Genesis of the Lijiahe banded carbonatite intrusion and associated iron mineralization, northwestern Yangtze Block, South China

Banded carbonatites commonly show a rhythmic alternation between calcite and magnetite layers, the latter often containing significant iron oxide (magnetite) mineralization. However, the petrogenesis of banded carbonatites and the mechanism responsible for iron mineralization remain poorly understood. In this study, we present detailed petrographic observations as well as mineralogical and geochemical data for the Lijiahe banded carbonatites in the northwestern Yangtze Block, central China. The banded carbonatites outcrop as small-scale stocks and dikes in the field, and they can be subdivided into three distinct types of layers: a calcite-rich layer, a mixed layer, and a magnetite-rich layer. Each layer consists of varying proportions of magnetite, calcite, and apatite, with minor amounts of olivine, phlogopite, and Nb-rich minerals. Apatite U-Pb dating results reveal that the Lijiahe banded carbonatites were formed at 778 ± 24 Ma. Three layers in the Lijiahe carbonatites all have identical Sr and Nd isotopic compositions [(87Sr/86Sr)i = 0.7039−0.7048; εNd(t) = +1.5 to +3.4], similar to the regional arc-related mafic rocks, suggesting derivation from a lithospheric mantle enriched by slab-derived materials. Both calcite and whole-rock compositions for the Lijiahe carbonatites are characterized by flat rare earth element (REE) patterns and low (La/Yb)N ratios (4.74−6.79 and 5.54−14.1, respectively), which is in stark contrast to global primary carbonatites and calcite. These distinctive flat light REE patterns may require magma generation from a garnet-poor, heavy REE−rich carbonated mantle source. Exhibiting distinct mineral assemblages and characteristic magmatic textures, the magnetite-rich layer has higher TFe2O3, Co, high field strength element (HFSE), Sn, and Zn contents but lower CaO, Sr, Ba, and REE contents than the mixed layer and the calcite-rich layer. Elevated δ13C (−0.6‰ to 0‰) and δ18O (17.6‰ to 18.4‰) values, combined with alkali depletion and replacement textures, record a late-stage carbo(hydro)thermal overprint. We propose that primary Fe oxide accumulation was predominantly governed by early magmatic processes, probably involving liquid immiscibility or density-driven mineral sorting. Subsequently, fluid-assisted metasomatism further enhanced enrichment in Fe and HFSEs (notably Nb) within the layered sequence.