Genesis And Prospecting Potential Evaluation Of Jinchangzi Gold Deposit In Weiningbeishan Area,Ningxia

Located in the eastern part of the North Qilian orogenic belt,the Weiningbeishan area is one of the areas with the best metallogenic conditions and the most metallogenic signs of gold polymetallic deposits in Ningxia.Many small gold deposits or mineralization points such as Au,Ag,Cu,Pb,Zn,Fe,Co have been discovered in this area,of which Jinchangzi gold deposit is the largest gold deposit discovered.Based on the detailed field geological survey and the previous research results,the Jinchangzi gold deposit has been studied in detail.Not only has the geological characteristics and metallogenic stages of gold deposits been identified,but also the precise dating has been carried out.By means of fluid inclusion,trace elements in pyrite micro-area,S-Pb-H-O-C isotope and other methods,the nature of ore-forming fluid,the source of ore-forming material,the occurrence state of elements and the genesis of gold deposits have been identified,and the ore-forming model has been established on the basis.The ore-controlling factors and prospecting indicators of gold deposits are systematically summarized.On the basis of building a comprehensive prospecting model based on geological,physical and chemical factors,combined with the law of deep variation of gold mineralization,the prospecting target areas are delineated and the prospecting potential of each target area is evaluated.Through the geochemical profile survey and deep drilling,the rationality of target area delineation has been verified,and on this basis,the next prospecting ideas have been put forward.The main achievements are as follows:（1）The gold ore body is mainly hosted in the interlayer fracture zone in the Qianheishan Formation and Chouniugou Formation of the Lower Carboniferous,which is distributed in east-west belt,and its occurrence is nearly consistent with the stratum.The shape of the ore body is mainly layered,lenticular and veined,which is obviously controlled by the structure.The types of ore can be divided into three types:primary ore,semi-oxidized ore and oxidized ore,of which the oxidized ore is the main one.The metal minerals in the ore mainly include natural gold,pyrite,galena,sphalerite,chalcopyrite,silver tetrahedrite,etc.The wall rock alteration is mainly of medium-low temperature alteration type.The gold mineralization process is divided into two periods and five stages,two of which are hydrothermal period and supergene oxidation period.The five stages are pyrite-quartz stage（I）,pyrite-sericite-quartz stage（II）,polymetallic sulphide-quartz-carbonate stage（III）,pyrite-carbonate-barite stage（IV）and supergene oxidation stage（V）.The first four stages are hydrothermal stage,and the second and third stages are the main mineralization stages.The first four stages are hydrothermal period,and the II and III stages are the main mineralization stage.（2）The results of sericite ⁴⁰Ar/³⁹Ar dating show that the gold mineralization age is198～200 Ma,which is mainly related to the regional tectonic activities in Indosinian period.There are four types of fluid inclusions in different metallogenic stages of hydrothermal period,which are aqueous solution inclusions,pure CO₂ inclusions,CO₂–H₂O inclusions and daughter crystal multiphase inclusions.The microscopic thermometry results show that the completely homogenous temperature of the ore-forming fluids is 171～396℃,the salinity is 1.30%～10.99%Na Cl equiv,and the density is 0.24～0.78 g/cm³,which are CO₂-H₂O-Na Cl systems with medium-low temperature,low salinity,low density,CO₂ rich,and a small amount of N₂.The hydrothermal quartzδD value is-66.0‰～-32.0‰,δ¹⁸O_V‐SMOW value is+19.7‰～+22.6‰,indicating that the ore-forming fluid is metamorphic fluid.The C isotope results show that in the late stage（Ⅳ）,calcite and sideriteδ¹³C value is-2.540‰～-0.736‰,indicating that the C in the ore-forming fluid is a mixed source.The fluids formed by metamorphism and dehydration of Ordovician-Carboniferous terrigenous clastic rocks and carbonate rocks may be the main source of ore-forming fluids.The results of sulfide in-situ S isotope at different mineralization stages,S isotope of single mineral of barite and whole rock of strata show that the totalδ³⁴S value is roughly equal to or slightly lower than+13.3‰～+15.5‰,higher than pyrite at different mineralization stagesδ³⁴S value（-12.3‰～+11.1‰）.The total fluidδ³⁴S value is consistent with the formation S isotope composition（average value is+11.32‰）,indicating that the ore-forming material mainly came from the formation,and the Pb isotope composition also reached the same conclusion.Through field observation and indoor microscopic analysis,five different phases of pyrite have been identified,which are fine-grained pyrite（Py0）in the stratum,pyrite（Py1）in the early quartz vein,pyrite（Py2）in the pyrite-semicite-quartz stage,pyrite（Py3）coexisting with polymetallic sulfide and pyrite（Py4）coexisting with carbonate and barite.There are obvious changes in the content of trace elements in pyrite at different stages.The content of Au in Py3 is the highest,followed by Py2,and there is an obvious positive correlation between Au and As in these two kinds of pyrite,indicating that the ore-forming fluid at the main mineralization stages is As-containing fluid.The results of trace elements in pyrite micro-area indicate that invisible Au is mainly present in pyrite in the form of solid solution,which is lattice gold or nano-grade inclusion gold,and other elements Co,Ni,Ag are mainly mixed in pyrite in the form of isomorphism or form Ag-Sb-Bi sulfide microinclusion in galena.The addition of deep fluids and immiscibility of fluids may be the important reasons for gold precipitation,and pyrite in sedimentary strata contributed less to the enrichment of gold.