The origin of high Cu/S sulfides in the Skaergaard Intrusion, East Greenland

Abstract

The Skaergaard Intrusion of East Greenland is unusual in that the trace sulfide assemblage is characterized by unusually high Cu/S ratios, containing abundant chalcopyrite and bornite (Cu5FeS4), the latter otherwise rare in most layered intrusions. Explanations for the high Cu/S ratio range from the parent magma inheriting low S concentration from the mantle source tosuggestions that S was lost to hydrothermal fluids as the intrusion cooled. The sulfide assemblage beneath the Au- and PGE-rich Platinova Reef show the common replacement of the sulfide assemblage with a Ti-poor magnetite. A non-equilibrium thermodynamic analysis of modal variations is consistent with the general reaction of chalcopyrite --> bornite + magnetite + S (lost) and that the original Cu:S mass ratio of the magma that crystallize the Lower Zone was~1:1. In addition, a comparison with other hot spot and flood basalt provinces suggest that these magmas begin to degas while still at mantle depths and can lose significant S to shallow level degassing. It is concluded that degassing of the parent Skaergaard magma prior to solidificationand continuous S loss during crystallization both contributed to the overall high Cu/S ratio of the Skaergaard system. More generally, variations in the Cu/S ratio of sulfides in basaltic systems may be a useful indicator of the extent of magma degassing.