| Summary: | <p>The Nemeiben Lake ultramafic pluton, in the Churchill Province
of the Canadian Shield in central Saskatchewan, is a small 1.6 km
circular plug largely composed of high alumina pyroxene cumulates
forming a series of grossly concentric layers of clinopyroxenite,
websterite, and wehrlite. Minor dunite and gabbro is present in the
northwestern reaches of the pluton. Disseminated primary magmatic
pyrrhotite, pentlandite, chalcopyrite, magnetite, and chromite occur
throughout the body and reach proportions of interest to prospectors
in the northern outcrops. A secondary assemblage of fine-grained
oxides, sulphides, and native metals occurs in serpentinized and uralitized
peridotites and pyroxenites. Primary opaque minerals are largely
altered to marcasite, violarite, and haematite. The pluton intruded a
supracrustal succession in the Early Aphebian before or in the early
stages of the Hudsonian orogenic events. The metamorphic peak during
Hudsonian time reached middle to upper amphibolite facies, granitized
the country rocks, and contributed to the amphibolization of the ultramafic
pluton.</p>
<p>The ultramafic rocks in the "La Range-Rottenstone" domain are
largely coeval with, and related to, the La Ronge-Group volcanics.
The Nemeiben Lake pluton may have been part of the feeder system for
Early Aphebian volcanics.</p>
<p>It is proposed the La Ronge and Rottenstone domains (Lewry and
Sibbald 1977), are parts of the same crustal zone which is progressively
exposed to deeper levels as one approaches the Needle Falls shear zone,
here interpreted as the westernmost fault zone due to compressional
and uplift forces caused by the closure of an ocean previously formed
between fragments of Archean crust.</p>
<p>The Ni-Cu prospect of the Nemeiben Lake pluton is not now economic
though standard technology for recovery of sulphide minerals in
pyroxenites would probably be adequate. Most base-metal minerals in
the altered peridotites belong to the fine-grained secondary assemblage
of opaques due to the formation of retrograde hydrous silicates and are
probably too fine-grained for standard recovery methods.</p>
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