The Study On The Tectonic-fluid System And Metallogenic Mechanism Of Shuangwang Gold Deposit In Shaanxi Province

Shuangwang gold deposit is located in Fengtai basin between the east Tsingling and West Tsingling. The gold mineralization is bound up with na-modificated brecciated zone in space. Shuangwang gold deposit is one of a large deposit in Tsingling areal which is significant with its special attitude and genetic.The NW Shiziling anticlinoria with some accompanying folds in wings composed the frame of the regional structure. The axial direction is NWW-SEE, and a few Pb-Zn and Au deposits (or spots) hosted in the plunging crown of the axis. Shang-Dan discordogenic fault and Shanyang-Fengzhen fault are the regional faults in this area. Regional magmatism has been found from Caledonian to Yanshan, and the largest magmation was occured in Indo-Chinese epoch. The lithology consists in acidic rock and neutral rock. Baoji rock mass, Taibai rock mass and Ningshan complex are the main magmatic bodies.Shuangwang gold deposit is located in the Wangjialeng basin which is the secondary basin of Fengtai basin. The deposit lies in the north branch of Yindonggou subsidiary anticline, Shiziling anticlinoria. It is seated in the area with the length of 11.5km between Hongyahe Wangjialeng and Taibaihe Wangjiazhuang, consisted by some breccial rocks in all size.The Devonian neritic microclastic and carbonate rock series is the main formation in this deposit area. The sequence from bottom to top of the formation includes the Lower Devonian Wangjialeng formation, Middle Devonian Gudaoling formation, Upper Devonian Xinghongpu formation and Jiuliping formation. The Xinghongpu formation is the host rock of Shuangwang deposit. The lineament of Shuangwang mining area is made up with a series of NW lineal folds and faults. NW Shiziling anticlinoria is the major fold which the auric brecciated zone contained in its north wings. The auric brecciated zone is sandwiched between the Wangjialeng fault (NW) and Xiushiya fault(NW) which are the main and boundary faults in this area. There have three NE postmineral faults disturbed orebodies that are large scald oblique-slip and strike-shift faults. Xiba rock mass is the main magmatic rock mass in the mining area. The early rock is quartz monzodiorite and the later is monzonitic granite. The zircon uranium/lead technique result displayed that the Xiba rock mass has been born from 219Ma to 218Ma in Indo-Chinese epoch. Besides Xiba rock mass, some SN trend lamprophyric dike and diabase dike are found on a large scale and densely in the mining area. These dikes are normal to the breccia rock, and some have been reformed by ore fluid. The zircon uranium-lead dating of representative basite dikes shows the concordia age is 242±17Ma. It means that the minerogenetic epoch of Shuangwang gold deposit is Indosinian.Albitization is particularly significant in Shuangwang gold deposit. The albitization zone generally cuts through strata with small-angle, meanwhile the spatial distribution of breccia body is limited in the scope of albitization, and the exposure widths of albitization zone and breccia body have a certain positive correlation. And the breccia body is gold mineralization body. The six gold-bearing breccia bodies in the deposit interruptedly distribute like band along NW direction, while the gold orebodies in breccia bodies named, in turn from west to east, KT2, KT6, KT5, KT7, KT9, KT8 ect., and the KT5 and KT8 have the biggest scale. Six hydrothermal mineralization stages can be divided by hydrothermal mineral paragenetic association, inclusion and cut through relation, which are stage Ⅰ.Pyrite-ferriferous Dolomite-Quartz-Albite, stage Ⅱ.Quartz-Pyrite-Albite-ferriferous Dolomite, stage Ⅲ. Pyrite-Calcite, stage Ⅳ.Pyrite, stage Ⅴ.Fluorite-Dickite-Calcite and stage Ⅵ.Gypsum-Anhydrite, and stage Ⅳ shared the closest relationship with gold mineralization.The gold orebodies in this deposit are mainly hosted in the breccia bodies through composite analysis of record compiling of KT5 orebody in 1420 level, record compiling of drill core in tunnels and gold grade. But the gold grades in different breccia bodies and different parts of the same berrica body both make big difference. And industrial orebodies just occupy a part of these berrica bodies, as orebodies can locate at the central or margin of berrica bodies and distribute intermittently as a whole. Berrica bodies, which left-stepped, oblique arrange along the strike in the plane and oblique arrange to east and dip toward south in the cross-section, have swelling and thinning