SLAB BREAKS AND EARLY MESOZOIC PAIRED ALKALINE PORPHYRY DEPOSIT BELTS (CU-AU ± AG-PT-PD-MO) IN THE CANADIAN CORDILLERA

Late Triassic to Early Jurassic porphyry deposits of British Columbia formed by subduction in island-arcs that are preserved in the Quesnel and Stikine terranes as paired alkalic and calc-alkalic porphyry belts that extend for ~2,000 km along the axis of the Canadian Cordillera. These arc terranes are joined at their northern ends, but are otherwise separated by relics of ocean basin and oceanic arc rocks collectively referred to as Cache Creek terrane. Most of the economic metal endowment (Cu-Au ± Ag-Mo) of this event occurred during a15 m.y. interval centered on 205 Ma.

We attribute this Late Triassic porphyry Cu bloom to a model wherein delivery of the Permo-Triassic Sitlika–Kutcho-Venables arc to the Stikine-Quesnel arc trench led to stalled subduction, disruption of normal calcalkaline magmatism and Cu-Mo mineralization, and an arc-parallel break in the slab. Hot sub-slab mantle migrated into the break causing a thermal spike in the overlying mantle wedge that produced high degree partial melts (as recorded by picrites) at ~205 Ma. By ~ 203 Ma, widening of the break and decay of the initial thermal spike raised temperatures across broad regions of the metasomatized mantle wedge and low degree partial melts generated from the most hydrated, metal-enriched portions produced the well-defined alkalic porphyry Cu-Au-Ag belt. Migration of the locus of Cu-Au and younger Au-rich porphyries towards the back-arc until ~195 Ma in southern Quesnel may track the leading edge of the slab gap. A younger suite of alkalic porphyries accompanied emplacement of the Quesnel arc onto the ancestral North American margin at ~186 Ma and a second slab break event. In this case, however, lack of a long-established subduction system resulted in a less intensely metasomatized mantle wedge and less metal-laden melts.

Slab breaks appear fundamental to mineralization in the Early Mesozoic arc terranes of Stikine and Quesnel, and may be keys to the origin of porphyry belts globally.