Geochronology and geochemistry of the Nantianwan mafic-ultramafic complex, Emeishan large igneous province: metallogenesis of magmatic Ni-Cu sulphide deposits and geodynamic setting

The Nantianwan mafic-ultramafic complex is situated in the northwest part of the Panxi district, southwest China. It consists predominantly of gabbros, gabbronorites, and lherzolites. LA-ICP-MS U-Pb zircon dating of the gabbronorites yields an age of 259.7 +/- 0.6 million years, consistent with the ages of other mafic-ultramafic intrusions in the Emeishan large igneous province (ELIP). Gabbronorites and lherzolites host Cu-Ni sulphide ores. Cumulus texture is pronounced in these rocks, containing magnesium-rich olivine (up to 81.4% forsterite). SiO2 contents of the lherzolites range from 42.93 to 44.18 wt.%, whereas those of the gabbronorites vary between 44.89 and 52.76 wt.%. Analysed samples have low rare earth element (REE) contents (23.22-30.16 ppm for lherzolites and 25.21-61.05 ppm for gabbronorites). Both lherzolites and gabbronorites have similar chondrite-normalized REE patterns, suggesting that they are comagmatic. All samples are slightly enriched in large ion lithophile elements (LILEs, e. g. Rb, Ba, and Sr) relative to high field strength elements (HFSEs, e. g. Nb, Ta, and Ti), very similar to those of ocean island basalts (OIBs). The presence of cumulus textures and geochemical signatures indicates that fractional crystallization played an important role in the petrogenesis of these rocks. Initial (Sr-87/Sr-86)(t) (t = 260 Ma) ratios and epsilon(Nd)(t) values of the mafic-ultramafic suite vary from 0.70542 to 0.70763, and -0.4 to 1.7, respectively. Compared to the Cu-Ni-bearing Baimazhai and Limahe intrusions in the ELIP, which were considerably contaminated by variable crustal materials, the Nantianwan complex exhibits much lower (Sr-87/Sr-86)(t). Their epsilon(Nd)(t) versus (Th/Nb)(PM) ratios also indicate that the ore-bearing magmas did not undergo significant crustal contamination. In combination with (Tb/Yb)(PM) versus (Yb/Sm)(PM) modelling, we infer that the magmas originated from an incompatible elements-enriched spinel-facies lherzolite that itself formed by interaction between the Emeishan plume and the lithospheric mantle. Most plots of NiO versus Fo contents of olivine suggest that sulphides are separated from the parental magma by liquid immiscibility, which is also supported by bulk-rock Cu/Zr ratios of the lherzolites (7.04-102.67) and gabbronorites (0.88-5.56). We suggest that the gabbronorites and lherzolites experienced undersaturation to oversaturation of sulphur; the latter may be due to fractional crystallization in a high-level magma chamber, accounting for the sulphide segregation.