GEOCHRONOLOGY AND GEOCHEMISTRY OF INTRUSIVE ROCKS IN THE NAHANNI REGION, SOUTHWESTERN NORTHWEST TERRITORIES AND SOUTHEASTERN YUKON

We have begun a comprehensive geochronological and geochemical study of mid-Cretaceous magmatism in the Mackenzie and Selwyn mountains of southwestern Northwest Territories and southeastern Yukon. The study area comprises the easternmost extent of the Tintina Gold Province (TGP), an elongate band of mid- and Late Cretaceous intrusions extending northwest from the study area across Yukon and into central Alaska. The TGP is characterized by precious and base metal mineralization genetically related to the Cretaceous intrusions. Many of the intrusions within our study area are spatially associated with known tungsten and/or Au-Cu-Sb-Bi metal and precious stone occurrences. Intrusions range from large composite batholiths to small multi-phase plutons or stocks. The majority of intrusions are reduced, subalkaline, slightly-moderately peraluminous (A/CNK: 1.0-1.2), and granitic to granodioritic (biotite-hornblende, biotite, and rare biotite-muscovite bearing). Multiple textural and compositional variations are observed, from fine-grained dioritic porphyry to medium-grained K-feldspar porphyritic granite to crowded K-feldspar megacrystic syenite. Evidence for the evolution of highly evolved phases within the intrusions includes marginal muscovite (+/- garnet or tourmaline) bearing phases, miarolitic cavities, aplite-pegmatite dykes, quartz veins/stockworks, and sheeted sub-vertical gossanous fractures. Initial U-Pb (ID-TIMS) and Ar-Ar geochronology reveals mid-Cretaceous ages of both Tay River and Tungsten plutonic suites (92-98 Ma) with short (0.5-1.5 m.y.) cooling periods. One exception to this is the Mine Stock intrusion associated with the Cantung W-(Cu-Au-Bi) skarn orebody, which cooled over a relatively long period (~3 m.y.). New U-Pb geochronology (LA-ICP-MS) has also identified several intrusions of Tombstone plutonic suite age (89-95 Ma) in the study area, which has implications for the type and style of local intrusion-related mineralization. Magmatism in the area has previously been interpreted to be S-type however intrusions typically lack volumetrically significant peraluminous mineralogies and compositions and the rare earth element primitive-mantle normalized profile reveals negative Nb, Ta, and Ti anomalies.