Micron-to-nanoscale investigation of Cu-Fe-Ni sulfide inclusions within laurite (Ru, Os)S2 from chromitites

Abstract

This paper provides a top-down nanoscale analysis of Cu-Ni-Fe sulfide inclusions in laurite from the Taitao ophiolite (Chile) and the Kevitsa mafic-ultramafic igneous intrusion (Finland). High-resolution transmission electron microscopy (HRTEM) reveal that Cu-Ni-Fe sulfide inclusions are euhedral to (sub)-anhedral (i.e., droplet-like) and form single, biphasic or polyphasic grains, made up of different polymorphs, polytypes and polysomes even within a single sulfide crystal. Tetragonal (I4\(\stackrel{-}{2}\)d) and cubic (F\(\stackrel{-}{4}\)3m) chalcopyrite (CuFeS₂) host frequent fringes of bornite (Cu₅FeS₄; cubic F\(\stackrel{-}{4}\)3m and/or orthorhombic Pbca) ± talnakhite (Cu₉(Fe, Ni)₈S₁₆; cubic I\(\stackrel{-}{4}\)3m) ± pyrrhotite (Fe_1 − xS; monoclinic C2/c polytype 4C and orthorhombic Cmca polytype 11C) ± pentlandite ((Ni, Fe)₉S₈; cubic Fm3m). Pentlandite hosts fringes of pyrrhotite, bornite and/or talnakhite. Laurite and Cu-Fe-Ni sulfide inclusions display coherent, semi-coherent and incoherent crystallographic orientation relationships (COR), defined by perfect edge-to-edge matching, as well as slight (2–4º) to significant (45º) lattice misfit. These COR suggest diverse mechanisms of crystal growth of Cu-Fe-Ni sulfide melt mechanically trapped by growing laurite. Meanwhile, the mutual COR within the sulfide inclusions discloses: (1) Fe-Ni-S melt solidified into MSS re-equilibrated after cooling into pyrrhotite ± pentlandite, (2) Cu-Ni-Fe-S melts crystallized into the quaternary solid solution spanning the compositional range between heazlewoodite [(Ni, Fe)_3±xS₂] (Hz_ss) and ISS [(Cu_1±x, Fe_1±y)S₂]. Additionally, nanocrystallites (50–100 nm) of Pt-S and iridarsenite (IrAsS) accompanying the sulfide inclusions spotlight the segregation of PGE-rich sulfide and arsenide melt earlier and/or contemporarily to laurite crystallization from the silicate magmas. Cobaltite (CoAsS)-gersdorffite (NiAsS) epitaxially overgrown on laurite further supports the segregation of arsenide melts at early stages of chromitite formation.