Origin of the high Pd/Pt ratio of the J-M Reef, Stillwater Complex Montana USA
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Abstract
The J-M Reef of the Stillwater Complex exhibits a high and consistent Pd/Pt ratio (~3.8). This ratio results from the equilibration of an immiscible sulfide liquid with a relatively high Pd/Pt silicate melt rather than an unusually Pd- and Pt-enriched parental melt. Numerical modeling suggests that the original silicate melt contained typical mantle-derived concentrations of Pd and Pt (~10–20 ppb Pd/Pt ~1). The partitioning of Pt and Pd between sulfide liquid and silicate melt alone cannot explain the consistently high Pd/Pt ratio across variable melt-to-sulfide mass ratios (R factors). Instead Pt-depletion caused by the early fractionation of Pt-alloy from S-undersaturated silicate magma likely established the high Pd/Pt signature. High Pd/Pt ratios can form through batch equilibration of sulfide liquid with silicate melt if partition coefficients are extremely high (>10⁶). Alternatively Pd enrichment may result from sulfide upgrading within the resident footwall mush under smaller partition coefficients (10⁴–10⁶) in this model the instantaneous R factors remain low (R ≈ 100–700). This limits the impact of Pt and Pd partitioning on sulfide composition and helps explain the Pd-enriched character of the J-M Reef.
| Publication type | Conference Paper |
|---|---|
| Publication Subtype | Abstract or summary |
| Title | Origin of the high Pd/Pt ratio of the J-M Reef, Stillwater Complex Montana USA |
| Volume | II |
| Year Published | 2025 |
| Language | English |
| Publisher | Society for Geology Applied to Mineral Deposits |
| Contributing office(s) | Geology, Minerals, Energy, and Geophysics Science Center |
| Description | 4 p. |
| First page | 603 |
| Last page | 606 |
| Conference Title | 18th SGA Biennial Meeting |
| Conference Location | Golden, CO |
| Conference Date | August 3-7, 2025 |
| Country | United States |
| State | Montana |
| Other Geospatial | J-M Reef Stillwater Complex |