Controlling factors for Co enrichment in mineral deposits: Insights from magnetite trace element big data

Abstract

Magnetite is a common mineral in mineral deposits and plays a crucial role in genetic interpretations. However, cobalt (Co) enrichment in magnetite and its effect on the distribution of Co between magnetite and sulfide in mineral deposits are still poorly understood. This study compiles a dataset of 9,218 trace element analyses of magnetite from 166 deposits worldwide. Using statistical analysis and machine learning-based feature importance evaluations, we investigate the controlling factors of Co content in magnetite across various deposit types. The results reveal a significant impact of magnetite on the economic Co resources within the deposits. The correlation analysis between Co and other elements in magnetite supports the conclusion that higher Co concentrations in magma and fluids, elevated temperatures, and lower oxygen fugacity are favorable conditions for the formation of Co-rich magnetite. Feature importance evaluations of Light Gradient Boosting Machine models were employed to identify the primary factors controlling Co enrichment in magnetite across porphyry, iron oxide-apatite, iron oxide-copper–gold, magmatic Ni-Cu sulfide, skarn, and banded iron formation deposits. These models exhibit strong performance in classifying Co content in magnetite across various deposit types, achieving accuracies of 0.91–0.94 and minimum area under the curve exceeding 0.969. The evaluations identify the composition of the magma and fluid as the primary controlling factors for Co content in magnetite, followed by temperature and oxygen fugacity. In addition, considering skarn and porphyry deposits as examples, skarn Fe and Cu deposits typically contain higher Co content in magnetite compared to skarn W, Sn, Pb, and Zn deposits. Similarly, porphyry Cu and Au deposits generally show higher Co content in magnetite than porphyry Pb and Zn deposits. These observations suggest that variations in the initial compositions of magmatic-hydrothermal systems exert a significant influence on Co enrichment in mineral deposits. Moreover, the extensive crystallization of magnetite at higher temperatures, which precedes the formation of sulfides, tends to reduce the amount of Co available for incorporation into the sulfide phase. This study underscores the importance of considering the distribution between magnetite and sulfides when evaluating Co resources in magnetite-bearing deposits.