The role of fluids in the formation of platinum-group minerals, Duluth Complex, Minnesota; mineralogic, textural, and chemical evidence

Abstract

Several new occurrences of platinum-group minerals (PGM), including sperrylite (PtAs 2 ), taimyrite ((Pd, Cu) 3 Sn), froodite (PdBi 2 ), michenerite (PdBiTe), and moncheite ((Pt,Bi) 2 Te 3 ) are recorded from drill core samples of the basal troctolites of the Duluth Complex. The first documentation of a vanadium-chromium hercynite (V 2 O 3 ranging from 5.14-5.23 wt %) in graphite-rich samples of the mineralized zone of the Duluth Complex is also reported.The PGM are associated predominantly with serpentinized olivine and secondary magnetite or with altered, calcic plagioclase ( approximately An 75 ), prehnite, actinolite, hornblende, and chlorite. The Pd bismutho-tellurides are associated with parkerite (Ni 3 Bi 2 S 2 ), tellurobismuthite (Bi 2 Te 3 ), bismuthinite (Bi 2 S 3 ), hessite (Ag 2 Te), native gold, and clausthalite (Pb(S,Se)) with low-temperature sulfides like valleriite (2(Fe,Cu)S . 3(Mg,Fe)(OH) 2 ), violarite (FeNi 2 S 4 ), heazlewoodite (Ni 3 S 2 ), and in some instances, graphite. These associations, especially the occurrence of the PGM in altered silicates, suggest that the PGM reported in this study were transported and deposited during a late magmatic hydrothermal event which could be related to the introduction of volatiles from the metasedimentary Virginia Formation and the Biwabik Iron-Formation. The high Cl contents of the serpentine, biotite, apatite, and graphite as well as the high An contents of the plagioclase in contact with the sulfides and PGM further suggest that chloride complexing was a possible mechanism for the transport and deposition of the PGM during the hydrothermal event. The occurrence of high vanadium-chromium hercynite is also consistent with this interpretation.