Geological Characteristics And Metallogenic Mechanism Of Orogenic Gold Deposits In The Southern Area Of Tibetan Plateau

Gold deposits,such as Bangbu,Mayum,Zhemulang,Nianzha,and Shangxu,have been recently discovered along the Indus-Yarlung Zangbo suture zone and the Bangong-Nujiang suture zone.These gold deposits are important for the further study of orogenic gold deposits in the southern area of Tibetan Plateau.The Shangxu gold deposit is located in south of the middle part of Bangong-Nujiang suture zone in northern Tibet.The orebodies,auriferous quartz veins,are hosted in the Middle-Lower Jurassic metamorphic flysh Mugagangri group and mainly controlled by NWW-trending faults.Wall rock alteration is not obvious.Native gold is dominantly distributed in the cracks of quartz,with minor in the cracks of pyrite.The ore consist of quartz,pyrite,galena,and sphalerite.Four stages of mineralization were identified,including quartz stage(S1),quartz-pyrite stage(S2),quartz-polysulfide stage(S3)and carbonate stage(S4).The precipitation of gold mainly occurs in S2 and S3.The ore-forming fluid was CO2-H2O-NaCl rich with homogenization temperatures of 142.0–409.8?(with peak values of 160–190? and 280–290?),salinities 2.74–9.74 wt% NaCl equivalent,and density 0.55–0.97 g/cm3.?18Ofluid(-0.1–8.4‰),?DV-SMOW(-123‰ to-89‰)and ? 13CV-PDB(-0.9‰ to 0.3‰)for quartz veins suggest ore-forming fluids were mainly metamorphic in origin,with some addition of formation water and meteoric water.The sulfides in S3 has a narrow ?34S range of-4.5 ‰ to-1.0 ‰ with an average of-3.1 ‰,which is consistent with the ?34S of pyrite from the wall rocks,indicating sulfur mainly derived from the metamorphic flysh Mugagangri Group(the youngest age of detrital zircons was 223 Ma).Pb isotopic data indicate that the lead source was highly mature and derived from the upper crust,maybe including the contribution of melange in orogenic belts.Metallogenic epoch is closed to the deformation and metamorphic processes of melange belt at Late Jurassic-Early Cretaceous.Gold mineralization was associated with collision of Lhasa and Qiangtang terranes.The Bangbu gold deposit is in the southeastern part of the Langjiexue accretionary wedge of the Indus-Yarlung Zangbo suture zone.Ore bodies(auriferous quartz veins)are controlled by the E-W-trending Qusong-Cuogu-Zhemulang brittle-ductile shear zone.Quartz veins at the deposit can be divided into three types: pre-metallogenic hook-like quartz veins,metallogenic auriferous quartz veins,and post-metallogenic N-S quartz veins.Four stages of mineralization in the auriferous quartz veins have been identified:(1)Stage S1 quartz+coarse-grained sulfides,(2)Stage S2 gold+fine-grained sulfides,(3)Stage S3 quartz+carbonates,and(4)Stage S4 quartz+ greigite.Fluid inclusions indicate the ore-forming fluid was in a CO2-H2O-NaCl-CH4-N2 system.?18Ofluid(2.0–7.9‰),low ?DV-SMOW(-104‰ to-49‰)and ?13CV-PDB(-11.7‰ to-7.0‰)for auriferous quartz veins suggest ore-forming fluids were mainly metamorphic in origin,with some addition of formation water.Quartz vein pyrite has ?34SV-CDT values of-0.1‰ to 3.4‰,indicating sulfur mainly derived from the deep metamorphic water.Pb isotopic data indicate Bangbu ore-forming materials were probably derived from the Langjiexue accretionary wedge.The Mayum gold deposit is in the Zhongba microcontinental block of the Indus-Yarlung Zangbo suture zone.Ore bodies(auriferous quartz veins)are hosted by light metasedimentary rocks of Devonian,and controlled by brittle-ductile shear zone.Three stages of mineralization in the auriferous quartz veins have been identified:(1)Stage S1 quartz+magnetite,(2)Stage S2 quartz+sulfides +gold,and(3)Stage S3 quartz+carbonates.Fluid inclusions indicate the ore-forming fluid was in a CO2-H2O-NaCl-CH4-N2 system.The ore-forming fluid was in a CO2-H2O-NaCl system,with homogenization temperatures of 240–280?(with peak values of 240–250? and 270–280?),salinities 2.02–3.76 wt% NaCl equivalent,and density 0.76–0.93 g/cm3.?18Ofluid(4.3–11.7‰),low ?DV-SMOW(-146‰ to-79‰)and ?13CV-PDB(0.3‰ to 0.4‰)for auriferous quartz veins suggest ore-forming fluids were mainly metamorphic in origin,with some addition of formation water.The total gas age of sericite from auriferous quartz veins is 59.5 Ma,implying that the gold mineralization was formed during the main collisional metallogenesis related to the Indo-Asian collision.The regional metallogenic model of orogenic gold deposits in the southern area of Tibetan Plateau is established on the basis of comprehensive analysis and contrast about geological characteristics and metallogenic mechanism of orogenic gold deposits in the southern area of Tibetan Plateau.The model can be explained as followings: CO2-dominant metamorphic fluids were generated by the syn-peak metamorphism caused by continental impact related to the Tibet-Himalayan orogeny or Qiangtang-Lhasa orogeny.These fluids were transported along the regional structures.The ore-forming materials were probably extracted from the deep metamorphic rocks when the CO2-rich metamorphic fluids migrated upward.Sulfur-bearing fluids were capable of transporting a significant amount of gold when they migrated through a complex fracture network.The second-or third-order structures provided suitable foci for gold precipitation.When the ore-forming fluids migrated into the second-or third-order structures,the pressure of the fluids dropped rapidly resulting in fluid boiling.During fluid boiling,reducing species such as CO2 and CH4 entered into the gas phase,which reduced the stability of the complex of gold.That precipitated gold efficiently.Fluid boiling,caused by declining pressure,was probably responsible for gold deposition in the second-or third-order structures.Based on comprehensive analysis and contrast about geological characteristics and metallogenic mechanism between orogenic gold deposits in the southern area of Tibetan Plateau(Shangxu,Bangbu,and Mayum)and typical orogenic gold deposits,we get some conclusions that:(1)orogenic gold deposits in the southern area of Tibetan Plateau were discovered in collisional orogens,but typical orogenic gold deposits were mostly formed in accretionary orogens;(2)orogenic gold deposits in the southern area of Tibetan Plateau(Shangxu,Bangbu,and Mayum)were formed relatively latter,but typical orogenic gold deposits were formed earlier;(3)the former and the latter were both controlled by the second-or third-order structures of large scale brittle-ductile shear zone;(4)the former were hosted in light metamorphic sedimentary rocks,while host rocks of the latter contained mafic and ultramatic volcanic rocks,intrusive rocks,BIF-chert,and greywacke;(5)the ore-forming fluids of orogenic gold deposits in the southern area of Tibetan Plateau,mainly metamorphic in origin,were in a CO2-H2O-NaCl±CH4 system,and characterized by CO2-rich,medium to low temperatures,and low salinity,which were similar to those of typical orogenic gold deposit;(6)the source of ore-forming material of the former and the latter were both controversial,namely,probably derived from wall rocks or deep origin.