The North Fork porphyry copper deposit of the Washington Cascades

The North Fork porphyry copper deposit in the Cascade volcanic arc of Washington displays copper mineralization confined to the potassic (biotite) and phyllic (sericite) alteration zones. No secondary potassium feldspars have been found in either alteration zone; moreover, chemical analyses indicate the potassic alteration zone contains a low K₂O content. Argillic and propylitic zones are also recognized, but these are barren of hypogene copper mineralization. Biotite-chalcopyrite intergrowths in the deposit have been given a 9.9 K-Ar age. Thus it is not only one of the youngest deposits dated in the western Cordillera, but it is associated with a volcanic arc which has no current Benioff zone seismic array or accompanying trench. The deposit appears to have developed during the period when coupling of the North American and Juan de Fuca plates probably inhibited subduction under the Cascade volcanic arc.