Petrological and geochemical studies of the Xiamaidi mafic–ultramafic intrusion in the Baoshan block, with implications for Ni exploration

Many major magmatic Ni-Cu-Co sulfide deposits worldwide occur in the magma plumbing systems of continental flood basalt. The Xiamaidi mafic–ultramafic intrusion (∼291 Ma) is associated with the Baoshan Late Paleozoic continental flood basalts in southwestern China and hosts important information about the role of magma differentiation in sulfide mineralization in the magma plumbing systems. We address this question by comparing the petrological and geochemical features between the Xiamaidi intrusion and the Daxueshan magmatic Ni-Cu-Co sulfide deposit. The Daxueshan deposit is hosted by a mafic–ultramafic intrusion (∼301 Ma) that is also associated with the Baoshan basalts. The olivine Fo contents of in both intrusions overlap (72–83 mol%), but the Xiamaidi intrusion tends to have slightly lower olivine Fo contents. The olivine of the Xiamaidi intrusive rocks is relatively depleted in Ni (∼600–1300 ppm) and shows no correlation with Fo contents, indicating that multiple pulses of parental magma were involved in the formation of this intrusion. The contents of platinum group elements (PGE) in the Xiamaidi intrusive rocks are about one order of magnitude lower than in the reported Baoshan basalts. The Cu/Pd ratios of the Xiamaidi intrusive rocks and the Daxueshan deposit are similar and are all significantly higher than the ratios of the Baoshan basalts (0.77 × 10⁴ to 4.0 × 10⁴), indicating that the parental magmas for both intrusions had experienced early sulfide segregation at greater depths. Compared to the Baoshan basalts, the Xiamaidi and Daxueshan intrusive rocks have larger negative Nb-Ta anomalies, lower ε_Nd(t) values (–5.7 to –7.9) and higher (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7111 to 0.7135) than the average compositions of the basalts [ε_Nd(t) = –1.9; (⁸⁷Sr/⁸⁶Sr)_i = 0.7073]. The differences can be explained by different mantle sources plus variable crustal contamination, with the parental magmas for the intrusive rocks derived predominantly from a subcontinental lithospheric mantle whereas the flood basalts derived mainly from the underlying mantle plume. The estimated amounts of crustal contamination in the parental magmas for the Xiamaidi and Daxueshan intrusions vary from 15 to 30 wt%. The whole-rock Sr-Nd isotopic and trace elemental data support the view that sulfide saturation in the parental magmas was mainly induced by crustal contamination. The most important implications for Ni exploration from this study are (1) at an intrusion scale, future exploration should focus on the feeders of the Xiamaidi and Daxueshan intrusive systems, and (2) at a regional scale, the ultramafic intrusions containing olivine with higher Fo contents than that in the Xiamaidi ultramafic rocks are the most favorable exploration targets in the Baoshan block.