Distribution of chalcophile elements during crystallisation and alteration of magmatic Ni-Cu-Co sulfide deposits in the anorthositic Espedalen Complex, Norway: pentlandite as an indicator for tracking metal tenors

Abstract

This study presents an investigation of the distribution of trace elements within base metal sulfides from magmatic Ni-Cu-Co deposits from the Proterozoic anorthositic Espedalen Complex, Norway. The mineralisation occurs both as primary, undeformed sulfides and as deformed sulfides hosted within shear zones. Distinct deposits yield Ni tenors ranging from 3 to 10%, allowing the assessment of whether the metal tenor of a deposit is reflected in its sulfide composition. The results allow constraining geological processes spanning from the compositional traits of the parental melts to late-magmatic fluid interactions. Notably, the sulfides exhibit a relatively low concentration of chalcophile elements compared to other Ni-Cu-Co deposits worldwide, particularly platinum-group elements (PGE). This is because these deposits formed from PGE-depleted magmas. Elements compatible with monosulfide solid solution (MSS; Co, Rh, Ru, Ir, Os and Se) are predominantly hosted by sulfides, whereas a smaller proportion of incompatible elements (Pb, Cd, Ag, Bi, Zn, In, Tl, As, Sn and Mo) is also accommodated by sulfides. Our findings for sulfide composition support for the first time a positive correlation between Se concentrations in pentlandite and whole-rock Ni tenors for both Espedalen and magmatic Ni-Cu-Co sulfide deposits worldwide. This is because of the more efficient collection of both Ni and Se by an immiscible sulfide liquid under high R-factor regimes, combined with the fact that Se concentrations in pentlandite remain largely undisturbed during post-cumulus processes as opposed to other trace elements. Consequently, Se concentrations in pentlandite may serve as a proxy for metal enrichment in magmatic sulfide deposits.