Orebody vertical structure and implications for ore-forming processes in the Xinxu bauxite deposit, Western Guangxi, China

The Xinxu bauxite orebodies, which are contained in Quaternary depressions overlying strata of Permian to Devonian age, were formed through the break-up, oxidation and re-sedimentation of a pre-existing Permian bauxite horizon. With progressive karsitification, the underlying strata of the Quaternary orebodies were eroded out downwards in sequence, gradually transiting from Permian to Devonian. Thus an older underlying stratum generally infers a higher degree of weathering. A total of 47 exploration wells, sampled each meter, were chosen from several orebodies to evaluate the vertical distribution of four major components (Al2O3, Fe2O3, SiO2 and loss-on-ignition) in the ores. The vertical evenness is quantified via a box-gliding multifractal model. The small width of the multifractal spectrum suggests that the vertical ore quality is even overall, indicating an intertwining break-up and mixing up processes during evolution of the Quaternary orebody. Although the four components show complex spatial distributions, topographic elevation displays a positive relationship with the mean Fe2O3 content, and a negative correlation with the vertical unevenness of Al2O3 and Fe2O3 in the wells. This is taken to reflect the more uniform oxidizing environment in the higher depressions, which facilitates a homogeneous and greater increase of Fe2O3 content throughout the orebody. In contrast, the redox conditions in lower depressions are less oxidizing and varied in space, inducing great variation in degree of Fe2O3 increase in the ores. This study also demonstrates that, in the Xinxu deposit, the ore-forming redox environment has more impact on the ore quality than the degree of weathering. As the contents show irregular vertical changes in the lower depressions, it follows that it is the ancient redox environment rather than the modern conditions which affect the orebody vertical structure. This clarifies the fact that the vertical structure of the orebody is jointly constrained by the physical mélange accumulation of ores and the differences between the ancient redox environments in the higher and lower locations. These factors should be the primary factors affecting the vertical structure of karst bauxite deposits.