Magnetic Investigations in the J-M Reef Section of the Stillwater Complex, Montana

Abstract:

The Stillwater J-M reef, the only economic platinum deposit in the USA, consists of a 0.5 to 4 m-thick stratiform zone of platinum group element (PGE)-rich sulfides in a layered mafic intrusion. The origin of this reef, purely magmatic or related to late-stage magmatic fluids, remains ambiguous. I propose to test these two genetic hypotheses using rock magnetism. Fractional crystallization trends deduced from petrological models would produce a sharp increase in magnetite and pyrrhotite content near the solidus. In contrast, percolation of sulfur-rich fluids through a crystal mush would produce a gradual increase in magnetite and pyrrhotite up to a fluid permeability barrier. Continuous logging of the magnetic properties of drillcores, combined with petrographic observations, will allow to test these two models.

Petrologic similarities between PGE reefs suggest that they share common physico-chemical origins, therefore, understanding the J-M reef genesis would have implications for other deposits such as the Bushveld Complex and the Great Dyke of Zimbabwe. The J-M reef formation has been explained by two alternative models: 1) magmatic model - magma replenishment causes thermal convection at the interface between two magmas, inducing PGE leaching by a sulfur-saturated magma, followed by precipitation of sulfide droplets; 2) fluid fluxing model - a sulfur-rich residual, late magmatic fluid migrates upward through the crystal mush leading to PGE concentration along a magmatic permeability barrier against the hanging wall. Both models account for the majority of geochemical and petrological observations and may not be fundamentally mutually exclusive. However, understanding the origin of PGE reefs would certainly benefit from new approaches.

Preliminary data shows systematic inch-scale cycling variations of magnetic susceptibility (K_m) in the hanging-wall that supports the magmatic model. The discovery of this magnetic cyclicity matters because this core does not display any macroscopically visible layering. Magnetic measurements (thermomagnetic experiments, magnetic hysteresis and first order reversal curves), in conjunction with detailed petrographic observations on selected specimens, will be performed to evaluate the significance of this cyclic feature.