Investigations of the Howland Reef of the Stillwater Complex, Minneapolis Adit area; stratigraphy, structure, and mineralization

Abstract

A platinum-group mineral deposit occurs within the lower part of the banded zone of the Stillwater Complex, a layered, tholeiitic intrusion of Archean age. The zone of platinum concentration, the Howland reef, is coincident with the reappearance of olivine in the crystallization of the complex. Host rocks to the mineralization are distinguished by variable proportions of olivine, bronzite, plagioclase, and augite. Phlogopite, hornblende, apatite, sulfide, and chrome spinel are accessory.Significant lateral and vertical variations in the composition and thickness of the Howland reef are evident over 700 m of underground development. There are superimposed large- and small-scale changes in mineral assemblages and rock textures suggesting that processes other than gravity sorting or crystal sedimentation were responsible for reef formation.Definition drilling in the Minneapolis adit and exploratory surface drilling have shown that the Howland reef is thickened locally by fault repetitions and elsewhere attenuated by the same system of high-angle, reverse faults.Significant concentrations of platinum-group elements (PGE) within the Howland reef are restricted to sulfide minerals. The sulfides show clear evidence of an origin as accumulated, immiscible sulfide liquid droplets.Sulfides occur generally as composite grains or globules, consisting mainly of pyrrhotite, pentlandite, chalcopyrite, and platinum-group minerals, with or without pyrite and assorted low-temperature alteration products. The dominant platinum-group-element-bearing minerals belong to the braggite-vysotskite [(Pt, Pd Ni)S] solid solution series.Consistent relationships between sulfide morphologies and the phase assemblages of host silicates provide convincing evidence that the immiscible sulfide droplets formed concurrently with olivine during the earliest stages of reef genesis.