Mineral Prospectivity Mapping of Weathering-Crust-Type Ilmenite Placer Deposits Using Nested Spatial Cross-Validation and Explainable Ensemble Learning: A Case Study of the Wuding–Luquan Area, Yunnan, China

Mineral prospectivity mapping of weathering-crust-type ilmenite placer deposits is challenged by the limited number of known mineral occurrences, the lack of reliable barren-site labels, and spatial autocorrelation. This study focuses on the Wuding–Luquan area of Yunnan Province, China, and develops an eight-predictor framework comprising elevation, slope, fault kernel density, an interpolated Ti-concentration predictor, FeOx, Al–OH and Mg/Fe–OH mineral anomalies, and distance to mafic rocks. Forty-three known ilmenite occurrences were used as positive samples, and 86 pseudo-absence/background samples were generated under spatial constraints. Random forest, XGBoost, and a multilayer perceptron, together with an equal-weight soft-voting ensemble, were used for mineral prospectivity mapping. Model performance and predictor contributions were evaluated using nested spatial cross-validation, SHapley Additive exPlanations (SHAP), permutation importance, and spatial-block bootstrap resampling. The equal-weight ensemble achieved the highest out-of-fold ROC-AUC (0.9202) and average precision (0.8580), while yielding the lowest log loss and Brier score. Ti was the most important predictor across all four modeling schemes, whereas Al–OH mineral anomalies, slope, and Mg/Fe–OH mineral anomalies also showed relatively high contributions in the equal-weight ensemble. A total of 379 measured TiO2 records showed weak positive and scale-dependent rank associations with predicted prospectivity, with median Spearman coefficients ranging from 0.133 to 0.325 across aggregation scales of 250–2000 m. This comparison was treated as an exploratory geochemical comparison rather than as independent validation of model performance. By integrating model probabilities with the distributions of mafic rocks, known mineral occurrences, faults, and measured TiO2, eight prospective areas (T1–T8) were delineated. The results provide a quantitative basis for regional mineral prospectivity assessment and exploration-target prioritization in the Wuding–Luquan area.