Mineralogy Of Au-carring Minerals And Ore Fluids Geochemical Characteristics Of Yangshan Gold Deposit, Western Qinling

The Yangshan gold deposit is located in Wenxian, Gansu Province, discovered by the Twelfth Team of Gold Exploration Branch of Armed Police. The explored reserves of Au in Yangshan are beyond 300 t and has attracted interests of researchers many years, which is regarded as the largest Carlin-type Au deposit in China, even in Asia. The ore deposits are hosted in Sanhekou Formation of Middle Devonian, a set of clastic rock-carbonate formation, and ore body distribution and occurrence are strictly controlled locally by Anchanghe-Guanyinba fault zone. The fault zone is 10-12 km long and 3-5 km wide, a part of regional fault system, which extends to join the Lueyang-Maqu fault zone in the west and extends to join the Mianlue suture belt in the east. The ore deposit is divided into 7 ore sections, namely, Zhangjiashan, Guanyinba(or called Yangshan), Gaoloushan, Anba, Getiaowan, Nishan and Tangbogou from east to west.This study investigated the residence state of Au in the main gold carriers, arsenian pyrite and arsenopyrite, based on the field investigation and systemically thin sections observation under optical microscope, and using jointly electron probe microanalysis(EPMA), high resolution transmission electron microscope(HRTEM) and X-ray photoelectron spectrometer(XPS). Alterations correlated with Mineralization are silification, decarbonatization, kaolinization, and sulfidation. Four ore types are identified in Yangshan, including altered siltstone, altered phyllite, altered cataclastic carbonate and granodiorite(dike). Arsenian pyrite and arsenopyrite are the main Au carrier minerals. Their Back-scattered electron(BSE) imagines showed clear zoned texture, which imply multiperiodic hydrothermal activities. The pyrites are the As-rich species, containing 1-10 wt% As in the overgrowth rim. The elemental mapping of arsenian pyrite and arsenopyrite showed that the microareas of As-rich contain higher Au, vice versa, i.e., the distribution of Au correlates positively with As.It is inferred that gold is present in solid solution(Au+1) mainly in arsenian pyrite and arenopyrite, and a small fraction of them present in natural nanoparticles(Au0) in Yangshan gold deposit by the model proposed by Reich et al.(2005). HRTEM studies did not show nanoparticles in the arsenopyrite grains with up to one percent of Au, however, discovered abundant lattice dislocations in the Au-rich microarea.Although two of the states of lattice-bounded Au and nanoparticles of natural gold are confirmed by numerous previous studies, the depositional mechanism of Au into sulfides remains unsolved up to now. Considering the intimate correlation between Au and As in sulfides, it is the key to know the valence state of As in them. We succeeded to obtain a pristine surface of arsenian pyrite from Yangshan gold deposit by sputtering specimen with Ar+1 beam in the vacuum chamber of XPS, and analyzed the valence state of As in it. The detailed analysis before and after sputtering shows that the peaks of As3+(around 43.0 ev) are present on the surface exposed to the air, but its strength lowered rapidly with lasting sputtering, and disappeared finally. In contrast, the peak of As-1(around 41.0 ev) keep its shape, position and strength nearly unchanged. The results indicated that no As in oxidation state exists on the pristine surface of arsenian pyrite, the peak of As3+ are produced while the specimen contacts with the air. Whereas the depth of penetration by XPS is about the uppermost 10 nm of specimen, therefore, it is unlikely that Au1+ together with As3+ coupled substitute 2Fe2+entering into pyrite lattice. The so called As3+-pyrite proposed by previous study maybe occur in some special geologic setting, however, it has no wide implication on the formation of varied gold deposits. Combining previous studies on Carlin-type Au deposit, we suggest that the most likely state of Au in Yangshan Au deposit is solid solution(also called lattice-bounded Au) with some nanoparticles(Au0).In conclusion, we suggest that most of Au in Yang-shan gold deposit are solid solution and minor fraction as nanoparticles present in arsenian pyrites and arsenopyrites(this need to be tested further by HRTEM), and the lattice defects, vacancies result from the substitution of As for S facilitates Au as a large size ion(Au+1=1.37?) incorporating into arsenian pyrite and arsenopyrite.In addition to Carlin-type gold mineralization, here we report coarse grained pyrite-bearing quartz vein recognized recently through the tunnel of PD051 in Anba district. Significant amount of CO2-rich fluid inclusions were found in quartz, and microthermometric analyses showed that the total homogenization temperature ranges from 221 ?C to 310 ?C, the salinity from 2.0 to 7.2 wt.% NaCl equiv. The results analyzed Helium and Argon isotopic compositions of fluid inclusions show that 3He/4He ratios are within 0.0~0.08 Ra, and 40Ar/36 Ar ratios varies from 434.1 to 863.0.These are typical characteristics of orogenic Au deposits. The new observations suggest that Yangshan gold deposit in deep segment has similar feature to orogenic gold deposit, which is associated with the continental margin geological processes along the boundary between the north and south China blocks. This is important for future exploration in the region and elsewhere along western Qinling belt.