Compositional variations of magnetite in different sulfide ore types in the Jinchuan Ni-Cu-PGE sulfide deposit, NW China: Insights into the mineralizing processes of conduit-type systems

Abstract

The trace-element composition of magnetite in Ni–Cu–platinum-group element (PGE) sulfide deposits is influenced by magmatic evolution, fractional crystallization of sulfide liquid, and post-magmatic processes, and, therefore, may record ore-forming processes. The Jinchuan Ni–Cu–(PGE) sulfide deposit in NW China contains a variety of sulfide ore types. Despite the capabilities of magnetite to serve as a petrogenetic indicator, its textural–compositional variability in these different sulfide ore types has received little attention. Five types of magnetite with distinct textures and chemical composition are identified in this deposit: (1) Cr-magnetite that crystallized from silicate magma, (2) reactional magnetite produced by the diffusion of oxygen from sulfide liquid to silicate melt, magnetite that crystallized from (3) MSS and (4) ISS, and (5) magnetite that precipitated from hydrothermal fluids. The compositional variability of Cr-magnetite in net-textured and disseminated ores records fractionation of multiple pulses of silicate magmas in a conduit system. Particularly, the higher Ni/Cr and lower V content of Cr-magnetite in disseminated ores relative to those in net-textured ores suggests that the silicate magmas from which the disseminated ores formed were more evolved and had higher fO2 than the magmas from which the net-textured ores formed. Compositional differences between Cr-magnetite and MSS magnetite suggest that magnetite that crystallized from sulfide liquids has higher Ni/Cr ratio, and lower concentrations of lithophile and chalcophile elements. Thus, magmatic magnetite with such compositional features is indicative of sulfide saturation in Ni–Cu–(PGE) sulfide deposits. Compositional differences between MSS and ISS magnetite record fractional crystallization of sulfide liquid. Particularly, the lower V content of ISS magnetite relative to that of MSS magnetite suggests an increase in fO2 during fractionation of the sulfide liquid. Accordingly, this results in greater diffusion of oxygen between the sulfide liquid and surrounding silicate magma, and the preferential occurrence of reactional magnetite in Cu-rich ores. Hydrothermal magnetite records the interaction of magmatic sulfides with oxidized hydrothermal fluids. Based on the lowest chalcophile element contents of this type of magnetite, this interaction did not affect the metal budget of the sulfide ores. Lastly, the similar chalcophile element contents, and overall similar multi-element variation patterns of MSS magnetite in disseminated and massive ores demonstrates that the sulfide liquid that formed the different ore types may have had similar chalcophile element contents. Therefore, the massive ores likely formed by the draining and coalescance of originally disseminated sulfide liquid in a mush zone. Taken together, the compositional variability of texturally distinct magnetite at Jinchuan document a range of magmatic–hydrothermal processes that formed and modified this mineralized system. This contribution continues to highlight the potential for magnetite to serve as a petrogenetic indicator for magmatic Ni-Cu-PGE deposits globally.