Sulfides, native metals, and associated trace minerals of the Skaergaard intrusion, Greenland: evidence for late hydrothermal fluids

Abstract

Sulfide assemblages, precious metals, transition metal alloys, and associated accessory phases were characterized throughout the Skaergaard intrusion to better constrain the sulfide saturation history of the intrusion and the role of late magmatic volatiles in modifying the Skaergaard metal budget and distribution. Sulfides in and below the Middle Zone of the Layered Series of the intrusion are readily replaced by low-Ti magnetite. The ratio Σ(low-Ti magnetite mode)/Σ(sulfide mode), indicating oxidation of sulfides, reaches maximum values in the Lower Zone of the Layered Series. Sulfide assemblages below the Middle Zone are typically accompanied by minor biotite, apatite, and rare calcite as well as trace compositionally distinctive clinopyroxene and orthopyroxene. The occurrence of Ag, Au, Pt, Cu, and metal alloys outside of the Middle Zone is further evidence of the Skaergaard intrusion parental magma being S-poor. Native Ag, commonly accompanied by trace amounts of Cl, occurs both in and below the Middle Zone. Evidence of coexisting precious metal + brine assemblages exists where native metals are accompanied by sylvite ± halite and Ag is accompanied by Ag halides. Ag occurrences in the Middle Zone are of irregular morphology with trace Cl ± S ± calcite. Further evidence supportive of a metal + brine assemblage is observed where Ag + quartz is found in an apparent open clinopyroxene-hosted fluid inclusion consisting of Na, Si, Cl, Ca, K, and S. Ag is used to model the behavior of precious and transition metals in the presence of an exsolving fluid phase. Numerical modeling suggests that, in a sulfide-bearing system, residual Ag concentrations and concentrations in the exsolved fluid are most affected at the point where sulfide is lost to a separating volatile fluid phase. It is suggested that, owing to the low-S nature of the Skaergaard system, fractional crystallization and early fluid saturation produced enrichment of Ag, with other precious and transition metals, in the interstitial silicate liquid much higher than normal due to delayed sulfide saturation. As this interstitial liquid evolved, Ag was lost to an exsolved volatile phase of high salinity and migrated upward. A similar process likely occurred for Au and other elements with high affinities for Cl.