（3）Jinchangzi gold deposit is an orogenic gold deposit,which is a metamorphic hot brine superimposed supergene transformation gold deposit affected by orogeny.The formation of gold deposit had roughly gone through three processes:initial enrichment of Au,early sulfide mineralization and late sulfide secondary oxidation enrichmen.Due to the remote influence of the orogeny of the Qinling orogenic belt（West Qinling）in the late Indosinian period,the Weiningbeishan area showed a strong tectonic compression environment during the Late Triassic.With the attenuation of stress and structural adjustment,a structural system transition from a compression environment to an extension environment occurred.The compressional tectonic event resulted in relatively strong deformation and regional metamorphism of the materials in the middle and deep crust to form metamorphic fluids,which extracted metal elements in the strata at different depths during the rising process to form ore-bearing fluids rich in Au and CO₂.The ore-bearing fluid migrated upward along the deep fault to the shallow surface brittle fault,resulting in the precipitation of gold due to the reduction of temperature and pressure.（4）The fault structure is the most important ore-controlling factor in this area.The EW direction tectonic system formed by the extrusion in the near SN direction during the Indosinian period is the spre-metallogenic and the metallogenic synchronous structure.This tectonic system mainly includes folds in the EW direction and faults in the EW direction,as well as the associated shear faults in the NE and NW directions.The Devonian and Carboniferous interface faults in the direction of near EW and the associated interlayer faults are the main ore-guiding and ore-bearing structures in the mining area.The NE direction fault mainly plays the role of communicating with the EW direction fault to make the ore body more easily enriched and locally expanded.The ore-control effect of strata is mainly manifested in specific stratigraphic horizon and lithologic combination.The main ore-controlling horizon is the second member of Qianheishan Formation and the first member of Chouniugou Formation of Lower Carboniferou.The ore-control effect of lithology is mainly manifested in that the ore body often occurs on the lithologic interface with different capabilities in the stratum.The main lithologic associations are calcareous siltstone and silicified quartz sandstone,sandy limestone and silty mudstone,limestone and silty mudstone,etc.The results of chronology and geochemistry of diorite porphyrite show that the diorite porphyrite intrusion time is 145～146 Ma,belonging to the high-potassium calc-alkaline I-type granite intruded at the end of the Jurassic,which is formed by the rapid upinvasion of the melt formed by partial melting of the ancient lower crust crystalline basement related to the Alxa block.The diorite porphyrite intrusion time is later than the mineralization event,and it did not provide ore-forming materials.The surface oxidation outcrops（“iron caps”）,hematite-limonization,silicification,sericitization,pyritization,jarosite and other surrounding rock alterations,Au-Ag-As-Sb elements combined geochemical anomalies and geophysical anomalies with medium resistivity and medium-high polarizability are the main signs of prospecting in this area.（5）Based on ore-controlling factors and prospecting indicators,a comprehensive prospecting model based on geological-physical-chemical factors are constructed.In combination with the law of deep variation of mineralization,6 prospecting targets are delineated on the surface（including 2 Grade I targets and 4 Grade II targets）,and 3prospecting targets are delineated in the deep.The prospecting potential of each delineated target area is evaluated.Through the geochemical profile survey and deep drilling,the rationality of the target delineation has been verified,which has played a good demonstration role for the future prospecting work in this area.On this basis,the ideas and directions for the next step of prospecting are put forward.It is believed that the prospecting work in this area should focus on oxidized ore bodies in the short term and primary ore bodies in the medium and long term.The focus of the next step should be on the prospecting work of the 2 Grade I targets,as well as the potential evaluation of the associated cobalt in the entire Weiningbeishan area.