THE ROLE OF LATE EOCENE TECTONISM AND FELSIC MAGMATISM IN THE GENESIS OF REDUCED PORPHYRY COPPER-GOLD MINERALIZATION AT THE NORTH FORK DEPOSIT, NORTH CENTRAL CASCADE MOUNTAIN RANGE, WASHINGTON, USA

The North Fork deposit (80.4 Mt reserve @ 0.44% Cu and 0.003 oz/ton Au) belongs to a suite of Eocene to Miocene porphyry Cu (Au) deposits situated on the western flank of the North Cascades Mountain Range, WA. Detailed field mapping (1:4800) and five new U-Pb (zircon) dates reveal that the deposit is hosted mainly in the Mount Persis andesite (38.9 +/-0.3 Ma), which is intruded by quartz monzodiorite-granodiorite (37.2 +/-0.1 Ma & 37.0 +/-0.2 Ma) and mafic latite porphyry (37.1 +/-0.2 Ma & 36.8 +/-0.2 Ma). Plutonic and volcanic rocks are calc-alkaline, weakly peraluminous, have highly fractionated REE-profiles (La/Yb_cn=4.9 to 9.2) and remarkably similar extended trace-element profiles. These geochemical data, together with an abundance of primary amphibole and ilmenite, indicate that intrusive rocks are reduced I-type granitoids that have intruded their own volcanic pile during construction of a late Eocene calc-alkaline volcanic arc. The association with reduced I-type granitoids, and hypogene mineralization characterized by abundant pyrrhotite but an absence of primary hematite and sulphate minerals, classify North Fork as a Reduced Porphyry Cu-Au (RPCG) deposit. The oldest structures measured in the 10 km2 deposit area are faults and fractures that strike 350-010° (NS). These are inferred to be associated with the initiation of slab rollback, crustal relaxation, and extension between 39 and 37 Ma. This style of deformation is broadly synchronous with the formation of the Mount Persis andesite and the oldest monzodiorites. Subsequent deformation (~37 Ma) is dominated by synthetic shear faults striking 320-340° (NNW). These structures are attributed to dextral movement of the Straight Creek fault located approximately 25 km east of the deposit. This style of deformation is spatially and temporally associated with intrusion of mafic latite porphyry and development of strong mineralization. The low-sulfidation epithermal Au veins situated 3 to 4 km SSE from the North Fork deposit are on strike with the NNW brittle structures suggesting that they may be part of the North Fork magmatic-hydrothermal system. Finally, normal faults striking 060-100° (EW) post-date the emplacement of volcanic and plutonic rocks consistent with brittle deformation resulting from plate rotations in the Oligocene-Miocene.