Platinum-group element geochemistry of the Heigutian FeTi oxide-bearing layered intrusion, Emeishan large igneous province, SW China

Abstract

The nature (e.g., whether or not depleted in platinum-group elements) and sulfide segregation history of the parental magmas of layered mafic-ultramafic intrusions in the Emeishan Large Igneous Province (ELIP), SW China, remain controversial, which limits our understanding of their mineralization potential and exploration of platinum-group elements (PGE). To address these issues, we investigate the PGE geochemistry of the Heigutian layered mafic-ultramafic intrusion, a representative FeTi oxide-bearing intrusion located in the central part of the ELIP. It comprises two distinct petrographic zones separated by a sharp contact. The lower zone is composed of olivine clinopyroxenite, magnetite gabbro, and medium-grained gabbro, while the upper zone consists of fine-grained gabbro. The olivine clinopyroxenite in the lower zone has a total PGE content of 28.8–31.7 ppb; the magnetite gabbro and medium-grained gabbro show much lower total PGE levels of 0.88–1.48 ppb and 0.09–0.18 ppb, respectively. The fine-grained gabbro in the upper zone exhibits a broader total PGE range of 2.01–32.27 ppb. The decoupling between Ru and Ir in the fine-grained gabbro, together with the negative Ru anomaly, suggests that the parental magma of the fine-grained gabbro did not undergo early-stage sulfide segregation but experienced significant removal of RuS₂ before emplacement. Given the mantle-normalized patterns of PGE analogous to those of the Emeishan PGE-undepleted high-Ti basalts and mantle-like Cu/Pd ratios (4 × 10³ to 7 × 10⁴) in the fine-grained gabbro, it can be inferred that the parental magmas were undepleted in PGE. The positive correlations between PGE and TiO_2, Ni, and Cr indicate in-situ sulfide immiscibility driven by fractional crystallization of olivine and FeTi oxides during the formation of the lower zone. This sulfide immiscibility could potentially contribute to PGE mineralization in the lower part of the intrusion. Given the similarities in PGE features between the Heigutian intrusion and other layered mafic-ultramafic intrusions, there is substantial potential for exploring PGE mineralization within coeval FeTi oxide-bearing layered intrusions in the ELIP.