GEOCHRONOLOGY AND GEOLOGY OF THE PINE PORPHYRY AU-CU (MO) DEPOSIT, TOODOGGONE DISTRICT, BRITISH COLUMBIA

The Pine porphyry Au-Cu (Mo) deposit (40 Mt @ 0.57 g/t Au & 0.15% Cu) is located in the Toodoggone district of north central British Columbia (northeastern Stikine terrane). Other large porphyry Au-Cu (Mo) deposits in the district include the Kemess South mine and the Kemess North deposit. Gold-copper mineralization at Pine is best developed within a 1 km² core of potassically altered (K-feldspar + magnetite), porphyritic quartz monzonite. Concentrically distributed around this mineralized core are broad zones of phyllic (quartz-sericite-pyrite) and propylitic (chlorite-epidote) alteration. Locally underlying the quartz monzonite is a phase of weakly mineralized granodiorite. Three generations of monzonite-syenite dykes and two generations of "mafic" dykes cut the mineralized quartz monzonite. Importantly, the oldest monzonite and diorite dykes are locally mineralized indicating that Au-Cu mineralization is syn-magmatic. Five new U-Pb (zircon) ages have been determined to help constrain the magmatic event(s) associated with Au-Cu mineralization at Pine. The host quartz monzonite is 197.6 ± 0.5 Ma, an age similar to that of other proximal felsic intrusions of the Black Lake suite, but slightly younger than the intrusions that host the Kemess South (199.6 ± 0.6 Ma) and Kemess North (202.7 ± 1.9 Ma) Au-Cu deposits. The oldest monzonite dykes at Pine are 193.8 ± 0.5 Ma thereby constraining the age of the mineralization event to between this age and 197.6 ± 0.5 Ma, the age of the host quartz monzonite intrusion. The end of the magmatic cycle is marked by 193.6 ± 0.4 Ma syenite dykes. The start of the magmatic cycle is represented by 200.9 ± 0.4 Ma trachyandesite ash flow tuff of the Toodoggone Formation, which hosts all the plutonic rocks associated with the Pine deposit. Six stages of alteration and vein formation are recognized at Pine with the bulk of the Au-Cu mineralization associated with comb-textured, quartz-magnetite-pyrite ± chalcopyrite veins that have intense phyllic (quartz-sericite ± chlorite) alteration halos. The Pine deposit has many geological similarities to the larger Kemess Au-Cu deposits and ongoing isotopic and fluid inclusion studies will further evaluate the physicochemical conditions of Au-Cu mineralization in all three deposits.