Geology and Noble-metal Geochemistry of the Lunar Creek Alaskan-type Complex, North-central British Colum[bia*

Alaskan-type complexes in British Columbia are potentially important hosts for economic concentrations of platinum-group elements (PGEs: Ruhlee, 1986; Evenchick er al.. IYXh) as well as other commodities such as chrome, nickel, cobalt, gold. jade and asbestos. To date, only the platinum-rich placers of the Tulameen complex have been exploited economically, yielding some 680 000 grams of impure platinum nuggets hetwcen the years IXXS and 1932 (O’Neil and Gunning, 1934). The source of the placer nuggets has been traced to chromitite horizons within the dunite core of the Tulameen complex (Nixon of al.. 199Oa), and the chromitite-PGE association appears to he one of the most favourahle exploration tergcts for lode occurrences of PGEs in British Columbia (Nixon and Hammack, 1990). The Lunar Creek complex is part of the “Polaris suite” of Alaskan-type intrusions, a series of mafic-ultramafic complexes concentrated in northern British Columbia along an arcuate zone around the northern and eastern margin of the Bowser basin (Figure 2-X-l). All lie within the allochthonous Quesnel and Stikine terranes which were amalgamated prior to accretion with the North American craton in the Mesozoic (Wheeler and McFerley, 1987) and form part of the lntermontane Supertemme. These intrusions are typically spatially associated, and believed to he coeval, with augitc-phyric volcanic rocks of the Upper Triassic Takla and Stuhini groups (Irvine, 1976). The size of individual intrusive bodies ranges from less than I square kilometre to 60 square kilomews, and outcrop patterns vary from round or elliptical to markedly elongate. Elongate intrusions have a northwesterly orientation, parallel to the tectonic grain of the region. These intrusions generally exhibit a crude concentric zonation such that ultramafic lithologies in the came grade outward into gahhroic lithologies at the margins. In all, nine Alaskan-type intrusions have been examined over the course of this project: the Polaris (Nixon E, al., IYYOd, e), Wrcde (Hammack of al., lY9Oa, b), Johanson (Nixon ef al., l99Oh), Turnagain (Nixon er ul., 1989a), Menard, Gnat Lakes. Hickman (Nixon ef ul., lYX9h) in northern British Columbia, and the Tulameen complex (Nixon and Ruhlee, 198X: Nixon, IYXX; Nixon ef ul.. 1989c, 1990a) in south-central British Columbia. Although similar in many respects, each has its own unique characteristics. Ultramafic lithologies characteristic of Alaskan-type intrusions lie along the join between olivine and clinopyroxene in Figure Z-X-2. This diagram essentially illustrates the IUGS classification scheme (Lt:Maitre, 19X9:’ with modifications to include the olivine wehrlite field (65 to 90 per cent olivinr and IO to 35 per cent clinopyroxcne) that was found to he useful for mapping purposes. Fieldwork at Lunar Creek was conducted during the summer of 1989, and this report summarizes tix geology of the previously published Open File map (Nixon ef al., 1990~). The project area is covered at a scale ‘of I:250 000 by the Finlay River map sheet (Y4E) and I:50 000 topography maps (Y4E/l3 and 14). Aeromagneric maps are not currently available.