Geochemistry, Volcanology and Physical Properties of the Late Triassic Nicola Arc and its Metallogenic Implications (NTS 92I, 93A, 93N)
- Thomas Bissig, Mineral Deposit Research Unit (UBC)

The Triassic Nicola Group and its northern continuation the Takla Group compose most of the Quesnel oceanic island arc terrane and host the majority of the known alkalic porphyry Cu-Au deposits in British Columbia. Despite their importance as metallotect, the Triassic arc sequences have not extensively been studied in the context of regional metallogeny.

This two year project will investigate along strike variations in the physical volcanology, geochemistry and related changes in physical properties of the Nicola Group and parts of the Takla Group. The goals are to identify characteristics indicative for proximity of known porphyry ore deposits and tectonomagmatic processes that led to porphyry ore formation.

The study will concentrate on five key areas within the Geoscience BC Quest geophysical surveys and its southern continuation. These are the areas around the Mount Milligan, Mount Polley and Afton/Ajax alkalic porphyry Cu-Au deposits, as well as two reasonably exposed areas within the Nicola arc without known alkalic porphyry deposits. The latter will include the Canim Lake area and exposures 50-100 km S of Kamloops.

We will obtain major and trace element whole rock geochemical data, backed up by limited microanalysis of mineral phases and geochronology (Fossils, U/Pb) where necessary (and if possible) to clarify stratigraphic relationships. We also plan to routinely analyze the samples by portable XRF to establish its use for field based large scale geochemical vectoring to mineralization.

Physical rock properties such as magnetic susceptibility, density and electrical conductivity will also be measured and related to petrographic observations and chemical variations. The rock property data provide the critical link between the geology of the Quesnel arc and the recently acquired high-quality airborne geophysical data (QUEST project), and will enhance the application of the geophysical inversion codes to map the geology under cover and consequently will allow for an improved understanding of the geology from the geophysics.

Field based observations and standard petrographical and geochemical techniques will provide prospectivity indices; this includes fingerprinting regionally significant changes in lithogeochemistry which reflects magmatic and/or hydrothermal processes related to alkalic porphyry ore formation.

Project Area

Posters and Presentations
2011: Variability in the Basaltic Rocks Hosting Cu-Au Porphyry Mineralization in the Quesnel Terrane (NTS 092, 093): Geochemistry, Stable Isotopes and Physical Properties
- Mineral Exploration Roundup Poster (pdf, 7.2MB)

2010: Geochemical and Physical Variations in the Late Triassic Nicola Arc and Metallogenetic Implications: Preliminary Results
- Mineral Exploration Roundup Poster (pdf, 4.53MB)
Technical Articles
2012: "Chemical variations of pyroxene and Fe-Ti--oxide crystals in basalts hosting Cu-Au porphyry mineralization in the Quesnel terrane, interior British Columbia"
- Summary of Activities 2011, Report 2012-1 p. 69-78 (pdf, 2.9 MB)

2011: "Variability in the basaltic rocks hosting copper-gold porphyry mineralization in the Quesnel terrane, south-central British Columbia: geochemistry, stable isotopes and physical properties"
- Summary of Activities 2010, Report 2011-1 p.123-132 (pdf, 7.7 MB)

2010: "Geochemical and physical variations in the Late Triassic Nicola Arc and metallogenic implications, central British Columbia (NTS 092P, 093A, N): preliminary results"
- Summary of Activities 2009, Report 2010-1 p.49-52 (pdf, 3.1 MB)

Final Deliverables
M.Sc. Thesis - S. Vaca
Variability in the Nicola/Takla Group basalts and implications for alkalic Cu-Au porphyry prospectivity in the Quesnel terrane, British Columbia, Canada - University of British Columbia, 2012 - Available digitally through UBC (pdf, 10.5 MB)

Abstract: The Late Triassic -- Early Jurassic Quesnel terrane located in the interior of British Columbia in Canada, is largely composed of rocks of the Nicola Group and equivalents, which are mostly represented by black to dark-green, clinopyroxene±plagioclase porphyritic basalts with variable Fe-Ti oxide content, and locally presenting analcime phenocrysts, interpreted as a volcano oceanic-arc sequence with calc-alkalic and alkalic signatures. The Quesnel terrane hosts most alkalic Cu-Au porphyry deposits of British Columbia. Petrography, whole rock and mineral chemistry of basaltic rocks, together with their physical properties (magnetic susceptibility and density), in context with regional geological and geophysical (gravity and magnetic) maps, allow to delineate prospective arc segments for comagmatic Cu-Au porphyry development within the Quesnel terrane. Incompatible element variations from whole rock geochemistry exhibit heterogeneities along the arc. For instance Ce/Yb ratios from 6.6 to 24.5 suggest that slight regional and local tectonic changes controlled the degree of partial melting in distinct portions of the mantle wedge, where the higher values represent a relatively compressive setting, resulting in volcanic products with high alkalinity, whereas the lower values denote extension and relatively low alkalinity. Primary Fe-Ti oxide inclusions in clinopyroxene within basalts showing Cr and higher Fe2+ contents, whole-rock Fe3+/Fe2+ ratios from 0.13 to 0.60 and magnetic susceptibilities <1.30x103SI units, indicate that basalts were produced from reduced source magmas; Fe-Ti oxides showing higher Fe3+ contents, Fe3+/Fe2+ ratios between 0.60 and ~3.50, and magnetic susceptibilities of 10.00 to 111.00x10-3SI units denote oxidized magmas; whereas Fe-Ti oxides with higher Fe3+ contents, Fe3+/Fe2+ ratios between ~3.50 and 7.10 together with magnetic susceptibilities <1.55x10-3SI units show secondary oxidation of Fe-rich minerals within rocks. Primary oxidized magmas are related to the formation of Cu-Au porphyry mineralization.Carbon and oxygen isotopic composition of secondary carbonate minerals within the low temperature alteration assemblage of the rocks, allow classifying basalts in oxidized and reduced as described above. Secondary carbonate minerals from primary oxidized basalts exhibit δ18OSMOW of +12.3 to +20.5‰, and δ13CPDB between -5.6 and +2.2‰ compared to the relatively reduced ones, which show δ18O from +10.6 to 14.2‰ and δ13C between -9.2 and -6.4‰.

 


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