Investigations of the Stillwater Complex; III, The Picket Pin Pt/Pd deposit

Univ. Wash., Dep. Geol. Sci., Seattle, WA, United States

The Picket Pin Pt/Pd deposit is a zone of disseminated, platinum-group element-bearing sulfide which occurs in the upper 150 m of Anorthosite subzone II (AN II), the thickest anorthosite member (ca. 600 m) of the Stillwater Complex. The sulfide zone is strata bound in the sense that it is traceable at the same stratigraphic interval over the 22 km of exposed strike length of the deposit. Although platinum-group element-bearing sulfides locally occur up to the contact with the overlying troctolite, sulfides most commonly occur in a poorly defined zone, the top of which occurs approximately 10 m below the top of Anorthosite subzone II, at a distinct textural and mineralogic transition. Below this transition, the anorthosite is characterized by relatively coarse cumulus plagioclase and up to 20 percent intercumulus pyroxene (augite and inverted pigeonite). Sulfides below the transition occur as podiform and lenticular accumulations which are conformable but discontinuous along strike. Below this zone, discordant sulfide-bearing pipes occur to a depth of 150 m in the footwall anorthosite. The platinum-group element-bearing sulfides are preferentially associated with pyroxene-poor, quartz- and apatite-bearing assemblages. The hanging wall consists of a medium-grained, largely monomineralic anorthosite member (ca. 10 m thick) overlain by troctolite of the Olivine-bearing subzone V. Sulfide within these two hanging-wall members is largely barren of platinum-group elements.The platinum-group element-bearing sulfide assemblage consists of pyrrhotite, chalcopyrite, and pentlandite. Geochemical trends suggest that arsenides and antimonides are the main host for the platinum-group elements. The composition of the platinum-group element-bearing sulfide is 6.1 wt percent Ni, 14.6 wt percent Cu, 0.2 wt percent Co, 1.4 ppm Ir, 35 ppm Pt, 44 ppm Pd, and 3.6 ppm Au. Whole-rock incompatible trace element abundances in sulfide-bearing rocks are high relative to those of average anorthosite in Anorthosite subzone II.The presence of transgressive zones of sulfide leading up to conformable sulfide-rich zones, the association of sulfide with late crystallizing minerals, and the Cu-rich nature of the sulfide assemblage are consistent with a model in which mineralizing solutions percolated upward during solidification of Anorthosite subzone II. It is proposed that the monomineralic medium-grained anorthosite member acted as an impermeable zone which trapped the migrating fluids at its lower contact. Calculated fluid compositions in equilibrium with the observed mineral assemblage show that these fluids were silica and alkali rich and that HCl and HF were important fluid species. The mutual exclusion of Fe-Ti oxides and sulfides indicates that the sulfides formed at a slightly higher f (sub S ₂ ) .

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