Gallium isotope fractionation in the Xiaoshanba bauxite deposit, central Guizhou Province, southwestern China

Abstract

We studied the gallium (Ga) isotopic composition of the Xiaoshanba bauxite deposit in central Guizhou Province, southwest China. Claystone with kaolinite as the main mineral has δ71GaIPGP values of − 0.22‰ to + 0.19‰ (−0.04‰ on average), whereas bauxite with diaspore as the main mineral has δ71GaIPGP values of − 0.60‰ to − 0.15‰ (−0.35‰ on average). Other samples with diaspore and kaolinite as the main minerals have δ71GaIPGP values between those of the claystone and bauxite (−0.20‰ on average). Dolostone from the Loushanguan Group has δ71GaIPGP values of − 0.28‰ to − 0.18‰ (−0.24‰ on average). Samples with kaolinite as the main mineral have higher δ71GaIPGP values than samples with diaspore as the main mineral. However, δ71GaIPGP values in the dolostone are lower than those of the claystone. Based on vertical variations in δ71GaIPGP values and Ga/Al ratios, we speculate that weathered dolostone from the Loushanguan Group may have higher δ71GaIPGP values than unweathered dolostone. In addition, for both of the Jiujialu and Wulongsi ore bodies, samples from the upper part of the profile are relatively enriched in heavier Ga isotopes than samples in the lower part. During the process of bauxite ore formation, supergene weathering and Ga re-precipitation could have led to enrichment in lighter Ga isotopes. This mechanism of isotopic enrichment has the potential to record the material source of the bauxite. The results of leaching experiments show that, during supergene weathering, Ga is more available to the leaching liquid than Al under neutral to weakly alkaline conditions, whereas Al is more soluble than Ga under weakly acidic conditions. In natural deposits, samples with a high degree of weathering show an obvious decoupling between Ga and Al; consequently, in a given sedimentary environment, samples with kaolinite as the main mineral have higher Ga concentrations than samples with diaspore as the main mineral. Clay minerals such as kaolinite can be important resources of Ga.