Yarden Gedalia, BSc
Y. Gedalia1, B. O'Driscoll1
1Department of Earth and Environmental Sciences, University of Ottawa,Ottawa, Ontario, Canada
The ~2.7 Ga Stillwater Igneous Complex (StIC, USA), particularly the J-M Reef, is the most enriched platinum-group element (PGE) deposit in the world. Precious metal enrichment in layered intrusions like the StIC is commonly hosted in chromitite seams. Even though numerous chromitite seams occur in the lower parts of this intrusion, chromitite has not traditionally been associated with the J-M Reef. The primary objective of this research is to study the petrogenesis of newly discovered and undocumented chromitite seams from the J-M Reef (at the Stillwater Mine) and compare their chromite chemistry to that of chromitite seams of the lowermost Peridotite Zone (PZ). A ~50 cm drill core containing one major (~15 cm) and three minor (~1 cm) chromitite seams hosted in coarse-grained anorthosite and gabbronorite was studied here. In addition to petrography, quantitative textural data were obtained from crystal size distribution (CSD) and apparent dihedral angle (ADA) measurements, as well as chromite chemistry by electron microprobe.
Three out of nine thin sections across the studied section were used for CSD and ADA analyses (comprising three subsampled areas per thin section from the main chromite seam totalling ~3000 CSD and ~2000 ADA measurements). Chromite seams only form embedded in coarse-grained intercumulus minerals (plagioclase, clinopyroxene, and orthopyroxene) with distinct grain boundaries, but chromite seams do not form together with cumulus plagioclase. Chromite grains form two distinct shapes: large and polygonal versus small and rounded. Thin (<1 mm) silicate rims are common around chromite grains. CSD mean grain sizes for the three samples are 0.138, 0.188, and 0.287 cm, whereas CSD volume phase abundances are 34.50, 37.44, and 41.32 vol.%. The CSD plot shapes exhibit kinks at distinct size fractions (i.e., different populations) in all samples. CSD slope values for the three samples are -6.780, -11.77 and -14.38 mm-1. Median average ADAs for chromite-chromite-clinopyroxene are 64.33, 79.54, and 84.61. For chromite-chromite-plagioclase, median average ADAs are 66.33, 71.97, and 81.11. The latter ADAs are farther from textural equilibrium, i.e., a single true dihedral angle value, for each sample.
Overall, we find that the fine-grained chromite occurs in higher volume abundances, exhibits steeper CSD slopes and reveals lower median ADAs (a lower degree of textural equilibrium). Coarser-grained chromites exhibit the converse characteristics. Rims around chromite show that a reactive interstitial melt existed coevally with chromite; the extent to which this postcumulus reaction drove the textural coarsening observed in the rocks is being investigated.