Arsenic-rich Cu-Ni-PGE mineralization in Wetlegs, Duluth Complex, St. Louis County, Minnesota, USA

• Published in: Canadian mineralogist Vol. 53; no. 1; pp. 105 - 132
• Main Authors: Raic, Sara, Mogessie, Aberra, Benko, Zsolt, Molnar, Ferenc, Hauck, Steven, Severson, Mark
• Format: Journal Article
• Language: English
• Published: Mineralogical Association of Canada 01-01-2015
• Subjects: arsenic; arsenides; bornite; chalcopyrite; cobaltite; copper ores; covellite; cubanite; digenite; Duluth Complex; Economic geology; gabbros; gersdorffite; hydrothermal alteration; igneous rocks; intrusions; layered intrusions; lollingite; maucherite; metal ores; metals; metamorphic rocks; metasedimentary rocks; metasomatism; mineralization; Mineralogy; Minnesota; nickel ores; nickeline; nonsilicates; P-T conditions; Paleoproterozoic; Partridge River Intrusion; pentlandite; platinum group; platinum minerals; platinum ores; plutonic rocks; Precambrian; Proterozoic; pyrrhotite; rammelsbergite; recrystallization; safflorite; Saint Louis County Minnesota; solid solution; sperrylite; stibiopalladinite; sulfides; sulfosalts; troctolite; United States; upper Precambrian; Virginia Formation; Wetlegs Deposit; yarrowite
• ISSN: 0008-4476; 1499-1276
• DOI: 10.3749/canmin.1400053

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 ppm) 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.