Metallogeny And Metallogenic Prognosis Of Gold Deposits In Southeastern Guizhou Province, China

The southeastern Guizhou district is located on the southwestern margin of the Xuefeng belt,which is on the southwestern part of the Jiangnan orogen.This district is one of the most important parts of the Xiang-Qian gold belt in South China,and it has a long history of gold mining and has long been treated as a promising gold prospect with great potential.Despite the wide spread gold deposits and occurrences,their sizes are small and the metallogenic types are simple.Mining has been active over the past decades,by contrast,the outcome of geological exploration is not so fruitful.Furthermore,the genetic type,geochronology,source of the gold,and source and evolution of the ore-forming fluids are still debated.In this paper,the author had undertaken geological survey to investigate all the gold deposits and occurrences in this district,and then selected the Bake?anticline-hosted?,the Hupen?shear zone-hosted?and the Pingqiu?both anticline-and shear zone-hosted?gold deposits for ore genesis study.Argon-argon dating on sericite samples gave high precise constraint on ore-forming ages,and in-situ LA-ICPMS and LA-MC-ICPMS performed on different pyrite types resulted in the trace elements and sulfur isotope variations,which helped to discuss the evolution of the ore-forming process.Such analysis,together with study on ore-forming fluids,helped to constrain the ore genesis.On the basis of study on geodynamic setting,ore-forming process,ore-controlling factors,spatial-temporal structures of the gold mineralization and the localization of orebodies,this paper summarized the mechanism of gold deposition and localization.Further,the author took out detailed analysis of the ore-controlling factors,spatial-temporal structures of the gold mineralization,alteration,mineral typomorphism,and geophysical and geochemical anomalies based on GIS platform,resulting in a series of prediction-evaluation vectors.And then the weights-of-evidence method based on MORPAS GIS was applied to delineated prospecting targets across the district.The results can be used to refine the genetic model,and also give implications for further exploration.The southeastern Guizhou district is dominantly hosted by the Neoproterozoic Xiajiang Group,a⁸000 m turbidite sequence which is composed of thickly-bedded sandy-slate,volcanic tuffaceous slate,and a few marble units.Gold mineralization is hosted in Neoproterozoic lower greenschist facies metamorphic rocks and controlled by fold-related structures.Gold deposits and occurrences are mainly located in six goldfields,i.e.,the Kentou–Laziping goldfield,the Moshan–Xiada goldfield,the Nanjia–Koudong goldfield,the Tianhuashan–Luoli goldfield,and the Gubang–Shuikou goldfield.On the basis of the principle differences on ore-forming process,characters of the orebodies,the ore-controlling structures,and the mineral associations,four mineralization types can be distinguished:fold-related lode type,shear zone-related lode type,disseminated type,and placer gold.The shears zones are distributed near the hinge zones of anticlines.The typical presentative of these shear zones are cloepacked cleavages,intensive phyllitization of the slate,and sheared folds.The rocks are almost vertical in the center of the shear zones.The geodynamic manifestation of the shearing is the rheological characteristic of semi-solid rocks,rocks in shear zones show tensile failures,allantoid structures,and the quartz veins show expansion in the hinge zones and reduction on the limbs.Under the microscopy,sheared structural lens,S-C structures,pyritic pressure shadows,and rotation,kink and domicile effect of chlorite can be observed.All the deformations indicate a S-N-trending thrust shear.Both the anticline-and shear zone-host lode veins are strictly controlled by the structures.Vein types of the anticline-hosted lode veins include bedding-parallel and discordant types,with saddle-reefs and their down limb extensions dominating but with lesser discordant types.Laminated veins fulfilled both the shear fissures and shear cracks.Four ore-forming stages could be distinguished across the whole district:?1?sericite–pyrite–arsenopyrite-quartz;?2?gold-pyrite–arsenopyrite–quartz;?3?gold-polymetallic sulfide–quartz;?4?pyrite–carbonates–quartz.Gold is mainly precipitated in the former three stages,and much of the gold is?m-to mm-sized grains and occurs as fracture-controlled isolated grains or filaments in