A Study On The Ore Genesis Of The Wuxu Ore Field In Hechi,Guangxi Province

The Wuxu ore field is one of the three major ore fields in the Danchi metallogenic belt and is also well known by its enrichment of polymetallic minerals such as lead,zinc,antimony,silver and mercury.In the past thirty years,geological exploration has shown that the ore reserves discovered in Wuxu ore field are far less than those of Dachang and Mangchang ore fields.It is thus that limited studies were focused on the ore genesis of the Wuxu ore field.Compared with the Dachang and Mangchang ore fields,the Wuxu ore field has some differences in the metallogenic elements and deposit characteristics,showing its unique metallogenic characteristics.Because the Jianzhupo deposit is the lead-zinc-antimony polymetallic deposit with the best metallogenic geological conditions and the largest metallogenic scale in the ore field,it is chosen as the case study.On the basis of fully combining the previous research results,and then a systematic study on the deposit geology,sulfur isotope,trace elements of sphalerite and fluid inclusions were carried out to constrain on the ore genesis of the deposit and provide a basis for the genesis and metallogenic theory of the Wuxu ore field,so as to better serve the deep and peripheral prospecting work of the mining area.The main results are shown as following:(1)According to the field geological features and the characteristics of mineral symbiosis combination at a hand specimen scale,and microscopic comprehensive observation under the microscope,the metallogenesis of Jianzhupo deposit could be divided into hydrothermal metallogenic period and supergene period.The hydrothermal mineralization period is divided into three mineralization stages:(I)the quartz-pyrite stage is the earliest stage of mineralization and minerals are mainly quartz and pyrite,with a small amount of sphalerite;(II)the quartz-sulfonate-sulfide stage is the major metallogenic stage,and quartz and pyrite occur along the vein wall,while sphalerite and jamesonite occur at middle part with a small amount of galena,stibnite and carbonate minerals;(III)the quartz-sphalerite-carb onate-stibnite stage is the late period of mineralization with a large amount of iron-manganese carbonate and stibnite in this stage.(2)The trace elements of sphalerite in the main ore-forming stage from the Jianzhupo deposit are relatively enriched in Fe,Cd,Mn,Cu,and In,while other elements such as Ga,Ag,Sn,and Pb are depleted.Meanwhile,the content of each trace element in sphalerite from the deposit is comparable to that of the sphalerite from the magmatic hydrothermal deposit.According to the Zn and Fe content of sphalerite and ore structures,it is shown that the ore-forming depth of the deposit is meso-shallow environment.The relative content between Ga and In in the lnGa-lnIn relationship discrimination diagram shows that the ore-forming fluid was derived from a magmatic hydrothermal fluid,resulting in the Zn/Cd,Cd/Fe,Cd/Mn and Ga/In ratios of sphalerite are similar to magmatic hydrothermal deposit.(3)The?³⁴S_VCDTvalues of sulfides in Jianzhupo deposit are varying from 3.9‰to 8.0‰,and sulfides among minerals were formed under thermal equilibrium conditions.Based on the?³⁴S_VCDTvalues of sphalerite and coexisting pyrite(?³⁴S_Py-Spvalues),the calculated?³⁴S?_S-fluidsvalue is+5.9‰,suggesting that sulfur was a homogeneous magmatic origin and was likely related to deep hidden granite,but we can not rule out sulfur contribution from the host rocks.This conclusion is consistent with previous studies.Therefore,the ore-forming materials of the Jianzhupo deposit dominantly came from granite with a small amount from wall rocks.(4)The fluid inclusions,in the minerals of main stage of mineralization,have several types:H₂O-NaCl system(type A),H₂O-NaCl-CO₂system(type B)and pure CO₂system(type C).Among them,the two-phase(gas and fluid)inclusions of H₂O-NaCl system(type A2)have the homogenization temperatures of 150?358?and mostly range in 160?200?,and have the salinities ranging from 0.87?9.86 wt.%NaCl equiv and mostly range in 4.00?6.00wt.%NaCl equiv;the inclusions of H₂O-NaCl-CO₂system(type B)have the homogenization temperatures of 151?436?and mostly range in 230?350?,and have the salinities of0.01?15.62 wt.%NaCl equiv and mostly range in 1.00?6.00 wt.%NaCl equiv.Although,the fluid inclusion of the Jianzhupo deposit have the homogenization temperature up to 436?and salinity up to 15.62 wt.%NaCl equiv,the average value and concentrated range are respectively lower than 358?and 6.00 wt.%NaCl equiv,consisting with the calculated the temperature of ore-forming fluid(114?377?)by sulfur isotope geothermometer.Thus,the ore-forming fluid of this deposit is of moderate-low temperature and low-salinity fluid.Besides,the fluid inclusions of CO₂system contain CH₄,N₂and organics of bitumen.(5)Taken together,we suggest that the Jianzhupo deposit if of a moderate-low temperature,low salinity and meso-epithermal magmatic filling deposit.Combining with the geological tectonic background and ore bearing stratum of Wuxu ore field,the genesis of Wuxu ore field was related to magmatic activities,and the ore-forming process could be summarized as“deep source-vertical migration through the NNW-trending faults-vein filled mineralization”,that is to say,the intermediate-acid magma provided the ore-forming materials,ore-forming fluid and heat source,and when the deep-source magma migrated upward through the NNW-trending faults and trapped by the Devonian strata,which are chemically inactive black banded mudstone,silty mudstone and carbonaceous mudstone and are physical barriers to seal the magmatic fluid,thus the ore-forming fluid was migrated through the NNW-trending fault zone and interstratified fracture zone in the shallow part of the crust.Due to the decreasing of fluid temperature and pressure,sulfides were precipitated to form the Jianzhupo deposit.In addition,the wall-rock alteration of silicification and carbonatization can be used as an indicator for ore exploration in ore field.