Trace and rare earth elements constraints on the sources of the Yunfeng paleo-karstic bauxite deposit
A number of Carboniferous paleo-karstic bauxite deposits occur in the Xiuwen-Qingzhen area, central Guizhou Province, which constitute an important part of the central Guizhou-Southern Chongqing bauxite belt in southwestern China. These deposits consist of lenticular and stratiform orebodies hosted in the Lower Carboniferous Jiujialu Formation, which lies unconformably above carbonate rocks (mainly dolomite) of the Middle and Upper Cambrian Loushanguan Group, Shilengshui Formation and Lower Cambrian Qingxudong Formation. Although it is generally agreed that the bauxite resulted from weathering of Cambrian rocks below the unconformity, it remains unclear whether the carbonates alone can provide sufficient material for the formation of the deposits. In this study, we examined the mineralogy and major, trace and rare earth elements geochemistry of bauxite and bauxitic clays from the Yunfeng deposit in the Xiuwen-Qingzhen area and compared them to previously published data of a paleo-weathering profile consisting of Loushanguan dolomite underlying the sub-Carboniferous unconformity and the "Black Rock Sequence" of the Lower Cambrian Niutitang Formation, in order to evaluate the sources of the bauxite. The bauxite ores are composed of diaspore as the main ore mineral, along with small amounts of pyrite, arsenopyrite, galena, stibnite, xenotime and uranium-bearing minerals, and are characterized by high contents of uranium and phosphorus. Chondrite-normalized REE patterns and various bivariant diagrams commonly used to study the provenance of bauxite including Eu/Eu* vs TiO2/Al2O3, Eu/Eu* vs Sm/Nd, TiO2 vs Al2O3, TiO2/Th, Th vs Ta, Th vs Nb and Nb vs Ta suggest that the Loushanguan dolomite and the Niutitang "Black Rock Sequence" are both potential source rocks. However, a comparison in U and Th contents between the paleo-weathering profile of the Loushanguan dolomite and the bauxite samples suggests that the dolomite alone cannot provide the materials for the bauxite ores. Instead, the "Black Rock Sequence" may have contributed most of the ore-forming materials, which are mainly transported from source regions where the Niutitang Formation was exposed, weathered and eroded. Therefore, the Yunfeng bauxite deposit is of mixed autochthonous and allochthonous origin.