| The newly discovered Jiadi large-size and Damaidi medium-size gold deposits are the typical representatives of Carlin-type Au deposits with basaltic rocks as the main host rocks.The two deposits are located in the Lianhuashan anticlinal core and the secondary knead fold belt in the southeast wing of the Lianhuashan anticline in the north section of Nanpanjiang-Youjiang Carlin-type gold metallogenic area.The gold ore bodies are layered and stratoid.The gold ore bodies including one occurs in the Shibianti(SBT)and one occurs in the P3?.The gold ore bodies are overlapped in space.The host rocks are mainly basaltic volcanic breccia,tuff,brecciated basaltic volcanic breccia and brecciated tuff,and a small amount of basalt and brecciated limestone.To investigate the metallogenic process of the Jiadi and Damaidi gold deposits,we carried out systematically analysis of the metallogenic geological background,ore deposit geology,major and trace elements analysis,petrography,mineralogy,major and trace elements of gold-bearing minerals,isotopes(H-O,C-O,S,Pb,Hg)and fluid inclusions.We also compared the results with the sedimentary rocks-hosted Carlin-type gold deposits in Southwest Guizhou.This paper reveals the sequence of mineral paragenesis,ore-forming fluid properties,source of ore-forming material,ore-forming process,and the important similarities and key differences compared with the gold deposits hosted in the sedimentary rock in Southwest Guizhou.This study establishes the ore-forming model of the basalt-containing gold deposits,summarizes the prospecting criteria of Carlin-type gold deposits in basalt distribution area,and analyzes the prospecting prospect.This research mainly obtains the following understandings.(1)The minerals in Jiadi and Damaidi gold deposits formed during three stages,including pre-mineralization,hydrothermal mineralization and local oxidation stages.The hydrothermal mineralization stage can be further divided into the main mineralization and late mineralization stages.The minerals in Emeishan basalt formed during pre-mineralization stage mainly include orthopyroxene,clinopyroxene,plagioclase,magnetite,and a small amount of ilmenite and apatite.The minerals formed in the main mineralization stage mainly include arsenopyrite,arsenopyrite,jasperoid quartz(locally quartz),illite,(Fe)-dolomite(locally Ca-Mg-siderite),rutile and apatite.These minerals are mainly disseminated in ore.The minerals formed in the late mineralization stage mainly include calcite,realgar,stibnite,quartz and orpiment.These minerals are mostly veined and fill in the open space near the orebody.Chlorite,hematite and limonite locally occur in the surface rocks which formed during supergene oxidation.(2)Gold mainly occurs in arsenian pyrite and arsenopyrite in form of invisible gold.The Au-bearing arsenian pyrite and arsenopyrite are enriched in Au,As,Sb,Hg,Tl,Cu and other ore-forming elements.The arsenian pyrite and arsenopyrite formed during sulfidation,which leads to gold precipitation and mineralization.Sulfurization is a key factor in the precipitation of gold and other elements.(3)The ore-forming elements(Au,As,Sb,Hg,Tl)were significantly added into the ore during Au mineralization,and a small amount of Bi,Te,Se,Cd and Ag were also added.The addition of a large amount of K2O may be related to the clayization.CaO content is unchanged,which indicates that de-calcification is insignificant.The contents of SiO2,Fe2O3,CaO,MgO,TiO2 and P2O5are basically unchanged,but their existing forms have changed.Before Au mineralization,Si,Ca and Mg mainly exist in silicate minerals(e.g.,orthopyroxene,plagioclase,clinopyroxene).After Au mineralization,Si mainly exists in quartz and illite;Ca and Mg mainly exist in(Fe)-dolomite.Before Au mineralization,Fe mainly exists in orthopyroxene,clinopyroxene,magnetite,and ilmenite;After Au mineralization,it mainly exists in pyrite,arsenopyrite,and(Fe)-dolomite.Ti mainly exists in ilmenite before Au mineralization,and mainly in rutile after Au mineralization.P mainly exists in the magmatic apatite before Au mineralization,and mainly in the hydrothermal and magmatic apatite after Au mineralization.(4)The?DV-SMOWvalue of ore-stage quartz from the Jiadi and Damaidi is-56?-81‰,and the?18OH2Ovalue is 10.9?12.5‰,the ore-forming fluid may be mainly magmatic hydrothermal.The?13C value of ore-stage dolomite is-3.24?