Why are Chromite-rich Rocks Enriched in Os, Ir, Ru and Rh? Constraints from the Stillwater Complex Chromitites.

English Abstract:

It is well documented that chromite-rich rocks from ophiolites and layered intrusions are enriched in IPGE (Os, Ir, Ru) and Rh. The classic model for achieving this collection is that tiny platinum-group minerals, in particular laurite (RuOsIr)S2, crystallize at the same time as the chromite and are included within the chromite. This model is supported by the presence of laurite in most chromitites from ophiolites and layered intrusions and by experimental work showing the crystallization of laurite in the boundary layers of chromite. However, recent in situ analysis of chromite from volcanic rocks have shown that chromite contains IPGE and Rh in solid solution, which raises the question of whether IPGE and Rh could have partitioned into chromite in ophiolites and layered intrusions. We have collected 41 samples from the Ultramafic series of the Stillwater Complex and carried out a study focused on the minerals hosting IPGE and Rh.

In situ analysis of the chromite show they contain only 3 to 11 ppb IPGE and Rh, insufficient for chromite to be the host IPGE and Rh. In 31 polished sections of chromitite 141 grains of laurite, 1-10 microns in diameter were observed. Using a combination of the laurite composition and the whole rock analyses we calculate that the laurites account for ~85 % of the Ru in the chromite layers and 40-50 % of the Ir and Os, but only 11 % of the Rh. Most of the Rh is present in small (<0.1 mm) patches of Pn-Ccp-Mil. The control of Ru and to a lesser extent Os and Ir by laurite supports the model of collection of IPGE by laurite. However, the concentration of Rh in the sulfides is more difficult to explain because the whole rock analyses show that IPGE AND Rh show strong correlations with each other and with Cr, whereas Rh does not correlate with S, Cu, Pd,or Se as it should do if it has been collected by a sulfide liquid. We suggest that initially the IPGE and importantly Rh partitioned into chromite. Subsequently a little base metal sulfide liquid percolated into the cumulate pile. During cooling there was an exchange of IPGE and Rh for Fe between chromite and sulfides resulting in some of the sulfide being converted to laurite and Rh diffusing into into Pn.