Stillwater Igneous Complex - Geology

Geology

The Stillwater complex is a large layered intrusion with many similarities to the Bushveld igneous complex of South Africa. The complex was intruded into existing gneisses during the Archean at about 2700 Mya. The region was subsequently intruded by a quartz monzonite stocks and underwent extensive metamorphism, faulting and folding during the Archean at about 2500 Mya. The area was intruded by north trending mafic dikes before being unconformably covered by a middle-Cambrian sedimentary rock sequence. The intrusion forms a linear body stretching some 30 miles (48 km) and striking roughly N 60 °W and dipping from 50° to near 90° to the northeast. The exposed thickness is around 18,000 feet (5500 m) with an additional estimated 5000 to 15,000 feet having been removed from the top by pre-Cambrian erosion.

The cumulate stratigraphy comprises three distinct zones:

• The basal zone consists of a chilled fine grained gabbro overlain by gabbro, norite and feldspar pyroxenites. Thickness up to 700 feet (210 m).
• The ultramafic zone is composed of a lower peridotite member consisting of alternating dunite, chromitite, harzburgite and bronzite pyroxenite. The upper third is massive bronzite pyroxenite. The ultramafic zone averages around 3500 feet (1100 m) in thickness.
• The banded zone is composed of alternating norite, gabbro and anorthosite. The banded zone has a maximum thickness of 14,000 feet (4300 m).