Investigations of the Stillwater Complex: III. The Picket Pin Pt/Pd deposit ( Montana, USA).
The Picket Pin Pt/Pd deposit is a zone of disseminated, Pt-group element-bearing sulfide which occurs in the upper 150 m of Anorthosite subzone II (AN II), the thickest anorthosite member (c 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. The Pt- group element-bearing sulfide assemblage consists of pyrrhotite, chalcopyrite, and pentlandite. Geochemical trends suggest that arsenides and antimoides are the main host for the Pt-group elements. The composition of the Pt-group element-bearing sulfide is 6.1 wt % Ni, 14.6 wt % Cu, 0.2 wt % Co, 1.4 ppm Ir, 35 ppm Pt, 44 ppm Pd, and 3.6 ppm Au. The presence of transgressive zones of sulfide leading up to conformable sulfide-rich zones, the association of sulfide with late crystalizing 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.-from Authors
Boudreau, AE; McCallum, IS
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