PETROCHEMISTRY OF SHOSHONITIC ROCKS ASSOCIATED WITH PORPHYRY COPPER-GOLD DEPOSITS OF CENTRAL QUESNELLIA, BRITISH-COLUMBIA, CANADA

Major and trace element petrochemistry has been determined for forty-six Late Triassic-Early Jurassic rocks representing the Mt. Milligan and Chuchi porphyry Cu-Au deposits, marginal phases of the Hogem Batholith, and Takla Group volcanic rocks. Monzonites and latites of the Mt. Milligan deposit have undergone moderate to strong potassic alteration, principally involving the addition of K-feldspar and biotite. They are characterized by high K2O (5-13 wt.%), low Sr (180-450 ppm), high LOI (3-7 wt.%), slight to moderate LREE enrichment (La(N)/SM(N) = 3.9, n = 5 for monzonites, La(N)/Sm(N) = 2.4, n = 5 for volcanic rocks); but have non-diagnostic compatible trace element, HREE and HFSE contents in comparison to less altered rocks in the region. Samples taken peripheral to the Chuchi deposit have undergone moderate potassic (K-feldspar) alteration. They have slightly enriched K2O (4-6 wt.%) and LREE (La(N)/Sm(N) = 3.0, n = 4 for monzonites; La(N)/Sm(N) = 2.7, n = 4 for volcanic rocks), but otherwise are similar to less-altered rocks nearby. An alteration index: K2O x 100/Sr, distinguishes mineralized from barren samples in this suite, with mineralized samples having values greater than unity. The entire suite has petrochemical characteristics of a shoshonitic arc suite, with relatively high alkalis (5-15 wt.%) over mafic to intermediate SiO2 contents (44-66 wt.%) and Mg numbers (48-77); accompanied by moderate LREE enrichment, low TiO2 (less than 1.1 wt.%), and low Ta (less than 1 ppm, n = 8) and Nb (all but one less than 20 ppm) contents. The volcanic rocks are comparable to broadly coeval shoshonites found elsewhere in Quesnellia and Stikinia, including in the Nicola and Stuhini Groups of southwestern and northern British Columbia, respectively. By comparison with the porphyry gold-bearing Tabir-Feni shoshonitic volcanic arc, and other porphyry Cu-Au-bearing arcs of the southwest Pacific, and considering regional tectonic features, the Late Triassic-Early Jurassic rocks of central Quesnellia may have formed as a result of arc reversal, with subduction of the adjacent Cache Creek and Slide Mountain oceanic terranes under the Quesnellia volcanic arc from the west and east, respectively.