INVESTIGATIONS OF THE STILLWATER COMPLEX: III. THE PICKET PIN Pt/Pd DEPOSIT.
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 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 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.
Boudreau, AE; McCallum, IS
Economic geology Lancaster, Pa.
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