Chromitites found in layered intrusions and ophiolite complexes are generally enriched in platinum-group elements (PGE), especially IPGE (i.e., Ir, Os, Ru)-bearing platinum-group minerals (PGM), and the chromitites are usually poor in base metal sulfide (BMS) minerals. The most common PGM observed is laurite [Ru(Os,Ir)S2], but how the laurite formed is not clearly understood. To address this problem we compare the differences in the composition and shape of PGM in the nine chromite layers (A to K) in the Stillwater Complex, Montana and then extend the study to examine laurites from the Bushveld Complex and ophiolites.
The most common PGM in the Stillwater chromitites is laurite, predominantly enclosed in chromite grains. In a few cases the laurite is accompanied by rarer and smaller PGM, including malanite [CuPtRh(±Ir)S] and Pt-Pd-sulfides. Interstitial to the chromite grains the PGM assemblage is quite different, dominantly PPGE (i.e., Pt, Pd, and Rh)-bearing, including Pd-Pb, Pt-Pd tellurides, sperrylite, platarsite, minor laurite, and one grain of Pd-Ge. The PGM grains enclosed in chromite formed by a different mechanism to the PGM grains outside chromite. During the crystallization of the chromite the magma was sulfide undersaturated and Ru, Os, Ir, and Rh partitioned into chromite thereby enriching the chromitite layers in IPGE. As the cumulate pile cooled, the fractionated silicate liquid became saturated in a BMS liquid and this migrated among the chromite grains. With further cooling the chromite grains sintered to form larger grains and in some case incorporated small grains of the BMS, which was converted to laurite by the exchange of IPGE and Fe plus Ni between the chromite and the BMS. In contrast, the BMS that was not included in chromite exsolved to form pentlandite, pyrrhotite, and PGM.
The shape and composition of the PGM within the chromite grains in the Stillwater chromitite layers is not uniform. Upper and lower layers contain laurites with rounded shapes and an Os content of 7–8%. In the sulfide inclusion-poor middle G layer, the laurites have 5% Os and a predominantly euhedral shape. It is likely that both rounded and euhedral laurites formed by subsolidus ejection of PGE from the chromite as it cooled and recrystallized. The rounded laurite formed in a more BMS and S-rich environment, whereas the euhedral laurite formed in an S-poor environment. Traces of Rh in laurite, PPGM, and BMS inclusions associated with laurite are most abundant in the uppermost layer K, suggesting that the upper-layer chromitites contained more PPGE in solid solution on crystallization. The average size of the laurite grains increases upwards from an average area of 6 μm2 in layer A to 21 μm2 in layer K. The larger size of the laurites from higher layers in the intrusion may be the result of them having had a longer period to cool, further from the basal contact. Rutile inclusions are most abundant in chromitite layer B and could be the result of a greater degree of contamination of the magma in the lower layers.
Comparison of the shape of Stillwater laurites with those in the Bushveld Complex chromitites reveals similarities with the Bushveld chromitites, as they also contain both euhedral and rounded laurites that are commonly associated with smaller PPGM. Ophiolitic laurites entirely enclosed in chromite are predominantly euhedral and sometimes zoned. Chromitites from ophiolites are generally PPGE-poor and although ophiolitic laurites also form composite PGM with other smaller PGM, these are usually Os- and Ir-rich rather than PPGE-rich. These laurites have a more variable and greater range of Os concentrations than those from the Stillwater and Bushveld Complexes. Most ophiolitic laurites probably formed by crystallizing directly from magma, but it is possible that some formed by diffusing from the chromite in a low fS2 environment.