Geology,mineralization And Genesis Of The Nibao,Shuiyindong And Yata Gold Deposits In SW Guizhou Province,China

Large areas of SW Guizhou Province host numerous sediment-hosted disseminated gold deposits which have been commonly conisdered to be analogues of Carlin-type gold in Nevada.Previous studies have detailed geological,alteration,and mineralization characters of major gold deposits in the area,yet a widely accepted genetic model is lacking.In this thesis,I present an integrated geological,mineralogical,geochemical,fluid inclusion,and geochronologica study of the Nibao,Shuiyindong and Yata gold deposits,which are the best representatives of three mineralization styles of SW Guizhou,aiming to provide a better understanding of nature,timing,processes and tectonic settings of gold ore formation that in turn lead to a robust genetic model.The Nibao and Shuiyindong deposits is mostly hosted in the hydrothermally altered breccias developed along unconformities between the Middle Permian Maokou Formation and the Upper Permian Longtan Formation,and in fracture zones within the “inpure carbonates” of the first and second units of the Permian Longtan Formation,with minor orebodies bing localized in reverse faults generally <500 m to the anxial traces of anticline.The host rocks are mainly composed of volcanic tuff,sandstone,calcareous siltstone,and bioclastic limestone.Gold mineralization of the Yata deposit is mostly hosted in the Middle Triassic Xuman Formation consisting of calcareous siltstone,sandstone,marlstone,and carbonaceous shale.Orebodies are mainly controlled by a group of folds and faults.Although the three deposits under investigation show distinctive host rocks and ore-controlling structures,they share common characteristics in mineralogy,alteration,and mileralization styles.The ore-related hydrothermal alteration consists of silicification,decarbonatization,carbonatization,sulfidation,argillization,and/or fluoritization,forming assemblage consisting of jasperoid and drusy quartz,ferrous calcite,dolomite,As-arsenian pyrite,arsenopyrite,stibnite,realgar,clay minerals(illite ± dickite ± sericite ± kaolinite),and fluorite.As mentioned above,the lithofacies of the host rocks of the selected three deposits are variety,but the ores present a similar variation of major elements,including high Si content,similar variation of Ca and Mg,high Fe and S content,and high Al and K content,which reflects the representative silicification,decarbonatization and carbonatization,sulfidation,and argillization,respectively.The variation of the trace element presents a large variation of Au,As,Hg,Sb,and Tl.Their changes upto 4 ~7 orders of magnitude,while the variation of other elements is commonly lower than 3 orders of magnitude,suggesting the former have more activity than later during ore-forming process.The compassion between the ores and the relative fresh host rock show the typical enrichment of Au,As,Hg,Sb,and Tl.There no obvious enrichment trend for other trace elements.In addition,the in-situ LA-ICP-MS trace element spot analysis and mapping work of the Au-bearing As-pyrite also presents the similar enrichment of Au,As,Hg,Sb,Tl(± Cu ± Pb),suggesting they are the typical ore-forming elements group for these gold deposits.A detailed mineragraphy study on the selected three deposits,including the high spacial resolution SEM observation,indicating that gold mainly occurs as “invisible gold”.The variation of Au and As content of As-pyrite is plotted in the typical Au-As variation of Carlin-type gold deposit,indicating they share a similar characteristic with the typical Carlin deposits.All these Au vs.As plots fall below the inferred solubility limit of invisible gold in pyrite(CAu = 0.02 × CAs + 4 ×10-5),indicating the gold is most probably occurred as homogeneous nanoparticle or solid solution within the structure of the As-pyrite.Gold mainly occurred as “invisible gold” is also confirmed by the absence of the abnormal bright spot by maps of Au distribution.The roughly positive correlation between Au and As in all ore-stage arsenian pyrite of the selected deposits suggests the enrichment of Au is associated with the As enrichment in pyrite.Two possible involved mechanism of As intruded in pyrite were discussed to provide a suitable accommodation for Au.Bitumen disseminations in high grade ores from the selected gold deposits show a consistent Ranman spectroscopic patterns and therefore are considered to have a similar origin.Baesd on the Raman spectroscopic data of bitumen,the early-stage ore fluids have calculated temperatures of 317 ~ 336 ?,which are significantly higher than paleogeothermal temperatures of 160 ~ 250 ? and homogenization temperatures of fluid inclusions of regional paleo-oil reservoirs(73 ~ 175 ?).The abnormally high temperatures of the early-stage ore fluids suggest that regional paleogeothermal field was insufficient to provide heat source for ore-forming fluid transportation and gold mineralization.The microthermometric data has demonstrated that the ore-stage fluids have the homogenization temperatures of ~ 220 ? with the salinities of ~ 4.2 wt.% NaCl equiv.and the late stage fluids have the homogenization temperatures of ~ 150 ? with the salinities of ~ 2.2 wt.% NaCl equiv.There presents an obvious cooling and salinities reduced during the ore-forming process.As for the source of ore fluids and other ore-forming materials of the selected gold deposits,a variety of geochemistry tracer methods were applied.Gas extracts from various hydrothermal minerals suggest the initial ore-forming fluids were derived from the concealed magmatic system.In the N2/Ar versus CO2/CH4 diagram,analyses from various minerals show a positive correlation extending from a convecting meteoric water field to magmatic fluid field,indicating that the inclusion fluids most likely represent mixture of gaseous species derived from deep-seated magma and convecting meteoric water.It is worth noting that one quartz sample deviates significantly from the main trend and approaches the organic component field,reflecting abnormal high contents of organic components in the fluid inclusions trapped during the ore-fluid migration.The