Metallogenic Mechanism And Prospecting Target Optimization Of Laowan Gold Deposit In Tongbai Orogen,Henan Province

Located in the middle part of the Qinling-Dabie orogenic belt in China,Tongbai area has successively experienced Paleozoic accretion orogeny and Mesozoic collision orogeny,and is also an important gold producer in China.The predecessors have carried out a lot of research and obtained important understanding about the genesis of these vein gold deposits,but the contribution of the ore-hosting metamorphic strata and the covel magmatism to the mineralization has been controversial.Therefore,I select the Laowan gold deposit,which is located in the middle part of the Tongbai orogen and ranks the third in gold reserves in China,as the research object,and carries out detailed field geological work and mineralogical observation.On the basis of regional relevant literatures and data of gold-silver mineralization,tectonic evolution,magmatism,metamorphism and deformations,the petrochronology and metallogenic geochronology,ore deposit geochemistry,rock geochemistry and isotope study were systematically carried out to reveal the ore-forming tectonic background,petrogenesis,relationship between ore,metamorphic strata and igneous rocks,and propose ore-forming mechanism and process.Combined with the analysis and processing of surface exploration geochemical data,the peripheral prospecting potential was comprehensively evaluated,and the further prospecting targets were identified.The Laowan gold metallogenic belt starts from Xiaxiaowan area in the west and ends to Beiyangzhuang area in the east,composed of several ore sections.The³⁹Ar/⁴⁰Ar age of hydrothermal sericite in the Shangshanghe ore section of the Laowan gold deposit is 142.0±1.4Ma,represents the same Cretaceous mineralization event as the quartz polymetallic sulfide stage of the Laowan ore section.The U-Pb age of monazite closely associated with gold-bearing pyrite in Xiaxiaowan area is 203±13Ma,indicating that Xiaxiaowan and Laowan areas were formed from different gold mineralization systems.There are 10 types of pyrite in the Laowan gold metallogenic belt.There are three types formed in diagenetic period,contain undeformed Py0 and deformed Py D from the Guishan Formation,and Py Z from the Mesozoic concealed granitic pluton.Both Py0 and Py D have high Co,Ni content and low Co/Ni ratio.Compared with Py0,Py D significantly enriched in Au,Ag,Cu,Pb,Zn,Sb,Bi and metal elements,indicating that metamorphism and deformation are conducive to the enrichment of metal elements.The Py Z is relatively deficient in trace elements and enriched in Bi.The content of Ni is very low and the ratio of Co/Ni is generally greater than 1.There are 6 generations of pyrite in the mineralization period,and it can be divided into 7 types according to its occurrence location:Py X in the Xiaxiaowan deposit show the core-edge structure and their compositions of trace elements are very similar to Py0.Py S,which was found in the wall rock of gold-bearing quartz vein from the Laowan gold deposit,show similar structures and trace element compositions with Py X,indicating that the ore-forming fluid that formed Py X also affected the Laowan area.The ore-forming elements in Py1,which formed in the pre-mineralization period of the Laowan gold deposit are stable in lattice,and the composition and distribution of trace elements are closer to Py0,Py X and Py S.The contents of Au,Ag and Bi in Py2 that formed in the quartz-pyrite stage of the early period of the main mineralization were significantly reduced,and its trace characteristics suggested that the precipitation in the ore-forming elements might mainly come from the dissolution-reprecipitation of Py1,and the micro-and submicroscopic inclusions of Au-Ag-Te began to appear,accompanied by the precipitation of natural gold,electrum,and gold-bearing tellurides,indicating the addition of mantle-derived fluids.In the late period of main mineralization,Py3 and Py4 which formed in the quartz-polymetallic sulphide stage are closely associated with chalcopyrite and galena,respectively.The ore-forming elements in Py3 are most enriched,and there is a relatively obvious