Origin of sulphides associated with the Spy Sill, Klu Property, Kluane Belt, Southwest Yukon

• Main Author: Bell, Cameron C.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/9391

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.