The petrology of the J-M Reef Pd-Pt deposit and the Lower Banded series of the Stillwater Complex, Montana, USA

The J-M Reef, of the Lower Banded series (LBS) in the Neoarchean Stillwater Complex, is the highest-grade platinum group element (PGE) deposit on Earth. The J-M Reef is a ~1.5 m thick stratiform accumulation of high tenor disseminated sulfide and platinum group minerals hosted in coarse-grained heteradcumulates called the Reef Package. The grades and sulfide tenors of the reef vary from one part of the Stillwater Complex to another due to variable amounts of silicate and sulfide liquid equilibration. The cumulates in the LBS can be modeled by batch crystallization of a komatiitic parental melt contaminated by lower crustal rocks. Emplacement of crystal-bearing slurries produces the noritic and gabbronoritic cumulates that account for most of the rocks in the LBS. The olivine-bearing rocks of the LBS are modeled by infiltration and partial melting of footwall gabbronorites by an influx of hot, dense contaminated komatiitic parental melt. This infiltration and reaction process dissolves gabbronorite mush and crystallizes olivine. Dissolution of plagioclase crystals at smaller size fractions during reaction with the infiltrating melt, produces flatter, convex crystal size distributions in the Reef Package. The infiltration process that produces the olivine-bearing rocks in the LBS and the coarse-grained rock in the Reef Package, can also produce high PGE tenor sulfide mineralization. Mass balance calculations for S and Pd show that sulfides hosted within the footwall gabbronorite can be dissolved and upgraded by the incoming, sulfide-undersaturated and PGE-undepleted melt. This infiltration and upgrading of footwall disseminated sulfide has implications for the origin of other reef-type PGE deposits where erosional contacts are observed between the ore-hosting rocks and their footwall (e.g., the Merensky Reef of the Bushveld Complex). The top of the Reef Package, and the top of economic reef mineralization, is defined by a change in rock fabric known as the hanging wall contact. The hanging wall contact is determined by the change from the coarse-grained textures in the Reef Package to foliated, finer-grained cumulates in the hanging wall. This contact represents the most important marker horizon in mine operations because it is always present, even when the J-M Reef is not.