A study of the trace sulfide mineral assemblages in the Stillwater Complex, Montana, USA

Abstract

The sulfide assemblages of the Stillwater Complex away from the well-studied ore zones are composed mainly of variable proportions of pyrrhotite, chalcopyrite, pentlandite, and ±pyrite. Excluding vein assemblages and those affected by greenschist and lower temperature alteration, the majority can be classified into two broad assemblages, defined here as pristine (multiphase, often globular in shape) or volatilebearing (multiphase, high-temperature, volatile-rich minerals such as biotite, hornblende, or an unmixed calcite-dolomite assemblage). The volatile-bearing assemblages are mainly found within and below the J-M reef, where native copper and sphalerite are also locally present. Pristine sulfides are found throughout the stratigraphy. Both groups can be affected by apparent S loss in the form of pyrite being converted to magnetite and chalcopyrite to a Cu-Fe-oxide (delafossite), with little to no silicate alteration. An upward trend from pentlandite-rich to pyrrhotite-rich to pyrite-rich assemblages is observed in the footwall rocks in upper GN-I, and the same trend repeats from just below the reef and continues into the overlying N-II and GN-II. Modeling suggests that the sulfide Ni in the Peridotite Zone is largely controlled by silicate Ni. When taken together, observations are most readily explained by the remobilization of selected elements by a hightemperature fluid with the apparent loss of S > Cu > Ni. This could concentrate ore metals by vapor refining, eventually producing a platinum group element-enriched sulfide ore zone, such as the J-M reef.

The Stillwater Complex, Montana – Overview and the significance of volatiles

“…Apart from the few sulfide-enriched zones noted by McCallum et al (1980) and noted on Fig. 3, overall bulk-rock S concentrations and sulfide modes are lower than expected from a S-saturated magma (Keays et al, 2012;Aird et al, 2016). There is extensive petrographic evidence of high-temperature S loss seen, for example, by the partial replacement of sulfides by Ti-free iron oxide in otherwise fresh rocks (e.g.…”

“…Abbreviations: pn = pentlandite; po = pyrrhotite; cp = chalcopyrite; py = pyrite. After Boudreau and McCallum (1989), Keays et al (2012) and Aird et al (2016). mafic minerals through the Lower Banded series show a systematic decrease in Mg# and plagioclase a decrease in An#.…”

“…2. More recent studies of the sulfide, halogen, and ore element trends in the complex were undertaken by Boudreau and McCallum (1989), Keays et al (2012), Page and Zientek (1987) and Aird et al (2016) and are shown in Fig. 3.…”

“…In the bulk S column, red circles labelled 'Anal.' are by whole-rock analysis (Keays et al, 2012), the blue circles labelled 'Mode' are bulk S calculated from the sulfide mode by Aird et al (2016) and the dashed pink line is the approximate sulfur concentration if the rocks were saturated in sulfide (Li and Ripley, 2009). Abbreviations: pn = pentlandite; po = pyrrhotite; cp = chalcopyrite; py = pyrite.…”

“…Barnes and Naldrett 1985;Godel and Barnes, 2008), have been generally well studied. Aird et al (2016) characterized the trace sulfide assemblages in the barren rocks of the complex. In brief, the sulfide assemblage associated with the Cl-rich part of the Peridotite zone are unusually rich in pentlandite, whereas pyrite increases up towards OB-I and again towards OB-II.…”