ARSENIC-RICH Cu-Ni-PGE MINERALIZATION IN WETLEGS, DULUTH COMPLEX, ST. LOUIS COUNTY, MINNESOTA, USA

Abstract

The magmatic Cu-Ni ± PGE (platinum group elements) sulfide deposit Wetlegs within the troctolitic Partridge River intrusion (PRI) of the 1.1 Ga Duluth Complex (northeastern Minnesota) contains disseminated low-grade Cu-Ni sulfide minerals distributed along its heterogeneous base. The main association consists of magmatic intercumulus pyrrhotite, chalcopyrite, pentlandite, and cubanite. Secondary copper-bearing sulfide minerals (bornite, covellite, yarrowite, and digenite) are found within variously altered troctolites. Several late stage arsenic-rich phases occur in altered troctolitic rocks and underlying metasedimentary footwall hosts (Virginia Formation). They appear as nickeline, maucherite, and diarsenides in the system rammelsbergite-safflorite-loellingite, and sulfarsenides of the cobaltite-gersdorffite solid-solution series. Based on their textural relationships and chemistries a three stage depositional history is suggested, starting with the formation of nickeline, followed by diarsenides enriched in Co (below 600 °C) and the late Co-rich sulfarsenides (between 550 °C and 400 °C) in the troctolitic host rock. Due to the higher Ni-and Fe-contents, higher formation temperatures are proposed for arsenides (below 625 °C) and sulfarsenides (~600 °C) in the recrystallized microveins of the footwall. The restricted appearance in altered troctolitic rocks and underlying recrystallized footwall metasediments (Virginia Formation) suggest a deposition of arsenic-rich phases by late-stage hydrothermal fluids. In addition, the metasedimentary Virginia Formation may be a potential source of elements necessary to form arsenides and sulfarsenides, which is further supported by elevated δ34S values (>9‰) for samples from the arsenide assemblages. The platinum-group minerals (PGMs; sperrylite, stibiopalladinite) and precious element-bearing phases are either associated with primary magmatic sulfides, or hydrothermally altered silicate patches (amphibole, chlorite, sericite, prehnite, carbonates, and serpentine). These associations suggest two sources for PGMs: fractionation of a sulfide-saturated melt, and a remobilization and re-deposition of platinum-group elements (PGEs) in the form of aqueous complexes during the late-stage hydrothermal events.