Ore Fluids And Metal Sources And Genesis Of The Daqiao Gold Deposit,Western Qinling Orogen

The western Qinling area is an important multi-metal metallogenic region in China.Daqiao gold deposit is located in the eastern part of the west Qinling orogenic belt,the transition between the northern edge of the south Qinling fold belt and the middle Qinling fold belt.The Minxian-Tanchang-Liangdang deep fault occurs in the north,and the Zhouqu-Chengxian regional fault occurs in the south.The Middle-Late Carboniferous and Triassic strata are mainly exposed in the region.NEE trending faults developed,and magmatic activities are weak with some Indosinian dikes distributed sporadically.The gold ore bodies are hosted in the siliceous breccia and siliceous rocks of the lower lithologic section of the Late Triassic formation.They are strictly controlled by siliceous breccias,and the ore bodies are distributed in NNE direction.The Daqiao gold deposit is mainly hosted in the siliceous rocks.Their chemical compositions consist of SiO₂,Al₂O_3,and Fe₂O₃.The mean content of SiO₂ is 80.21wt%.The ore rocks still contain other trace elements such as As,Ba,Sr.The grades of Au range from 0.06 to 9.27?g/g.There is no correlation between Au and other trace elements.The trace element characteristics of the ores indicate that the genesis of siliceous rocks is dissimiarity in Daqiao gold deposit.The sedimentary environment of siliceous rocks in Daqiao gold deposit is determined to be continental slope or marginal sea.A large amount of terrigenous debris was added during the formation of siliceous rocks.In the process of normal sedimentation,it is affected by submarine hydrothermal process,which may be the mixed product of terrestrial normal sedimentation and hot water sedimentation,and it is mainly controlled by terrestrial normal sedimentation.The microscopic observation shows that pyrite is disseminated in the ore and mainly consists of euhedral pyrite,subhedral-anhedral pyrite,with a small amount of framboidal pyrite.Analysis using electron probe micro analysis?EPMA?shows that pyrite contains Au generally.That indicates pyrite is one of the main Au-bearing minerals in Daqiao gold deposit.The contents of As are little.There was no correlations between Au and Cr,Tl,Pb,Co,Ag,As and S.The Co/Ni ratios in pyrite are generally more than 1,indicating that the ore fluids may be derived from magmatic hydrothermal fluid.The solubility curves of Au-As in Au-bearing pyrite show that gold may occurs in nano-scale native gold?Au⁰?and lattice gold?Au⁺?in the Daqiao gold deposit.Nano-SIMS elemental mapping analysis shows that most of the Au-bearing pyrites have multi-generation and multi-stage textures.The distribution of elements from the core to the outer zone displays a certain regularity:The contents of As are lowest in the cores,and are highest in the middle zone.Au distributed hetergenously in pyrite,mainly distributed in the outermost zone.Elemental mapping and line analysis showed that Au was positively correlated with³⁴S/³²S,suggesting that heavy sulfur(³⁴S)may help carry Au.Au-rich outermost zone is the main metallogenic event.The?³⁴S values of pyrites in Daqiao gold deposit range from-24.32‰to 14.66‰,which may be mainly derived from the mixture of magmatic sulfur,sedimentary sulfur,and seawater sulfur in different degrees.Compared with some gold deposits in the western Qinling area,the Au-bearing pyrites have obviously different sulfur isotopic compositions in Daqiao gold deposit,which are characterized by rich heavy sulfur.The?³⁰Si values of the siliceous rocks in Daqiao gold deposit range from-0.54‰to 0.2‰.This is between the?³⁰Si value of the hot water sedimentary siliceous rocks and the?³⁰Si value of the siliceous rocks deposited in the deep sea environment.This is similar to the?³⁰Si value of dissolved silicon in seawater and the?³⁰Si value of the hydrothermal sedimentary siliceous rocks in Qinling Devonian.The results indicate that the siliceous rocks of Daqiao gold deposit may form in deep-sea environment.This is hydrothermal sedimentary.The deep magmatic hydrothermal solution may bring some ore-forming materials.The ore-forming materials of Daqiao gold deposit likely derive from two sources:one is the Triassic gold-bearing strata.The other is that the deep magmatic hydrothermal fluid which carries a large amount of ore-forming materials invading along the fault,enriching the gold and favorable ore-forming materials in the formation,and move to the slip position of the fault and fold.With the change of physical and chemical conditions,SiO₂ supersaturated precipitates and cements the broken breccia to form silicified breccia,and gold deposits are formed by gold precipitation and enrichment.The genesis of Daqiao gold deposit should belong to sedimentary-reformation type.