Atypical chromitite deposit in the Gaositai mafic-ultramafic complex, North China Craton: Geochemical, geochronological, and isotopic systematics

The Gaositai mafic-ultramafic complex in the North China Craton hosts a significant chromitite deposit featuring morphological and geochemical characteristics that deviate from the usual characteristics of typical high-Cr and low-Al podiform chromitites. Field observations and microscopic studies reveal notable variations in textural layering, mineralogy, and geochemical composition, challenging conventional features of stratiform chromitite deposits. Zircon U[sbnd]Pb ages indicate that chromitite is most likely Carboniferous-Permian in age (330–275 Ma), but also contains numerous inherited zircons with concordant ages as old as 2544 Ma, reflecting a complex geological evolution. Post-formation metasomatic or metamorphic imprints can be evident by concordant zircon ∼247 Ma. Variable zircon ε_Hf(t) values (−15 to +9.6) and depleted mantle model ages (432 to 2819 Ma) attest the presence of heterogeneous or reworked source materials. The PGE and trace element characteristics of the Gaositai chromitite are comparable to the geochemical features of Bushveld and Great Dyke deposits. Pyrite, millerite, norsethite, galena, and PGM (mainly Os[sbnd]Ir alloys) inclusions are ubiquitous in chromite. Galena inclusions with homogenous Pb-isotopic ratios (²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb) suggest a mixed sources of lead. In contrast, pyrite inclusions display significant variations in δ³⁴S_V-CDT values (−0.11 ‰ to 18.31‰) indicative of a crustal origin of sulfur. Bulk-rock Re[sbnd]Os isotope data of 0.1264–0.1294 shows ¹⁸⁷Os/¹⁸⁸Os ratios similar to abyssal peridotites, implying a potential connection between chromitite and the host ultramafic rocks. Highly disparate mantle model ages (55 to 2141 Ma) and re-depletion ages (28 to 692 Ma) suggest disturbance of the Re[sbnd]Os isotopic system during post-formational metasomatic or metamorphic events. These variations may, alternatively, reflect heterogeneity by subduction components. The obtained results do not support the Alaskan-type genesis of the host complex, but point towards an atypical stratiform type.