February 27, 2014
The Stillwater Complex in the Beartooth Mountains of Montana is a classic example of a layered intrusion - the solidified remains of a large (~ 4000 cu km) lower-crustal body into which one or more pulses of iron- and magnesium- rich magma intruded. As the magma solidified, its thermal, mechanical, and chemical conditions changed, producing both a visually striking layered appearance and the only platinum deposit currently being mined in the US. Here we use micro- and macro-scale rock magnetic and paleomagnetic information to constrain the mechanical and thermal evolution of the Stillwater Complex, providing a perspective missing from traditional petrologic studies. In particular, magnetic remanence and susceptibility anisotropy from three transects through the Middle Banded Series of the Stillwater Complex exhibit a foliation that increases with stratigraphic height up to the top of Olivine Bearing Zone III, as well as a subtle but distinct magnetic lineation. The foliation likely records crystal mush compaction, but the lineation - only identified at one location prior to this study - suggests shear of the crystal mush prior to compaction. The consistency of the lineation over long stratigraphic intervals may result from the slope of the magma chamber floor. Magnetic lineation in layered intrusions may thus be a useful tool in identifying other features related to magma chamber floor topography, such as the "potholes" associated with thick platinum-group-element mineralized zones.