phenomenon. The occurrence of gold orebodies have the same changing features, with the orebodies thickness is wider in the northwest part and thinner or non-development in the southeast part in the plane. And the strike of orebodies in the deposit pitch to east too. The distribution of berrica bodies and orebodies both show the control of compression and twist of right lateral oblique shear.Trend-surface analysis has been done on statistics of orebody thickness and gold grade of KT5 above 1420 level. The isograms of orebody thickness and gold grade are both concentrated around 1600 level and 1500 level, and distribute beaded-like. But the two peak centers are inconsistent, with the high-value area of grade iosgram locates at the gradient belt of orebodies thickness isogram. Combined with the occurrence and distribution of pyrite of main metallogenic stage, the paper proposes that the enrichment of gold mineralization is controlled by wave change of faults’ occurrence, berrica bodies and gold mineralization are restricted by the activity of faults, and the berrica bodies and gold mineralization are products of different stage of minerogenetic structures.The occurrence and composition characteristics of the dominant hydrothermal minerals in the ore deposit, such as albite, dolomite (ankerite) and pyrite, are systematically studied. We consider that there are different types of albitites in this deposit, including albite originated from hot water sediments, albite which occured as cement, and hydrothermal albite veins. It is proposed that there are albite carbonate breccias in the hydrothermal sedimentary albite, and there exists the albite veins. There are significant differences between the compositions of the albite with different occurrences, indicating that they are products of different stages of fluids. The bimodal composition characteristics also imply two different sources of albite, which means the hydrothermal albite overlapped the preexisting albite in the protoliths. The analyses of dolomites (ankerite) show that with the evolution of hydrothermal fluids, the dolomites (ankerite) grew increasingly lighter, larger and with less Fe content. While the analyses of the pyrites indicate that the pyrites in this deposit are generally enriched in Fe and depleted in S and other elements. The Co/Ni ratios of the pyrites are between 0.6 and 2.7, indicating their hydrothermal origin.The analyses of the rare earth elements indicate that the altered wall rocks all display HREE enrichment. The wall rocks with different alteration degrees are in consistent with the ∑REE content and the Ce/Ce* and Eu/Eu* ratios, generally show the characteristics of the protoniths. The rare earth elements curve of the albite veins is characterized by flat type and significant Eu negative anomaly, while that of the ankerite shows left-leaning characteristics, indicating that it may have certain differences between the ore-forming fluids they represent. Compared with the weakly altered wall rocks, the strongly altered wall rocks are obviously depleted in LREE and enriched in HREE, consistent with the ankerite, indicating that the wall rocks are strongly altered by CO32-rich fluids.The isotopic ratios of the main hydrothermal minerals, ores, veins, and Lamprophyre walls are analyzed. The Pb isotopic ratios of the Xiba granite, the Lamprophyre wall and the hydrothermal minerals are near the area for the orogenic belts, indicating they are closely associated with the orogenesis. Whereas the Pb isotopic ratios of the albite veins are far from the orogenic field, implying less association between the genisis of the albite veins and orogenesis. The Pb isotopic ratios of the Xiba granite have both characteristics of the mantle-derived and magmatism-derived, while that of the hydrothermal ankerite, pyrites and albite veins show characteristics of mixture of the magmatism-derived and upper crustal-derived. According to the Pb isotopic diagram, the Xiba granite and the hydrothermal products (except the hydrothermal albitite veins) display a good linear distribution, implying that they may be genetically associated, which means that the Xiba granite may have provided part of the ore-forming materials. In the C-0 isotopic diagram, the ankerite plot within the junction of the sedimentary carbonates field and the mantle originated fluid field, and the cultellations are more adjacent to the sedimentary carbonates field. The Sr isotope also indicates its crustal origin. The Yb/Ca-Yb/La diagram suggests that they are far from the magma carbonate field and may be hydrothermal in origin. The H-0 