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

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 degrees C) and the late Co-rich sulfarsenides (between 550 degrees C and 400 degrees C) in the troctolitic host rock. Due to the higher Ni- and Fe-contents, higher formation temperatures are proposed for arsenides (below 625 degrees C) and sulfarsenides (similar to 600 degrees 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 delta S-34 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.