| Summary: | Ni-Cu-PGE rich sulphide mineralization and associated ultramafic sills on the
Klu property in Southwest Yukon were studied to determine their origin in the
context of regional and worldwide examples of various Ni-Cu-PGE deposits. This
sulphide mineralization is related to the Spy Sill, a 100 m thick ultramafic sill of
Triassic age, which is believed to be co-magmatic with the Triassic Nikolai
basalt. A number of reasonably well studied Ni-Cu-PGE deposits and showings
(including the Wellgreen and Canalask deposits) also occur in the Kluane Belt.
The host intrusions to these deposits and showings suffer variable degrees
tectonism, serpentinization and overburden cover which can obscure the
characteristics of the intrusions and associated Ni-Cu-PGE mineralization. The
Spy Sill on the Klu property provides an opportunity to study one of the least
tectonized and serpentinized intrusions in the Kluane Belt. In addition, an entire
section through the Spy Sill is well exposed from top to base.
Another ultramafic sill occurs on the Klu property, which appears to be depleted
in Ni, Cu, Pt and Pd relative to the Spy Sill. The two intrusions have similar
ranges of MgO (25 to 35%) in their peridotite phases, but the Lewis Intrusion has
Ni values in the 300 to 800 ppm range, whereas the Spy Sill has Ni values in the
1,100 to 1,400 ppm range. Cu, Pt and Pd show similar levels of depletion. The
depletion in whole rock metal contents in the Lewis Intrusion is mirrored by low
Ni levels in olivine grains relative to their Fo # (Fo 78 to 80.7 with Ni from 0.05 to
0.11%).
The geochemistry of the Nikolai basalt and Kluane-type intrusions on the Klu
property indicate that these rocks have originated from a common parental melt.
Strong co-genetic trends are displayed in plots of immobile, incompatible
elements between samples from the Nikolai basalt and various phases of the
mafic to ultramafic Kluane-type intrusions. A gabbro phase of the Spy Sill,
marginal to the main peridotite part of the sill, displays geochemical evidence of
contamination (including elevated Ce/Yb, Ba/Zr and Th). No evidence of Ni, Cu
or Co depletion similar to that observed in the Noril'sk area of Russia is evident
in any particular part of the Nikolai basalt. However, the Nikolai basalt does
display significant overall depletion in Cu and Co relative to the basalt occurring
in the western facies of the terrane, the Karmutsen basalt. The parental
composition predicted by olivine compositions from the two intrusions, is close to
close to that of the Nikolai basalt (averages of Mg# 55.3 and 57.9 for the two
sections).
Ni-Cu-PGE rich marginal gabbro contains higher MgO levels (average of 12.8%)
than non-mineralized marginal gabbro. Within the suite of mineralized marginal
gabbro, there is no correlation between MgO and Ni. The highest Ni and Co
levels occur in massive sulphide lenses at the lower marginal gabbro-sediment
contact. The highest Cu, Pt and Pd levels occur in massive sulphide lenses in
the footwall sediment. Ni, Cu and Co display an overall increase with increasing
S contents in Ni-Cu-PGE rich samples whereas Pt and Pd values correlate
poorly with S.
Ni-PGE-Cu-Au values in 100% sulphide/chondrite for the Spy showings show
patterns which are most similar to those from flood basalt-related deposits. Pd/lr
versus Ni/Cu and Ni/Pd versus Cu/lr plots suggest PGE distribution has an
overall control from olivine and chromite fractionation and a local control from
sulphide fractionation.
Ultramafic sills and related gabbro sills on the Klu property are emplaced within
siltstone, shale and limestone of the Permian Hasen Creek Formation. This
formation also contains Ni-Cu-PGE barren sulphide layers close to the
stratigraphic level of the Spy Sill. Sulphur isotopic data from Ni-Cu-PGE
mineralization and potential sulphur sources show that the sulphur isotopic
signature of the showings is the result of mixing between mantle and
synsedimentary sulphur. The various types of Ni-Cu-PGE mineralization on the
Klu property have median δ³⁴S values between -1.5 and -7 0/00. Ni-Cu-PGE
barren synsedimentary sulphide from the Hasen Creek Formation is the
probable sulphur contaminant.
Ni-Cu-PGE rich material appears to be related to an early magma pulse.
Specific flows of Ni-Cu-PGE depleted magma are not recorded in the Nikolai
basalt due to ancient (Triassic) erosion of such depleted flows or later tectonism.
Also, there may be a lack of connectivity between the mineralized sills and
magma that reached the surface. Metals were stripped from part of a lower
magma chamber by sulphide equilibration; this magma is represented by the
Lewis Intrusion. During sill emplacement, synsedimentary sulphur was
assimilated from sulphidic layers in the Hasen Creek Formation.
Most Ni-Cu-PGE mineralization is associated with marginal gabbro pulses. The
magma represented by the Lewis Intrusion is either an earlier sill (before the Spy
Sill) or the leading edge of a magma pulse that also formed the Spy Sill which
contains magma with normal or elevated Ni-Cu-PGE levels.
No economically significant quantities of Ni-Cu-PGE rich sulphide are known on
the Klu property, but the nature of the mineralization suggests a process capable
of producing Ni-Cu-PGE enriched sulphide took place. This is known to have
occurred on a larger scale at the nearby Wellgreen deposit.
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