quartz,galena,sphalerite,pyrite,and wallrock.The major sulfide phases are arsenopyrite and pyrite,with minor sphalerite,galena,chalcopyrite,and rare pyrrhotite,marcasite,and tetrahedrite.Gangue minerals include quartz,sericite,carbonates and chlorite.The siliciclastic nature of the metasedimentary rocks has a rather limiting effect on their response to the interaction with hydrothermal fluids.Only weak and rare hydrothermal alteration is developed in zones with less than 1 to 3 m in width.The alteration is characterized by the discoloration of the metasedimentary rocks,due to the alteration including silicification,sericitization,and sulfidation.Sulfidation of the wallrocks is manifested by disseminated euhedral arsenopyrite and pyrite porphyroblasts?up to 2 cm?.Three types of fluid inclusions could be distinguished from the auriferous veins across this district:?1?pure CO₂ inclusions;?2?carbonic–aqueous fluid inclusions;and?3?aqueous inclusions.Fluid inclusions from the Pingqiu deposit yielded final homogenization temperatures of 171–396°C,and calculated salinities of 0.2–9.86 wt%NaCl equiv.Among that,fluid inclusions from the anticline-and shear zone-hosted veins gave highly overlapped ranges,indicating the same fluid reserves.Laser Raman analysis and quantitative analyses of volatile composition in bulk fluid inclusions indicated that the gas phase of fluid inclusions is dominated by H₂O and CO₂,followed by lesser amounts of N₂,CH₄,C₄H₆,SO₂ and H₂S.Fluid inclusions from the Bake deposit yielded final homogenization temperatures of 143–391°C,and calculated salinities of 0.88–15.37wt%NaCl equiv.Fluid inclusions from the Hupen deposit yielded final homogenization temperatures of 164–336°C,and calculated salinities of 2.07–10.86 wt%NaCl equiv.The fluid inclusion study suggests an ore-forming aqueous-carbonic fluid,with minor CH₄,trace N₂,0.2–15.37 wt%NaCl equiv.at low–moderate ore-forming temperatures.Hydrogen isotope values from the Pingqiu,Bake and Hupen yielded-59.0‰–-46.9‰,and the calculated?¹⁸O_H2O values range between-3.52–9‰,and all these values fall into the range of most orogenic gold deposits.The?D and calculated?¹⁸O_H2O of ore-forming fluids are clustered tightly and most plot are within the overlap of the magmatic and metamorphic field,and some analyses move to meteoric water.LA-ICPMS analysis performed on syngenetic framboidal pyrite in Pingqiu revealed that it is rich in a series of elements including Ti,V,Mn,Ni,Cu,As,Se,Ag,Sn,W,Pb and Bi,whereas the metamorphic pyrite has elevated As and W,but other element concentrations decline apparently.Gold and arsenic concentrations are closely correlated in hydrothermal pyrite from early stages and show apparent increase,ranging from 0.31to 74.5 ppm and 4410 to 26200 ppm,respectively.Hydrothermal pyrite from late stage show a significant increase in Sb concentrations,and bigger Co/Ni ratios than other pyrite types.Variations in?³⁴S values are consistent with changing fluid compositions.Measured?³⁴S values for Py0 and Py1 are very similar and vary from+12.41‰to+16.99‰,indicating that syngenetic-diagenetic pyrite was produced by bacteriogenic reduction of seawater sulfate.Py2 from alteration zones,and Py3 and Py4 from early quartz vein stages,have?³⁴S values in the narrow range of-2.48‰to+1.73‰.The latest,Sb-rich Py5 shows a shift to more positive?³⁴S values varying from+7.56‰to+22.98‰,which reflects a change in fluids compared to the earlier hydrothermal pyrite.Similarly,LA-ICPMS analysis performed on syngenetic framboidal pyrite in Bake revealed that it is rich in almost all the elements analyzed.However,Au and As concentrations from hydrothermal pyrite are much lower than those of the hydrothermal pyrite in Pingqiu.Diagenetic pyrite has?³⁴S values in the narrow range of+9.27‰to+10.75‰,whereas hydrothermal pyrite shows a broad range?+6.81‰to+17.42‰?,based on LA-MC-ICPMS analyses.High-resolution stepwised ⁴⁰Ar/³⁹Ar dating was performed on two hydrothermal sericite samples from the Pingqiu deposit.Both the samples yield well-defined age plateaus with nearly 80%39Ar and plateau ages of 425.7±1.7 Ma?MSWD=2.48?and425.2±1.3 Ma?MSWD=1.20?.Overall,the similar setting,paragenesis of vein minerals,fluid chemistry,and wallrock alteration developed across the whole gold district suggest a similar setting and ore genesis for these deposits.Thus,they should have been formed contemporaneously.Combined the ages with other regional tectonic-thermal event,the author inferred that gold mineralization was post-peak regional metamorphism