Epithermal Gold Mineralization In The Northern Lesser Xing'an Range Area,NE China

The Lesser Xing'an Range is one economically important area of northeast China,with endowed mineralization of Au,Mo,Ag,Pb,and Zn etc.,and a spectrum of deposit types comprise porphyry,skarn,and epithermal types etc..Porphyry Mo and skarn polymetallic deposits generally occur in areas where granitic intrusions are prodominant,whereas epithermal gold deposits are developed in volcanic basins.We recently did a lot of works on epithermal gold deposits in Lesser Xing'an Range area,as Tuanjiegou,Sandaowanzi,Dongan,Gaosongshan,and Yongxin.Considering the issue on Au mineralization of epithermal system,exemplified by the Dongan,Gaosongshan,and Tuanjiegou,a combination of field mapping and sampling with geochronology,fluid microthermometry,and geochemistry has been conducted in this study,with an aim to decipher the magmatic-hydrothermal processes that have essential controls on the mineralizing capability and eventually ore depositions at shallow-crust levels.We also quantified the paleodepths of ore deposition,which indirectly provide constraints to the erosion and preservation degrees of individual deposit.1.Ore veins are generally hosted in the coeval andesitic to rhyolitic suite and felsic porphyry of early Cretaceous in age,and the distribution of veins is largely controlled by the normal faults related to the volcanic activities,and locally by the brecciaed zones being in association with hydrothermal eruption.Ore zonation vertically grades from upper thin vein-filling breccias through intermediate quartz bands and hydrothermal eruption breccias to lower sulfide-rich quartz veinlets,with increasing proportions of pyrite and marcasite with increasing depths.Alteration system centered on the ore veins comprises the innermost pervasive phyllic alteration,outer intermediate argillic alteration,and distal propylitic shell.Multiple events of fluid pulses are recognized based on the crosscut relationships,including quartz-pyrite-hematite stage(I),ivory massive quartz(II),gray quartz-sulfides stage(III),and clear quartz-carbonate±fluorite stage(IV).2.Inclusion microthermometry results show the Au and Ag precipitation from low temperature,dilute,near-neutral,reduced hydrothermal fluids.The inclusion types in hosting gangue minerals are relatively simple and dominated by liquid-rich inclusions with minor vapor-rich ones.Except for Tuanjiegou system associated with relatively high temperature fluids(167?353?),the fluids in epithermal systems have temperatures of<300?and salinities of<10 wt.%Na Cl equiv.They are initially exsolved from volatile-saturated melts,and are weakly oxidized.After these stored fluids are liberated from reservoir,a transition to reduced state occurred during magmatic fluids ascent along fractures and mixing of meteoric water.Evidence of textures and mineralogy suggests the cooperation among boiling,water-rock interaction,desulfurization,and mixing of two fluid components that drove the Au precipitation due to the lower metallic solubility in the late near neutral,reduced fluids compared to the early weakly oxidized,acidic magmatic fluids.3.Dates of magmatic zircon and hydrothermal minerals suggest that the epithermal precious metal mineralization occurred in a short duration between 110 Ma and 100 Ma.Andesite-rhyolite-granite porphyry assemblage at Gaosongshan have ages of 108-102Ma,overlapping with an age of 98±1.6 Ma of hydrothermal quartz within errors.The emplacement timing of Andesite-rhyolite-granite porphyry suite is at 110-107 Ma,coincident with the hydrothermal sericite of 107.24±0.55 Ma.Dates of dacite-granodiorite porphyry suite at Tuanjiegou span a range from 110 Ma to 103 Ma,consistent with an age of 113.8±4.0 Ma of pyrite collected from ores.4.A genetic bond of Au-Ag mineralization with andesitic to rhyolitic eruption is emphasized here.These resulting volcanic rocks have calc-alkaline affinities and are arc-related,characterized by enriched LREE and LILEs and depleted HREE and HFSEs.A combination of depleted both Sr-Nd isotope and Hf isotope and whole-rock geochemistry suggests that andesitic magmas are sourced from the mixing between crust-and mantle-derived melts and subsequently experienced fractionation of silicate minerals to produce the more felsic melts during the magmas ascent.The segregation of fluids from melts contributes the primary fertile solutions into hydrothermal system.5.We determined that the highly evolved granitic melts are hydrous(>4.0 wt.%H₂O)and oxidized(FMQ>2.0±2.0)using the mineral oxybarometers,facilitating the transports and concentrations of Au in melts.By contrast,the ore-related rhyolitic magmas in the Gaosongshan system is relatively dry(?2.5 wt.%H₂O)and reduced(FMQ 0.12±1.26).Magmas parental to the Tuanjiegou hydrothermal system were stored at 3.42-4.31km levels and became volatile-saturated,ensuring abundant hydrothermal fluids to be liberated.6.A framework of magmatic-hydrothermal processes in Creataceous is summarized here.The decompressional setting is related to the deep subduction of Paleo-Pacific slab downward Eurasian Continent at Early Cretaceous,and as a result,the fluid-modified lithosphere mantle was remelted to produce the mafic magmas that then underplated at the crust and mantle boundary due to the density similarity.A mixing between heat mafic melts and felsic melts generated by the heating-derived remelting of lower crust materials must have occurred to produce the intermediate(andesitic)melts.Magmatic evolution totally towards silicic compositions is dominated by the fractional crystallization of silicate minerals during ascent,and the incompatible Au would have been concentrated during hosting melts ascent.High-silica melts were eventually stored at shallow-crustal levels(?4 km)and reached the H₂O-saturation boundary to exsolve hydrothermal solutions.Low-density fluids rapidly transported upward along the fractures and experienced the extensive boiling,water-rock interaction,mixing with cool meteoric waters,and desulfurization.These processes led to the dramatic changes in physicochemical environments and thus the deposition of Au and boiling-indicator minerals(e.g.,adularia and bladed carbonate).7.Fluid geobarometer and apatite low temperature thermochronology are used to determine the paleodepths of ores in individual low sulfidation deposit in the Lesser Xing'an Range area.Under hydroclastic pressures,fluids in the Gaosongshan system is deposited at pressures of 2.3±0.9 MPa,equivalent to a hydrostatic depth of about 200-300 meters below paleowater stable,whereas the Dongan fluids at 3.6±1.1 MPa pressures,with a paleodepth between 300 m and 400 m.An elevated pressure of 7.6±1.9MPa characterize the Tuanjiegou ores,corresponding to a?900 m paleodepth.These paleodepth estimates are consistant with the time-temperature modelling of apatite low temperature thermochronology,such as the maximum emplacement depths of<0.7 km for Dongan granite porphyry and of 1.0 km for Tuanjiegou granodiorite porphyry.