Variability in the Nicola/Takla Group basalts and implications for alkalic Cu-Au porphyry prospectivity in the Quesnel terrane, British Columbia, Canada

• Main Author: Vaca, Santiago
• Language: English
• Published: University of British Columbia 2012
• Online Access: http://hdl.handle.net/2429/43192

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 <1.30x10⁻³SI 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‰. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate