian system,and is strictly controlled by stratigraphy and fault structure.The ore bodies are mostly layered,lamellar,lenticular and podded.The ore fabric is mainly layered,disseminated,massive structure,granular,metasomatism(metasomatism residual),vein(mesh vein)structure,oolitic and colloidal structure(pyrite),with obvious characteristics of early sedimentary mineralization and late hydrothermal transformation.The metallic minerals are sphalerite,galena,pyrite and arsenopyrite.Gangue minerals are calcite,quartz and barite.Surrounding rock alteration,such as calcite,barite,sericite and silicification,is developed.The ore-forming process can be divided into four stages.They are the sedimentary period(stratified beige sphalerite-pyrite-galena-calcite-calcite),the hydrothermal superimposed stage I(reddish-brown fine grained sphalerite-galena-calcite-barite-quartz-arsenopyrite),the hydrothermal superimposed stage II(green or yellow sphalerite-galena-calcite-pyrite)and the later mineralization period(calcite–sphalerite-quartz-galena-pyrite).The fluid inclusions are mainly R-L type,followed by R-V type,P-L type and P-V type.The composition analysis of single inclusion LA-ICP-MS showed that the fluid mainly enriched Pb,Li,Na,K,Rb,Cs,Ba,Sb,and the content of Sb was much higher than that of the continental crust of China.It suggests that this area may have the potential to find Sb resources.The metallogenic fluid temperature of Mengxing Pb-Zn deposit was between 174～362°C(concentrated at 200～240°C),the salinity was 1.06%～17.34%Na Cl eqv(concentrated in 6～16%Na Cl eqv),the density was 0.666～1.02 g/cm~3,the pressure was 7.10?10~5 Pa～179.0?10~5 Pa,the p H was 5.97～6.71(weakly acidic),and the Eh was-0.09～0.08 V(weak reduction-weak oxidation).There may be two main mineralizing fluid systems in this deposit:medium-low temperature(180～260°C),medium-low salinity(5%～18.8%Na Cl eqv,the average value is about 10 Na Cl eqv)in the early(sedimentary period),weak acidity,weak oxidation fluid;In the later stage,there are medium-high temperature(280～360°C),low salinity(2%～10%Na Cl eqv,the average value is about 6%Na Cl eqv),weak acidity,weak reducing fluid mixing or transformation.Both stage of mineralizing fluids are critical to the formation of Mengxing lead-zinc deposit.The S isotope shows that the sulfur in Mengxing deposit has two main sources:Marine sulfate and evaporite in sedimentary period,and magmatic hydrothermal solution in modification period.The analysis of C-O isotopes,H-O isotopes and fluid inclusions indicates that the ore-forming fluid of Mengxing Pb-Zn deposit may be a mixed source of seawater,basin brine and organic water during the depositional period,which was modified by magmatic hydrothermal superposition in the later period,showing the characteristics of metamorphic brine on the whole.Water-rock reaction and fluid cooling are the main causes of metal element unloading.The geological characteristics,orebody occurrence,ore fabric,metallogenic temperature,ore-forming fluid properties,ore-forming materials and fluid sources of Mengxing Pb-Zn deposit are comprehensively analyzed,and then compared with the typical SEDEX type Pb-Zn deposit and MVT type Pb-Zn deposit,the final conclusion is that Mengxing Pb-Zn deposit is a sedimentary(such as SEDEX type)-hydrothermal superposition transformation type Pb-Zn deposit. |
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