-6.15‰,indicating that the carbon is dominantly sourced from the deep mantle.The?18OH2Ovalue is 8.27?12.06‰,suggesting that the ore-forming hydrothermal may be mainly magmatic hydrothermal,but the metamorphic water can't be excluded.The?34S value of stibnite is-0.90?-1.90‰,the sulfur in ore-forming fluids may mainly come from deep magma.The Pb isotopic composition of stibnite shows the Pb is mainly sourced from the orogenic belt,with minor mixture of crust Pb.The?202Hg value of the whole rock is-0.63?1.38‰,and the?199Hg value is-0.02?-0.12‰,the whole-rock Hg isotope value suggests a magmatic source of Hg.According to the H-O,C-O,S,Pb,Hg isotope compositions,the ore fluid for the Jiadi and Damaidi may be mainly released from the deep magma.The magmatic hydrothermal reacted with the host rock during ascenting and mineralization,resulting in the isotope composition exhibiting a mixture of magma and sedimentary rock.(5)The ore-forming fluids of Jiadi and Damaidi gold deposits has the characteristics of low temperature(150?210?),medium-low salinity(8?12 wt.%NaCl eq.),low density(0.69?0.94g/cm3).(6)Jiadi and Damaidi gold deposits may belong to the same metallogenic system with Guizhou sediment-hosted Carlin-type Au deposits and other Carlin-type gold deposits in the area.They were formed in the same regional metallogenic event,and these gold deposits are most likely the products of magmatic hydrothermal mineralization in the same region.(7)According to the above results,this study revealed the metallogenic process and established the metallogenic model of the basalt-containing gold deposits.Based on H-O,C-O,S,Pb,Hg isotopic analysis and gravity and magnetic data,we suggest that the ore-forming fluids were released from the deep buried granitic magma.Ore fluids are enriched in ore-forming elements including As Au,As,Sb,Hg,Tl and volatile components including CH4 and CO2,and have the characteristics of high to ultra-high pressure.During Yanshanian,ore-forming fluids upwelled to the regional structural detachment surface between P2m and P3? along the regional faults.Some ore-forming fluids migrated laterally and reacted with the rocks near the regional tectonic detachment surface by water-rock reaction to form the SBT.Some ore-forming fluids migrated upward along the fault to the tuff or basaltic breccia in the interlayer fracture zone of P3?,and the ore-forming fluids kept migrating laterally due to the high porosity of the rocks.When the ore fluid converged at the culmination(e.g.,the anticlinal core,the dome),the ore-forming fluids reacted with the Fe-rich basaltic rocks.Orthopyroxene,clinopyroxene,plagioclase,magnetite,ilmenite and other minerals of basaltic rocks dissolved and released Fe2+ and etc.The released Fe2+ combined with S and As in the ore-forming fluid to form arsenian pyrite and arsenopyrite.The Au-HS complex was decomposed and Au was incorporated into the arsenian pyrite and arsenopyrite in form of invisible gold.Sulfurization results in gold-bearing arsenian pyrite and arsenopyrite,which lead to gold precipitation and enrichment,respectively forming gold orebodies in SBT and P3?.Meanwhile,the water-rock reaction also led to the formation of jasperoid quartz(locally quartz),(Fe)-dolomite(locally calcium-magnesium siderite),illite,rutile,and apatite.In the late ore stage,calcite,realgar,stibnite,quartz,and orpiment occurred as veins and filled in the open space near the orebody.(8)The prospecting criteria for Carlin-type Au deposits in basalt distribution area mainly include geochemical criteria(Au,As,Sb,Hg combination anomaly),the criteria of oxidized gold deposits,structural criteria(structural culmination,such as Lianhuashan anticline,SBT,and interlayer fracture alteration zone of Emeishan basalt formation),stratigraphic criteria(Upper Permian Emeishan basalt formation(P3?)),lithologic criteria(basaltic volcanic breccia and tuff which contains high porosity,complex lithology,and is enriched in Fe),and alteration criteria(silicification,pyritization,arsenopyrite,dolomitization and clayization).(9)The basaltic rock is a potential host rock for Carlin-type Au mineralization.The Lianhuashan anticline area or even the whole basalt distributed area with similar geological characteristics has a prospect of prospecting for Carlin-type gold deposits,for example,Shachang,Shangzhai and Hudu are favorable prospecting targets for Carlin-type gold deposits. |
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