Abstract

Drilling by the Stillwater Mining Company in the Minneapolis adit area of the Stillwater Complex has afforded a three-dimensional view of platinum-group element mineralization in the J-M reef. Stratigraphically, mineralization occurs at four levels ("zones") relative to the base of olivine-bearing zone I: (1) Footwall zone below olivine-bearing zone I, (2) Basal zone, straddling the basal olivine-bearing zone I contact, (3) Main zone within olivine-bearing zone I, and (4) Upper zone within olivine-bearing zone I but above the Main zone. Usually only one zone is present, but up to three stacked zones do occur. In lateral dimension, i.e., in the plane of the layering, all four zones are characterized by elongate or arcuate patterns of mineralization; the mineralized areas are irregularly spaced at approximately 60 to 75 m. The basal olivine-bearing zone I contact shows depressions and highs of similar dimension, but the position of mineralization does not correlate with the topography of the contact.We propose that olivine-bearing zone I formed as a result of injection of a buoyant plume of a secondary melt (magma II), which was hotter and less dense than the resident gabbroic melt (magma I'). Cooling of magma II increased its density causing it to fall to the floor as inverse diapirs; this sulfide-saturated magma (II) scavenged platinum-group elements from magma I' during descent. Upon reaching the floor, the diapirs coalesced and pooled as a hot layer. With cooling, convection within the layer eventually became laminar and ordered convection cells formed. Resorption of bronzite and crystallization of olivine in the hot upwelling centers of these cells caused local depressions in the basal olivine-bearing zone I contact. Sulfides accumulated along the cooler downfalling peripheries, yielding the crude honeycomb pattern observed. Periodic turbulence with subsequent calming caused shifting of the convection cells and formation of the discrete stratigraphic levels of mineralization.

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