C-H-O isotope data of ore fluid confirm the possible magmatic source of the initial ore-forming fluid and subsequent mixing with the shallow convecting meteoric water and basinal fluids.In the ?DH2O vs.?18OH2O diagram,the calculated ?18OH2O values for quartz(220?)and calcite(150?)and measured ?DH2O values of fluid inclusions extend between three end members: metamorphic(± magmatic)waters(14 ‰,-15 ‰),organic water(14 ‰,-135 ‰),and local meteoric water(-10‰,-72‰).Significantly,calculated values for sericite and dickite(220?)plot near the metamorphic(± magmatic)box,indicating a possible metamorphic(± magmatic)origin for initial ore fluid.In the ?13CPDB vs.?18OV-SMOW diagram,analyses from numerous ore-related calcite samples has generally lower ?13CPDB and ?18OV-SMOW values that define an obvious inclined array that may be indicative of reactions between the host rocks and fluids containing CO2 derived from mantle/ magmatism or the oxidation of the sedimentary organic carbon.Arsenic,commonly gold-bearing,pyrite,quartz,calcite,and fluorite,which typically formed in different paragenetic stages of gold mineralization,have comparable species,composition,and variation patterns of hydrocarbons,indicating minimum decomposition or restructuring of organic ionic compounds during the evolution of ore-fluids.Thus,the lower ?13CPDB and ?18OV-SMOW values may indicate a mantle/ magmatic origin of CO2.Noble gas and ion extracts from the fluid inclusion of various hydrothermal minerals show signals of mantle/ magmatic-derived fluids.Helium is highly enriched relative to Ne and Ar in fluid inclusion extracts,and 8 of 24 samples have the He/Ne ratios lower than the ratio of(He/Ne)air(0.3)that indicate the contamination of air,whereas other 16 samples have the higher He/Ne ratios typical of crustal/ mantle He.All the helium extracts has Rc/Ra values extend between 0.01 that is typical of a crustal component and 0.3 that corresponds to 4% mantle He(R/Ra = 8),with an exception for quartz of the Nibao deposit that has Rc/Ra = 1.44 indicating up to 24% mantle-derived He injection.The ion extracts show the high Na/Cl and low Cl/Br ratios extend from a basinal brine into the metamorphic field.Extracts from two fluorites and one milky quartz have low Na/Cl and high Cl/Br ratios are typical of a magmatic source.In addition,the Na/Ca and Na/K ratios of fluid inclusion extracts provide evidence for mixture of magmatic hydrothermal fluids and basinal water.Extracts from three realgar samples plots near the 400 ? granite-derived fluids apex,suggesting the end member of magmatic origin.In-situ LA-MC-ICP-MS pyrite sulfur isotope analyses reveal a large ?34S variation of-49.59~16.85 ‰.The pre-ore stage pyrite shows very scattered ?34S values,ranging from-49.59 to 16.85 ‰,indicating that H2 S was generated by biogenic sulfate reduction in open and sulfate limited systems during sedimentary and/ or diagenesis.The ore stage pyrite has ?34S of-1.48 ~ 9.95 ‰(mean = ~3 ‰),indicating a magmatic source or average sulfur isotope in sedimentary rocks.Line scanning of the single zoned pyrite show a distinctive decrease of ?34S values,the enrichment of 32 S,and move toward to “0 ‰”,the similar scenario was revealed in Nevada's deposits has been interpreted as a result of the injection of magmatic sulfur.Collectively,these results demonstrate the heart,initial fluids and other ore-forming components may have derived from a concealed magma system,with significant input of the shallow convecting meteoric water,basinal(organic)water and air.In this study,hydrothermal fluorite samples of Nibao from the Au-bearing altered breccia within the regional unconformity zone have the Sm-Nd isochron ages of 126 ~ 122 Ma,realgar-bearing calcite vein of Shuiyindong has the Sm-Nd isochron age of 146.5 ± 3.3 Ma,realgar-bearing quartz vein of Yata has the Rb-Sr isochron age of 148.5 ± 4.1 Ma,and the Au-bearing As-pyrite associated illite has the Rb-Sr isochron age of 204.6 ± 5.2 Ma.Together with the published geochronological data,it is suggested that the SW Guizhou province underwent two independent gold mineralization events in the late Triassic(~ 200 Ma)and late Jurassic to early Cretaceous(148 ~ 122 Ma),respectively.Combined with the regional geological setting,the former is consistent with the closure of the branch of the Palaeo-Tethys,namely the collision of the IndoChina block and the South China block.Gold precipitated during a period of post-collision extension.The latter is consistent with the lithospheric extension of the South China block,it may have been associated with the subduction of the paleo-Pacific plate.Gold mineralization was developed during the regional lithospheric extension cause by the tearing or retreat of the subducting slab during the late Jurassic to early Cretaceous.Collectively,the SW Guizhou province records two independent gold mineralization events in the late Triassic and late Jurassic to early Cretaceous.Combined with the regional tectonic evolution and the geology of the typical gold deposits,the ore-forming process is summarized and described as below: In the late Triassic(~ 200 Ma),following the closure of the branch of the Palaeo-Tethy,the IndoChina block collided with the South China block as marked by the Songma Suture.During the extension after post-collision,the initial ore forming fluids derived from the concealed magma chamber upward along the regional crustal faults or boundaries faluts of the isolated carbonate platforms or other basin internal faults,and then variably mixed with the shallow convecting meteoric water,basinal(organic)water and air.when it migrates to the shallow regional unconformity zone,interbeded structure or reverse faults associated with anticline.The gold mineralization processing when the fluids dissolved and sulfphidized carbonate wall rocks,leading to the precipiation of gold-bearing pyrite.In the late Jurassic to early Cretaceous(148 ~ 122 Ma),similar initial ore forming fluids exsolved from the concealed magma chamber and then migrated upward aong the pre-existing channel to the similar ore-controlled structures and unloading by a similar reaction mechanism between the fluids and wall rocks.