Fig. 3: Petrological and geochemical evidence for the magmatic origin of sulfides from BM and CS.

a Back-scattered (BSE) image of a primary sulfide grain hosted by CS1 gabbronorite (Gab) domain displaying (lighter) lamellae of troilite (Tro) exsolved from pyrrhotite (Po; darker) associated with small elongated crystals of magmatic low-temperature (LT) pentlandite (Pn). b The typical sulfide assemblage in a CS1 pyroxenite (Px) including Po, massive grains of chalcopyrite (Ccp) and high-temperature Pn (HT-Pn). c Sulfide assemblage in a CS2 Px displaying partial replacement of primary grains (relict Po) by massive pyrite (Py) with Ccp crystals or blebs enclosed within Py. d Association of sulfides and massive, irregular-shaped ilmenite (Ilm) and magnetite (Mag) in a CS1 Px. Images b to d are reflected light images. e Binary diagram of bulk S versus Se indicating S addition to CS1 and CS2, and S loss in the top CS3 site. The mantle S/Se range is adapted from Eckstrand and Hulbert⁶⁶. The black dashed lines denote the S/Se ratios. f Binary plot of Sb/Se and As/Se ratios in selected Pn. The inset indicates the effect of processes controlling these ratios. The colored fields are adapted from Mansur et al.³⁵. High Sb/Se and As/Se values suggest an external input of As and Sb in the sulfide liquid by crustal assimilation. To plot Pn composition from the VS8 site, the As and Sb concentrations are calculated as 0.75 of the detection limit. g Binary diagram of Cd/Se versus Ni/Se in Ccp. h Binary diagram of Co/Sb versus Se/As in Py, enabling to discriminate between magmatic and hydrothermal Py. The black dashed lines in diagrams (g) and (h) separate the magmatic and hydrothermal compositional fields^68,69, respectively; the colored fields correspond to the Ccp and Py compositions from the magmatic deposits³⁵. All sampling sites abbreviations refer to Fig. 1.