SULFIDE MINERALS OF THE MIDDLE BANDED SERIES OF THE STILLWATER COMPLEX, MONTANA

Models of the origin of the platinum-group element deposits in layered intrusions such as those of the 2.7 billion year old Stillwater Complex range from strictly magmatic to hydro-magmatic. Meurer et al (1999 Lithos 47:143-156) found evidence for Cl-rich fluids moving through the Middle Banded Series (MBS) of the Stillwater complex. They suggested that the ore metals in the MBS were remobilized and fractionated by a vapor refining process based on geochemical trends within this 1750m section. The whole rock analysis by Meurer et al shows modification of Cu content in olivine-bearing zones III and IV (OB-III and –IV) that broadly matches models of chromatographic separation by a Cl-rich fluid moving upward through the column. This project has looked for petrographic evidence of high temperature remobilization of sulfide minerals in the MBS to confirm Meurer et al’s findings.

The MBS includes troctolites and olivine-bearing gabbros and gabbronorites which contain single phase sulfides in addition to multiphase sulfides composed of combinations of chalcopyrite, pentlandite, pyrite, and pyrrhotite. The observed assemblages of sulfides range from fresh to locally affected by greenschist alteration; the major focus was on the fresh sulfides. These assemblages differ from those expected in magmatic crystal fractionation and settling models where Ni is quickly depleted and Cu becomes dominant. Instead, the profiles of the Ni- and Cu-containing sulfides pentlandite and chalcopyrite indicate modification of sulfides during cooling, possibly by late magmatic vapor that caused remobilization of ore elements. The pentlandite percentage of the sulfides in the MBS fluctuates considerably but does not decrease overall as expected, while the chalcopyrite percentage shows no marked increase. Samples in the lowest section of OB-IV contained exclusively pyrite. Higher in the OB-IV pentlandite or chalcopyrite became the major sulfide elements and were often present in a multiphase sulfide grain with pyrite or pyrrhotite. The variations seen in these sulfide percentages are too rapid to be the result of fractional crystallization, but are consistent with vapor remobilization of Cu in the cooling crystal pile.