LATE TRIASSIC TO EARLY JURASSIC BASALTS AND INTRUSIONS RELATED TO ALKALIC AND CALC-ALKALIC PORPHYRY CU-AU DEPOSITS OF QUESNEL TERRANE, SOUTH-CENTRAL BRITISH COLUMBIA: PETROCHEMISTRY, TECTONICS AND METALLOGENY

The Quesnel and the along-strike equivalent Stikine oceanic arc terranes of British Columbia are well endowed in alkalic porphyry Cu-Au and calc-alkalic porphyry Cu-Mo and Cu-Au deposits. Mesozoic supracrustal rocks of the Quesnel terrane are largely Upper Triassic basaltic oceanic arc assemblages of the Nicola group and the along-strike equivalent Takla group (hereafter Nicola group).

The Nicola group is remarkably homogeneous over 1300 km strike length and is largely composed of clinopyroxene-phyric basalts and clastic rocks derived therefrom. However, subtle compositional variations across and along strike are mimicked by the style of porphyry mineralization hosted in the corresponding segments of the arc. In the southern Quesnel terrane of British Columbia a western, calc-alkalic domain of Nicola group basalts hosts the ~210 Ma Guichon Creek batholith and the calc-alkalic Highland Valley porphyry Cu deposit. Further east, stratigraphically higher shoshonitic rocks host the ~201-205 Ma Iron Mask batholith and alkalic porphyry Cu-Au mineralization at Afton and Ajax, as well as the ~ 201.5-205 Ma Copper Mountain stock and related alkalic porphyry Cu mineralization. Nicola group basalts of the central Quesnel terrane, in contrast, are, generally younger, more alkalic and oxidized towards the west. This region hosts the 201.5 to 205 Ma Mount Polley alkalic porphyry Cu-Au deposit, which is broadly coeval with the feldspathoid-bearing alkali basalt host rocks. The 196-197 Ma calc-alkalic Woodjam porphyry Cu-Au (-Mo) prospect, some 35 km S of Mount Polley, is part of a slightly younger magmatic belt and is hosted by ~204-198 Ma calc-alkalic Nicola group basaltic and andesitic volcanic and sedimentary rocks. Calc-alkalic porphyry deposits are related to intrusions with depleted MREE and HREE and Sr/Y > 40 which indicate melt evolution under hydrous, high-P comditions. For alkalic porphyry deposits a relationship to rocks with such geochemical characteristics cannot be demonstrated. This suggests that alkalic porphyry deposits have been emplaced in trans-tensional tectonic settings and relatively thin crust whereas calc-alkalic counterparts were emplaced in transpressional settings during crustal thickening which led to melt evolution in the lower crustal MASH zone at higher-P.