According to this U.S. Bureau of Mines study, platinum-group elements (pge) are preferentially associated with chromian spinels, including high-iron chromite, magnesian-chromian hercynite, and chromian magnetite in clinopyroxene-rich upper stratigraphic portions of the tonsina Ultramafic complex in southern Alaska. Ratios of pt:pt+pd are proportional to chromium content in the associated chromian spinels. The maximum pge content of geochemical rock samples from Dust Mountain is greater than 20,000 ppb (>0.6 Oz/ton) with minor gold values. The maximum pge content of tabled chromite concentrates is 3,430 ppb (0.1 Oz/ton) at Dust Mountain, 1,029 ppb (0.03 Oz/ton) at Sheep Hill, and 1,749 ppb (0.051 Oz/ton) at Bernard Mountain. High-iron chromite at Dust Mountain yields tabled chromite concentrates with cr:fe ratios between 0.5 and 1.2. Lower pge contents are found in more chromium-rich chromitites from Sheep Hill and Bernard Mountain that yield concentrates wth cr:fe ratios between 1.0 and 2.8. Platinum-group minerals include amalgams, arsenides, pge alloys, and sulfides. These minerals are between 3 and 40 um in size and are concentrated along grain boundaries and fractures of metasomatically and hydrothermally altered chromian spinel grains.
Proc. Symp. Process Mineral., Metall. Soc. Ann. Mtg, Feb, 1987; Metall. Soc., 1988, PP. 165-195