Cryptic variation in laurite inclusions in cumulus chromite from the upper main chromitite, section 225-180E, of the Bird River sill provides significant evidence that platinum-group minerals are trapped during chromite precipitation and are not exsolved from chromite. Only one laurite grain was found outside chromite in about 100 specimens. Laurite varies in composition from (Ru (sub 0.96) Ir (sub 0.03) Os (sub 0.01) )S 2 to (Ru (sub 0.80) Ir (sub 0.04) Os (sub 0.16) )S 2 . In general, there is little variation in Ir. However, in large grains, strong zonation from core to rim is evident; (Ru (sub 0.92) Ir (sub 0.03) Os (sub 0.06) )S 2 in the core area of one grain ranges to the most Os-rich values measured. Euhedral laurite, enclosed partly by chromite and partly by a pargasitic amphibole inclusion, is similarly but less strikingly zoned. The microprobe data show that there are cycles in the Os content of laurite through the stratigraphic section which are directly related to increases in Mn, and to a lesser extent, the Mg/(Mg + Fe) ratio of the host chromite. The Ti content of chromite varies directly with the Cr/(Cr + Al) ratio but inversely with the Os content of laurite. These variations are more likely related to a cyclic appearance of a second fluid phase in the magma than to injections of new batches of magma.

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