and occurred during the later stages of the Caledonian orogeny.Three periods of mineralization could be recognized,i.e.,mineralization during fold,mineralization during shear zone,and mineralization during supergene enrichment.During the early stage of the Caledonian orogeny,the Meso-to Neoproterozoic units experienced intensive folding deformation,resulting in not only a series of NE-trending anticlines and their parasitic folds on the limbs,but also crustal thickening and geothermal gradient increasing,which trigger the metamorphic devolatilization in the underlying Mesoproterozoic units.The released ore-forming fluids are low-salinity CO₂-H₂O-H₂S at low–moderate temperatures.Gold in the ore-forming fluid is dissolved and transported at the onset of devolatilization as Au?HS?₂^-.With more and more fluids released into intragranular fractures,the fluid pressure increases.When fluid pressure became higher than hydrostatic pressure,the fluids were upward flow through fractures and deposited at detachments in anticlines,and thus formed the anticline-hosted quartz vein system.Along with the ongoing compression,the anticlines became tighter,making the early-formed quartz veins to be thicker in the hinge zones and thinner on the limbs.In the late stage of the Caledonian orogeny,the horizontal tectonics stress became weak and continued fold tightening after gold mineralization resulted in a series of NE-trending shear zones cutting the vein-bearing folds?Fig.2?,which are inferred to have developed to accommodate the stress regime.Quartz veins hosted by shear zones are small and lack in sulfides,making the ore grades to be very low.The shear zones made the ore-forming system to be open and the ore-fluids were mixed with meteoric water,and also a significant increase in Sb concentrations.The supergene enrichment led to the further enrichment locally,however,this was largely confined to the early-formed mineralization.This paper also looks into the spatial occurrence styles and localization patterns of the orebodies.Gold mineralization is structurally confined to NE-trending anticlines and shear zones.The first-order NE-trending anticlines host the ore belt,whereas the detachments in anticlines and shear zones host the quartz vein systems.Tuffaceous-rich units,especially the contact between tuff and sandstone or slate,tend to be the best place for deposition.Quartz vein systems in anticlines show equidistant distribution patterns with intervals between 50 and 60 meters.Within the anticlines,auriferous quartz veins developed preferentially in dilational jogs at the hinge zones,and changes in strike at the transition of two parasitic folds.Native gold is more common in intersection of shear zones and anticlines.The observed concentrations of visible gold occur at grain boundaries and in fractures in hydrothermal pyrite and arsenopyrite.By summarizing the metallogeny and analyzing the geophysical and geochemical information,the ore-bearing units,1:200,000 gravity,1:10,000,000 aeromagnetic data,and 1:200,000 stream sediment survey data were chosen to set up the evidence comprehensive information prospecting model and quantitatively outlined the probable Au prospect in this region.Ten prospects were achieved for further study,i.e.,two A-class prospects,four B-class prospects and four C-class prospects.According to our practical working conditions,and the more geological data could be achieved,this study choose the Pingqiu and Hupen as the typical sample for deposit-scale metallogenic prognosis.When combined the patterns of structural control of ore deposition,and the actual conditions of the mining,this study put forward four targeting areas in the deep and another four outside the present mining area.And on the basis of tectono-geochemical study,this study set up the model of primary superimposed halo in Hupen deposit.In order to evaluate the Au concentrations in shear zones,bulk sampling and fire-assaying method were introduced.According to the fire-assaying method,ore grades of shear zone-hosted veins range between 0.13 g/t and 9.02 g/t,with half of the samples are above 1 g/t.By contrast,if we took the traditional method,namely analyzed 200 g samples by atomic absorption spectroscopy,the ore grades were extremely low except one extremely high value,which was obtained from the sample located at the contact between the auriferous quartz vein and the host rocks.From the fire-assaying method,a promising gold potential can be placed to the shear zone-hosted quartz vein systems.