PORPHYRY COUSINS? GEOLOGICAL COMPARISON OF THE KEMESS SOUTH AND KEMESS NORTH PORPHYRY AU-CU (MO) DEPOSITS, TOODOGGONE DISTRICT, BRITISH COLUMBIA

The Kemess South (KS) mine (109.4 Mt reserve @ 0.71 g/t Au & 0.23% Cu) and the Kemess North (KN) deposit (407 Mt resource @ 0.41 g/t Au & 0.22% Cu) occur 7 km apart and are the largest known porphyry Au-Cu ± Mo systems in the Toodoggone. Mineralization at KN and KS is temporally and spatially associated with two high-K, calc-alkaline, monzonite porphyry intrusions that were emplaced at ca. 202 Ma and 200 Ma, respectively. Both intrusions are tabular (shallow-dipping) and cut and locally mineralize Late Triassic Takla Group andesite. At KN, overlying ca. 199 Ma Hazelton Group volcaniclastic units are mineralized, whereas at KS, younger (ca. 194 Ma) Hazelton Group rocks unconformably overlie the monzonite and are barren, with the exception of small detrital Au and remobilized Cu zones. KS and KN have similar vein-types with comparable paragenetic histories. Mineralized veins mostly contain quartz, pyrite, and chalcopyrite. Magnetite and molybdenite are less common; the latter is more abundant at KS. Both deposits have late-stage pyrite stringer veins cut by successive post-mineralization carbonate-rich, anhydrite-rich, and chlorite-rich veins. Both deposits display early potassic alteration replaced by phyllic (quartz-sericite ± pyrite) alteration; at KN the early potassic phase is biotite whereas at KS it is K-feldspar. The KN phyllic alteration event destroys most of the early potassic alteration. Unlike KN, KS displays a second potassic (K-feldspar) alteration that replaces phyllic alteration implying a later, thermally prograding episode of alteration-mineralization. A near-surface “broken zone” in the Takla Group andesite at KN is caused by the recent hydration of hypogene anhydrite to secondary gypsum and its partial removal via modern weathering processes. This creates a classic “gypsum line” in the core similar to that developed in the Pine porphyry Au-Cu-Mo deposit located 17 km due north. At KS, any cover (and broken zone) has been completely removed by erosion. Post-mineralization faults control the distribution and morphology of the ore bodies. E-W striking faults truncate the northern areas of both ore bodies with later NW- to NE-striking normal faults producing horst-and-graben block shuffling. These features are consistent with KS and KN experiencing similar far-field stresses after formation.