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
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.
|Title||Precious and base metal mineralization within the lower stratigraphy of the Stillwater Complex: New targets defined and old targets revisited|
|Authors||Craig Bow, Mike Ostenson, Justin Modroo, Allen K. Andersen|
|Publication Type||Conference Paper|
|Publication Subtype||Conference Paper|
|Record Source||USGS Publications Warehouse|
|USGS Organization||Geology, Minerals, Energy, and Geophysics Science Center|
Allen Andersen, PhD
Allen Andersen, PhD