Sedimentological And Metallogenic Study Of Bauxite Deposits In Guizhou And Guangxi Provinces,South China

Bauxite is a kind of residual rock with minable aluminum content,the product of intense continental subaerial weathering under the humid tropical or subtropical environment.Geologically,the occurance of bauxite deposit indicates sedimentary discontinuousness,hot and humid(paleo-)climate,and tectonic uplift.Formation of bauxite deposit is the rare event in Earth history,requiring the integration of several environmental factors.The distribution of bauxite deposit in the geological history and the global scale,from the first occurance of bauxite deposit in Cambrian,to Middle-Late Devonian,Early Carboniferous,Late Permian,Late Triassic – Early Jurassic,Late Cretaceous,Eocene – Miocene and Holocene,there were seven globally large bauxite metallogenic periods,linked with the high temperature in the low latitude,unstable sea levels and vertically stable tectonic background.Based on the new stratigraphic and geochemical works on Permian bauxite deposit in Guizhou and Guangxi provinces,South China,sedimentological and geochemical methods have been utilized on three typical bauxite deposits in Guizhou and Guangxi areas in this study,trying to combine the bauxitic metallogenesis with global paleoclimatic and local paleogeographic changes.Two cases in this study include Lower Permian Dazhuyuan Formation bauxite deposit in Wuchuan-Zheng'an-Daozhen area,North Guizhou and Upper Permian Heshan Formation bauxite deposit in West Guangxi.Bauxite deposits in North Guizhou and Zunyi locate on the northside and eastside of Qianzhong Uplift,respectively.Bauxite deposit in North Guizhou is sedimentary-type,it overlies the Upper Carboniferous Huanglong Formation carbonate or Lower Silurian Hanjiadian Formation shale,and covered by Middle Permian Liangshan Formation mudstone.The age of the Dazhuyuan Formation is Asselian – Artinskian.In West Guangxi,the bauxite deposit occurs at bottom of the Heshan Formation,covered by the mudstone Member and overlies the Middle Permian Maokou Formation carbonate.The deposital age of this bauxite deposit is Capitanian – Wuchiapian.It is also a karst-type bauxite deposit.Bauxite deposit in North Guizhou formed in an Early Permian northwards facing bay.The bay linked with open sea with channel,and it is surrounded by erosional areas in other three directions.High quality bauxite concentrates in the tidal wetland area.High CIA values and diaspore+kaolinite dominated mineral composition indicate strong chemical weathering under hot and humid paleoclimate.PAHs(polycyclic aromatic hydrocarbon)are detected in this bauxite deposit,implying dry and wet seasonal differences during the metallogenesis.By reviewing the former paleosalinity data,we believe the high-Al claystone within the Dazhuyuan Formation formed in the marine environment,whereas the bauxite ores formed in the terrestrial environment.Positive Ce anomalies are common in the bauxite ore samples,whereas high-Al claystone samples show negative Ce anomalies.In weathering profiles,positive and negative Ce anomalies indicate oxic and anoxic environments,respectively.In this case,there is a redox environment change between bauxite ore and high-Al claystone samples.This is further supported by the isoprenoid extracted from the high-Al claystone samples in the Dazhuyuan Formation,pristine(Pr)/ phytane(Ph)ratios are less than 1,indicating the low Eh(suboxic or anoxic)environment.The acid environment dominated the bauxite metallogenic process,resulting in activations and removals of REEs(rare earth elements)and HFSEs(high field strength elements).C27 rearranged sterane / C27 sterane ratios in high-Al claystone samples range from 0.31 to 0.37,indicating at least one third sterane rearranged in the acid environment.Both decomposition of organic matters and microbial activities can produce organic acid,oxidation of sulfide in weathering process can also form inorganic acid.The bauxite samples contain double-peak n-alkanes distributions.As to the terpane composition,tricyclic terpanes are with wide carbon distribution,but their contents are lower than hopances.Evidences from n-alkanes and terpanes indicate the organic matter in bauxite deposit come from both terrestrial plant and bacteria/algae.The detrital zircon geochronological results give good contrain on the provenance of this bauxite deposit,similarity of zircon U-Pb age distributions in the Dazhuyuan Formation bauxite samples and the underlying Hanjiadian Formation sandstone samples indicate the Hanjiadian Formation provided the main source for the Dazhuyuan Formation.Moreover,the ultimate source of Dazhuyuan and Hanjiadian formations is the pre-Silurian strata in Cathaysia Block.In the Dazhuyuan Formation,numerous evidences left by laterization,transportation and colloid process indicate the parautochthonous and / or allochthonous origins for the Dazhuyuan bauxite deposit.Syn-sedimentary leaching process are also recoreded in the bauxite deposit,one or several layers of loose beds indicate syn-sedimentary exporsures during metallogenic process.The mass balance calculation results demonstrate most of elements in the bauxite deposit are lost during intense weathering,losing amounts of silicon and iron are more than aluminum,and then aluminum is relatively enriched.In West Guangxi,bauxite deposit is at the bottom of the Upper Permian Heshan Formation.During Permian,the Youjiang Basin was located at the continental margin of the Yangtze Block.Isolated carbonate platforms were separated by the interplatform troughs in this basin.All bauxite deposits in the Youjiang Basin are found on the isolated carbonate platform areas.Both field observations and tarce element indicators support the Heshan bauxite deposit were formed in the terrestrial environment during the exposure of the carbonate platforms.CIA values in bauxite ore samples range from 97 to 100,mineral composition of bauxite ore are dominated by Al-minerals(diaspore and boehmite)and kaolinite,both of these evidences indicate a hot and humid paleoclimate in the tropical area.From bottom to top in the Heshan bauxite deposit,colors of bauxitic layer change from red to black gray,and Ce anomalies change from positive to negative,implying the redox environment changed from oxic to suboxic(anoxic)during the metallogenic process.According to the detritial zircon U-Pb ages in the Heshan bauxite deposit,the main age peak is at 263 – 262 Ma,the trace elements in zircon grains indicate their origins are within plate / anorogenic and arc-related / orogenic,provenance diversity shows in the bauxite ore samples on different carbonate platforms,.Combined with the tectonic and paleogeographic background of the Youjiang Basin,we believe the ELIP(Emeishan Large Inogenous Province)and arc system between South China and Indochina blocks provided volcanic ash for the Heshan bauxite deposit,the transportation of clasts was dominated with atmospheric transport.Quaternary Supergene weathering and leaching processes effectively improved the qualities of bauxite ore.From Carboniferous to Permian,massive bauxite metallogenic events in South China are controlled by the paleoclimatic and paleogeographic factors.In Devonian,the South China Block separated from the Gondwana supercontinent,and continuously shifted to the Northern Hemisphere.During Permo-Carboniferous,South China Block was located in the equatorial position and far away from continents.Tropical climate and isolated position were both favorable for bauxite metallogenesis,troptical climate can provide the indispensable high temperature and humid for weathering and leaching processes,isolated paleogeographical condition can preserve the weathering residue and block the premature clasts input.As a conclusion,the paleogeographic factor would control the distribution and thickness of the bauxite deposit,the weathering and leaching processes would control the quality of the bauxite ore.