Precious and base metal mineralization within the lower stratigraphy of the Stillwater Complex: New targets defined and old targets revisited

Abstract

Group Ten Metals is exploring for base and precious metals within the Stillwater Complex, a 2.7 Ga layered mafic/ultramafic intrusion, host to the world-class Sibanye platinum group element (PGE) mines. Group Ten controls approximately 25 kilometers of strike length of prospective mafic and ultramafic rocks in the western portion of the Complex. The property includes multiple target types and individual prospects. Stillwater Complex magmatic stratigraphy is divided into three major series based on the proportions of cumulus minerals—Basal, Ultramafic and Banded. Group Ten explores primarily within the lower third of the intrusion, from the Basal Series at the footwall contact, upwards into the Ultramafic Series which is divided into a lower Peridotite Zone and an upper Bronzitite Zone. Rocks of the Peridotite Zone are repetitive, laterally continuous and layered sequences of olivine, chromite and pyroxene cumulates. This model was developed in the eastern portion of the Complex. Mapping in the western portion of the Complex indicates that the Peridotite Zone is thinner than to the east and with cyclic units not as regularly developed. Group Ten has reviewed data from over 50 years of historical exploration in light of different commodity focus, land positions, analytical menus, and petrogenetic models to define a series of exploration targets. A six hole, 1,600 meter diamond drilling program targeting the Iron Mountain sector was completed in 2019; results are discussed in more detail below. At Iron Mountain the primary targets are Ni-Cu-PGE magmatic sulfides located within the Basal Series and lower Peridotite Zone, near the basal contact of the Complex. Important objectives included establishing the PGE tenor of sulfide mineralization drilled by AMAX in the 1970’s and penetrating basement rock rafts known to conceal mineralized (but not previously analyzed) ultramafic rocks beneath. Drilling also tested the PGE-enriched A-B chromitite package which rests stratigraphically above the previously tested zones. At Chrome Mountain, magmatic layering was disturbed or destroyed over large areas along a WNW axis that may be an artifact of early magma chamber deformation or result from much later, Laramide tectonism. Correlation of marker units, for example chromitite seams, are problematic in the disturbed area; these rocks are termed the Hybrid Unit. In addition, normal igneous stratigraphy appears to be intruded by discordant dunite masses, pyroxenite pegmatoids and magmatic breccias. This unusual assemblage of rock types is accompanied by significant and previously underappreciated PGE mineralization related to chromite schlieren and minor but persistent base metal sulfides.