NEW GEOCHRONOLOGIC CONSTRAINTS ON MESOZOIC QUESNELLIA VOLCANO-STRATIGRAPHY AND MINERALIZATION, WITH POSSIBLE CORRELATIVES IN WESTERN USA

Quesnellia is among the largest accreted arc terranes in North America. Mesozoic magmatic components include a belt of alkalic intrusions associated with rich Cu-Ag-Au±Mo porphyry deposits that is globally unique. Most historical mineral production is from southern Quesnellia where the arc rocks are known as the Nicola Group and historic work predates the wide application of modern geochronologic techniques. With the benefit of modern geochronology, we establish a stratigraphic framework as an aid to mineral exploration in other parts of Quesnellia, and possibly the coeval Huntington arc, Oregon, where alkalic porphyry deposits have not yet been discovered.

New U-Pb CA-TIMS dates extend igneous limits of the Nicola Gp., formerly considered Carnian and Norian (~237 - 209 Ma) to ~239 (Ladinian, Middle Triassic) through ~202 Ma (Rhaetian, latest Triassic). Nicola arc is mainly mafic to intermediate in composition, characterized by thick coarse augite-phyric flows and breccia (pre-223 Ma); up-section they also include hornblende with cooling ages of 210-202 Ma. However, initial magmatism was rhyolitic, with repeat explosive, quartz-bearing intervals at ~223-224 Ma and ~202 Ma. Mineralized granitoid intrusive pulses at ~205 and ~210 Ma lack dated extrusive equivalents, consistent with the idea that copper is lost from the magma system when it vents.

By 223 Ma, the arc had built to near sea level and small patch reefs formed. Except where preserved in down-dropped fault blocks, much subaerial arc construction was lost to erosion enhanced during a regional tectonic episode ~210 Ma. Sediments shed from the 25 km wide arc were deposited as an adjacent 20 km wide clastic apron to the east. On the arc, preserved sections deposited between 208 and 201 Ma include thick beds of oxidized, maroon conglomerate containing clasts derived from Nicola arc strata as young as Norian (~210 Ma). Volcanism during and following this deformational interval was variable: strongly alkalic basalts to calc alkaline apatite-biotite-quartz-phyric pyroclastic strata (~202 Ma) with associated copper mineralization, and a distinctive coarse bladed plagioclase-phyric succession. Emplacement of 210-205 Ma alkalic intrusions hosting Cu-Ag-Au±Mo porphyry mineralization are probably genetically related to this period of arc destabilization.