Au-Ag-Te-Bi harmonic association,showing the temperature of the ore-forming fluid is gradually reduced and the ore-forming materials are gradually exhausted.During the mineralization period,pyrite generally showed a transition trend from Py0 to Py Z.However,the Pengjialaozhuang and Beiyangzhuang ore sections only retain pyrite in the early period of gold mineralization,with Py3 and Py4 are absent.Pyrite in-situ sulfur isotope shows that theδ³⁴S value is 0.88-3.65‰,which is close to Py0,showing a signal of mantle-derived magmatism and intermediate-mafic igneous rocks.The absence of Triassic magmatism indicates that the sulfur of Xiaxiaowan gold deposit comes from the Guishan Formation.In-situ pyriteδ³⁴S value obtainef from the Laowan gold deposit are 0.73-6.72‰.Py1 have similarδ³⁴S values to Py0,while Py2 and Py3 gradually trend to Mesozoic granitic pluton indicates that both the Guishan Formation and the Mesozoic magmatism provided sulfur for gold mineralization,and the proportion of magmatic sulfur gradually increased.The lead isotope composition of Py X shows the characteristics of orogenic belt,indicating that its genesis is related to regional metamorphism,and the ore-forming material are mainly from the Guishan Formation.In the Laowan gold deposit,pyrites from the early period of mineralization show the characteristics of the transition between mantle and orogenic lead,which are similar to those of the Guishan Formation,but pyrites from the late period of the main mineralization has mantle-like lead isotopic compositions,and close to those of the Mesozoic igneous rocks,indicating that both the Guishan Formation and Mesozoic igneous rock contribute to the ore-forming fluid and materials of Laowan gold mineralization.With the progress of mineralization,the proportion of sulfur and lead from Mesozoic igneous rocks gradually increased.The protoliths of the amphibolite and mica schist from the ore-hosting Guishan Formation,are the early Ordovician continental tholeiite and the Devonian active continental forearc margin mature upper crust sediments,respectively.Their protoliths may carry a large amount of gold during the emplacement and deposition process.Multi-period metamorphism and deformation are conducive to the further enrichment of gold,Fe²⁺rich basalts can also provide enough reductant for the gold mineralization.Therefore,the Guishan Formation can be a potential gold source.Three period of magmatism were recognized in the Laowan gold metallogenic belt:Neoproterozoic,Paleozoic and Mesozoic.The Neoproterozoic magmatism（ca.750-683 Ma）intruded in the south of the Laowan fault,both of which are LREE-enriched peraluminous granites with strongly Eu depletion.Rb is strongly enriched and Ba is relatively depleted.They are all S-type granite and formed in the partial melting of Paleoproterozoic upper crust sediments caused by the underplating of basaltic magma under the background of continuous subduction in the Neoproterozoic,their lead isotope show that the southern Qinling continent has not been completely split at this time.The Paleozoic magmatism is represented by two types of dykes,granite-porphyry and granodiorite,which are filled in the Songpao fault.Their zircon U-Pb ages are 424.4±1.7 Ma（MSWD=1.1）and 380.0±1.3 Ma（MSWD=0.8）,respectively.Granodiorite is enriched with large ion lithophile elements（LILEs）and depleted with high field strength elements（HFSEs）,while granite porphyry has a higher degree of evolution and more crust source components,both of which are derived from remelting of the Qinling Group and mixed with depleted mantle-derived magma,which were formed during and after the accretion orogeny.The Mesozoic magmatic rocks mainly composed of a lamprophyre and granitoids,which were represented by Laowan pluton,concealed granitic pluton and granite breccia in lamprophyre.The apatite U-Pb age of the lamprophyre is 136.5±7.8Ma（MSWD=1.3）,and the zircon U-Pb age of the three granites is 134.2±1.1Ma（MSWD=0.6）,129.8±0.7Ma（MSWD=1.3）and 135.9±0.7Ma（MWSD=1.1）,respectively.The lamprophyre is enriched in LREE and LILEs and has a high Th/Ta value and significantly lower Nb/U ratio than MORB and OIB,which proves the existing of enriched