Petrology of the G and H Chromitite Zones in the Mountain View Area of the Stillwater Complex, Montana

Abstract

The Ultramafic Series of the Stillwater Complex in the Mountain View area of the intrusion consists of 17 cyclic units that have been numbered stratigraphically. A typical unit has olivine cumulates at the base, olivine–bronzite cumulates at intermediate levels, and bronzite cumulates at the top. Most cyclic units also have chromite-rich layers near their base, the thickest being the G and H chromitite zones in units 10 and 11. The G and H zones are each separated from the top of the underlying cyclic unit by 1–3 m of coarse-grained olivine cumulate and pegmatite; and they are both succeeded by thinner chromitite zones, respectively called the hanging wall G (HWG) and the hanging wall H (HWH) zones, situated ˜20 m and 5 m above them. The G and H chromitite zones feature rhythmic sequences of thin layers that tend to progress upward from massive chromitite through chromite–olivine cumulate to olivine–chromite cumulate (the last with the minerals in approximately cotectic proportions of about 98:2). In cyclic units 10 and 11, variations of Mg/Fe in the olivine and bronzite, and of Ni in the olivine, are small and show no clear stratigraphic fractionation trends. The abundance of Cr in the chromite in unit 10 does have a fractionation trend, however, being generally highest at the bottom of the unit and lowest at the top, with a regression at the HWG zone. In general, Cr in chromite is highest at the base of a rhythmic unit and decreases upward, but it shows no overall decline through successive rhythmic units; Fe3 exhibits opposite variation, being lowest in the massive chromite, and highest in the disseminated grains. The G and H chromitite zones, in the Mountain View area, each contain enough chromite to form a single layer of massive chromitite ˜ 1 m thick. If their formation involved removal of only 30% of the Cr in the parental magmatic liquid (estimated concentration, 600 ppm), then this liquid could have amounted volumetrically to an areally equivalent layer at least 2000 m thick. Model calculations demonstrate that such a large volume of liquid is consistent with the small variations of Mg/Fe in the pyroxenes and olivines in the Stillwater cyclic units. We postulate that the G and H chromitite zones and cyclic units that host them formed in response to the entry of new pulses of primitive magmatic liquid into the Stillwater chamber. From experimental observations, we infer that these pulses produced fountains in which the primitive liquid mixed with residual fractionated liquids, yielding hybrids that were compositionally within the chromite liquidus field (or volume) and that were supercooled (supersaturated ) with respect to the oxide mineral. These effects may have been enhanced by low fO2 (oxygen fugacity) in the primitive liquid and(or) by high fO2 of the fractionated liquid. The hybrid liquids probably collected at the bottom of the chamber in a zoned layer that then divided into double-diffusive convecting layers. In these circumstances, the lowest chromite-rich layer in a rhythmic sequence could have formed from the lowest double-diffusive liquid layer, and the next could then have formed when this liquid mixed with the liquid layer above it—and so on up the sequence. We argue that the thick G and H chromitite zones are situated toward the top of the Ultramafic Series because that level marks when the compositional contrasts between the injected primitive liquid and the residual fractionated liquids in the chamber were greatest.