Existence Form And Enrichment Mechanism Of Lithium In Bauxitic Rocks Of The Wuzhengdao Belt,Northern Guizhou

The region of Wuchuan,Zhengan and Daozhen counties,referred as the Wuzhengdao,is located in northern Guizhou province.It consists of the southern part of ore concentration area of bauxite deposits,named as Southern Chongqing-Central Guizhou bauxite metallogenic belt,and is one of the 103 key bases of energy and resources in China.The Permian bauxite deposits in the Wuzhengdao are characterized by high Li-enrichment,but comprehensive and in-depth works remain lack.Moreover,there is no corresponding planning and design for utilization of the associated Li resources in the newly bauxitic mining due to being short of its existence form and distribution.Therefore,to understand the Li mineralization in the region and to promote a full utilization of the bauxite resources,this study has carried out a comprehensive study on genesis of the regional bauxite deposits,existing form of Li element and its enrichment mechanism.Profiles of a section of bauxitic claystone in Chicun(Daozhen)and a section of bauxitic horizon in Dazhuyuan(Wuchuan)were selected as the objects of this thesis.In addition,some typical ores of Carboniferous bauxite in the central Guizhou belt were also selected for comparison.Contents of this work comprise element geochemistry of the profiles,Li isotope analysis of the bauxitic horizons and basement rocks in eastern Guizhou regarded as potential provenance of the bauxitic deposits,and experiments based on X-Ray Powder Diffraction(XRD)of clay minerals hosted by the bauxitic rocks.Geochemical features of elements along the profiles are illustrated.Li enrichment behavior and its spatial relationship with the bauxitic rocks are discussed consequently;finally,Li exiting form in the ores is identified.A model for regional bauxitization and the mechanism of Li enrichment are accordingly proposed.Bauxitic horizon of the Chicun profile comprises bauxitic claystone in the middle and aluminum claystone in the upper and the lower layers.It is revealed that Al₂O₃contents of the samples are negatively correlated along the horison with those of SiO₂,TFe₂O₃ and most other major and trace elements,but is positively correlated with Li.It is suggested that Al and Li were enriched synchronously during the mineralization as indicated by their coherence relationship.By contrast,Li and Al₂O₃ contents are negatively corelated in ore layer(Al₂O₃>55%)of the sandwiched horizon of the Dazhuyuan bauxite deposit,suggestive of a decoupled relationship,but their positive correlations retain in the upper and the lower claystone layers,resambling that in the Chicun profile.Intense enrichment of Li,W,Mo and some HFSE,related to their abundances in the upper continental crust,are pervasive in the Wuzhengdao deposits,and Li and W contents have reached Chinese industrial grades.In fact,these elemental enrichments are also common in the central Guizhou bauxite belt and in some early Trassic strata in SW China.It is suggested that igneous suites with mineralization of rear elements have played a significant role in the provenance of late Paleozoic bauxites in Guizhou,and this kind of derivations possibly have lasted from the Carboniferous to the Permian.Discriminations by immobile elements reveal that mafic suites(in form of volcanic lava or ashes)likely dominated the provenance of the bauxitic rocks.The basaltic rocks are chemically unsaturated in SiO₂ which may promote the decomposition of kaolinite into aluminum hydroxides,their mafic compositions are also conducive to later bauxitization due to high contents of iron.Separation and purification of clay minerals from bauxitic samples was carried out.The pure powders were saturated by ethylene glycol and followed by stepwise heating.The pure clay minerals collected at each step were analyzed by XRD scan.It reveals that lithium enriched in bauxitic rocks exist in Li-mineral of cookeite,rather than other clay minerals or in form of clay absorption.In contrast to the lateritic horizons which are topped by iron crust,Fe enrichments occure at the lower layer in the bauzitic horisons.Moreover,a large mass of pyrite,which usually formed under reduce condition,is found in mineral assemblages typifying bauxite ores,which formd under intense oxidation condition.Based along with re-distribution behaviors of elements with diverse geochemical characteristics,a new genetic model for the bauxitic ores in northern Guizhou is accordingly proposed.South China where Guizhou province located in was situated in tropics during the late Paleozoic,where laterization was well developed;coastal plain formed along the northern margin of Guizhou and frequent cycles of transgression and regression during the Permian caused a transition of weathering profiles from laterization to bauxitization:massive pyrite in the horizons formed in a reducing environment during the transgression through combining of iron enriched in the laterite profile and sulfur from the soaking and penetrating seawater;during the regression,oxidation of pyrite caused strong acid medium and induced the bauxitization.It is suggested that regional enrichment and mineralization of lithium occurred during the laterization of coastal plain,ore-forming materials were sourced mostly by Li deposits hosted by igneous suites via weathering,transportation and deposition,whereas the bauxitization occurred lately resulting in significant dissolution and migration of lithium from ore layers of the horizons,and reprecipitation in distance in form of massive type of ores with Li enrichment due to aqueous dilution.