mantle related to subduction fluid.The Mesozoic granitoids are all adakitic,which are enriched in Rb and U and depleted in Nb and Ta.They show characteristics of magmatism related to subduction and have relatively enriched Sr-Nd-Hf isotope composition.It is inferred that the lamprohphyre,concealed granitic pluton and granite breccias are derived from the enriched mantle metasomatized by the lower crust of the North Qinling units,while the Laowan pluton are derived from the enriched mantle source area metasomatized by the lower crust of the South Qinling untis.The composition of zircon trace elements and apatite volatiles shows that the Mesozoic magmatic rocks from the enriched mantle have more oxygen fugacity（ΔFMQ+0.65 toΔFMQ+3.83）suitable for gold precipitation,and volatile components enriched in SO₃and depleted in Cl,which show potential for the formation of large gold deposits.The relationship between composite orogeny and gold mineralization in the Laowan gold metallogenic belt can be summarized as follows:a large amount of metal elements were carried when the protolith of the Guishan Formation was formd in the Paleozoic Ordovician,and then further enriched by the greenschist-amphibolite facies metamorphism in the Carboniferous,as well as the regional strike-slip shear and ductile deformation in the Late Permian.The shallow metamorphism occurred in the Xiaxiaowan area during the Triassic by the plate withdrawal and uplift extracted the ore-forming materials in the Guishan Formation,thus forming the Xiaxiaowan gold deposit.In the early Mesozoic,transition of regional tectonic environment from compression to extension produced a brittle-shear fault system in the Laowan area.Subsequently,the partial melting of metasomatized lithospheric mantle formed the alkaline magma with high oxygen fugacity,SO₃-enriched volatile.The magma then migrated upward along the Songpao fault and other regional trans-lithospheric fault systems,and occurred continuously fractionation and evolution,bringing a large number of hydrothermal fluids,ore-forming elements and heat,activating the pre-enriched metal elements in the Guishan Formation and forming the ore-bearing hydrothermal fluid from Pengjialaozhuang to Beiyangzhuang area.The ore-bearing fluids migrated along and filled the fault system,forming early pure-white quartz vein.With the further upwelling and evolution of magma,the conceal granitic pluton under the Guishan Formation in the Laowan-Shangshanhe ore block was formed,which together with the ductile-shearting to cause the difference of the ore-control structure in the Laowan-Shangshanhe ore sections.Te-Bi component and reducing sulfur in mantle-derived magma can efficiently extract the ore-forming elements in the fluid and existing minerals,and form gold-bearing hydrothermal fluids with the dissolved volatiles,and then inject it into the ore-forming fluid.This mechanism caused the main gold mineralization.With the process of mineralization and the depletion of the ore-forming material from the Guishan Formation,the proportion of mantle-derived magmatic material is increasing generally.The addition of atmospheric water and rise of ore-forming fluids also cause the temperature decreases and the pressure changes,which lead to the highly efficient deposition of Au,forming high-grade gold bodies in the Laowan-Shangshanghe sections.The Pengjialaozhuang and Beiyangzhuang sections at both ends are far away from the magmatic hydrothermal center,which mains less reconstruction of mantle-derived magmatic hydrothermal fluid,lead to poor gold grade.A prospecting prediction was carried out for the Xiaxiaowan-Laowan gold metallogenic belt and its sunrrounding areas,in which three A-level and two B-level metallogenic favorable areas were identified using an Evidence Weighting Model.Based on the analysis of the surface geochemical exploration of the Laowan gold deposit,the main ore-forming element association is identified as Au-Ag-Cu-As-Sb-S.The Spectrum-Area model is established with reference to the location of ore-bearing drills to effectively separate the background and anomalies.Combined with the metallogenic model,the prospecting targets are optimized.