isotopes indicate that the ore-forming fluids are mainly from strata. The δ 34S values are between 5‰ to 13‰, probably imply that they are originated from mixture of deep and shallow strata. In summary, the isotope studies generally suggest that the ore-forming materials and fluids are mainly originated from crust, the magmatism and the deep fluids may also contribute to the ore-forming process. The ore-forming process was closely associated with the orogenesis.The analyses of the compositions of the ore-forming fluids indicate that the cations in the early fluid are dominated by Na+, and the contents of the Mg2+ and Ca2+ gradually increase with the evolution of the hydrothermal process. The anions are dominated by HCO3-, Cl-, SO42-HCO3-, Cl-, SO42- and F-, in which the content of the HCO3- is highest. The gas phase composition is mainly CO2 and H2O, with subordinate H2, CH4 and CO. The homogenization temperatures of the ore-forming fluids range from 80-440 ℃, and the ore-forming pressures range from 500-50MPa, decreasing gradually from early phase to late phase. The density of the fluids varies from 0.72-0.96g/cm3,and gradually increase from early to late. The pH value of the early fluids is alkaline or weak alkaline-partial acid, while it turns into acid during the ore-forming phase. The fo2 and fH2o decrease gradually from early to late, while the fco2 values of the second and third phase fluids are the largest. According to the mineral assemblages and contents, combined with the element geochemistry characteristics, we propose that the Au in this deposit most likely migrates in form of NaAuHCO3 and Na[Au(HS)2]. The expansion of the fluids in the breccia body, and the changes of the physical and chemical conditions, finally resulted in the deposition of the ore-forming elements.The tectonic-fluid system and metallogenic mechanism of the Shuangwang gold deposit are based on synthesis of the fluid inclusion data with relevant structural information. The Lixian-shanyang fracture is the NWW-trending syngenetic fault, with strike length of>400 km albitite-carbonatite breccia zone, which is the controlling factor of the alkaline fluid for diagenesis and mineralization in this area. The Fengtai (first order) basin is controlled by Lixian-Shanyang fault and Zhen’an-Banyanzhen fault. The Fengtai basin was separated into more secondary and third small-sized basins futher by and NWW-and NE-trending syngenetic faults and archicontinents. Shuangwang gold deposit is located in the Shuangwang third platform basin, which is a secondary unit in Taibai secondary basin. The tectonic-fluid system and metallogenic mechanism involving in: ①basin and the syngenetic fault are the fundamental environment for the diagenesis, providing passageway for high content of the Na+ and CO32-fluids. The hydrothermal sedimentation basin may have provided abundant ore-forming materials, and different-order syn-faults in the area offer foundation and passageway for ore-forming fluids. ②Indo-chinese epoch folds orogen in geotectonic events, the early stage fractures and stratum were controlled by the strike-slip structures, which caused by NS-trending principal stress. The competent rich-sodic rocks are easy to broken and growth NWW-trending fractures along with the albitite zone. The high content of the Na+ fluids migrated at the fault zone and make natrium alteration in the wall rock further. The early stage deformation was marked by dextral shear, offering the prototype of the breccia belt. In the middle stage, seismic pumping and the hydraulic fracturing were essential in the breccia formation, which make the extension main breccia body. The high content of the CO32- Fluids with internal pressure promoted brecciated, which means that expansion of tensile breccia free up more space in breccia body and also providing passageway for fluids activity. Accompanied with the brecciation drawing to a close, the fractures grew the formation. Breccia formed in this process, whereby the fractures cut the breccia body or follow the breccia. Fractures frequently were observed in the breccia body and contact zones, as well as the main passageway for fluid migration and the favorable position for ore formation. Constructional surface is continuous irregularity shape and shown a wavy change, which is an important factor for gold precipitated. The fluid was characterized by the high content of sulphur, and pyrite with rich gold precipitated in this stage. In the later stage, there was the intrusion of near NS-trending mafic dykes cracking of continental earth crust, and influenced by ore-forming fluid.