Syenite-hosted gold mineralization and hydrothermal alteration at the Young-Davidson deposit, Matachewan, Ontario

Abstract

The syenite-hosted Young-Davidson (YD) gold deposit is located at the western extension of the Cadillac-Larder-Lake deformation zone (CLLDZ), southwest of Kirkland Lake, in the southern Abitibi greenstone belt, Canada. Gold is predominantly hosted by syenite, and to a lesser extent by Timiskaming sedimentary rocks and mafic volcanic rocks in veins and/or small pervasively altered shear zones related to the CLLDZ. Historical gold production was mainly from quartz veins and disseminated pyrite hosted in altered syenite. Recent drilling by AuRico Gold Corporation (formerly Northgate Minerals Co.) has expanded the underground resource to 14.6 million tonnes (measured and indicated) at an average grade of 2.03 g/t Au and an underground reserve of 39.1 million tonnes (proven and probable) at an average grade of 2.79 g/t Au, where most of the gold is hosted in the syenite. Three generations of veins have been identified based on crosscutting relationships mapped at an underground ore crosscut: V1 boudinaged iron-carbonate veins, V2 folded quartz-pyrite veinlets and V3 planar quartz-carbonate veins. Structural characterization of the vein sets indicates that V2 and V3 extensional vein arrays developed during the D2 deformation, which generated the steeply dipping east-west striking penetrative S2 foliation that is found elsewhere along the CLLDZ. Petrographic analysis of mineralized syenite shows that native gold grains are most abundant in zones of intense potassic-hematite-pyrite alteration as inclusions and along cracks in vein-related and disseminated pyrite. Volcanic-hosted gold is mainly located in quartz-iron-carbonate veins hosted in narrow sheared and pervasively albite-iron-carbonate-pyrite altered volcanic rocks. Whole rock lithogeochemical analyses and mass balance calculations show that mineralized syenite is characterized by enrichments in K, S, Na, Ba and W whereas mineralized volcanic rocks are characterized by strong enrichments in Na, Al, Si, K, Fe, S, Ba and W. Geochemical contouring of 279 analyses of drill core along two N-S sections through the YD syenite show broad cross section scale correlation of gold mineralization with K2O, S and Ba. Sulfur is the most strongly correlated with gold mineralization; however, the most intense sulfidation is generally coincident with zones containing coarse altered feldspars and a fine-grained groundmass of iron-carbonate, Na- and K-feldspar and minor quartz. Bulk sulfur isotopic analyses of syenite-hosted pyrite grains indicate that the dominant source of sulfur is magmatic but there is also a component that has interacted with the reduced Archean atmosphere. Pyrite grains in syenite and volcanic rocks contain up to 15,000 ppm Co, 4,000 ppm Ni and 4,200 ppm As. Chemical mapping of Co, Ni and As from different vein associated pyrite grains show: 1) Arsenic in pyrite is low, < 1,000 ppm; 2) Pyrite shows Co-Ni zoning and gold in V1 vein pyrite is associated with Co-Ni rich pyrite overgrowths of earlier corroded pyrite grains; 3) gold along fractures in V2 vein pyrite is also associated with Co-Ni enrichment; and 4) gold in V3 veins crosscuts Co-Ni patterns in pyrite, which suggests that the V3 style of mineralization has precipitated by different mechanisms. Pyrite mapping combined with alteration assemblages, mineralization characteristics and mineral chemistry suggests that the syenite-hosted mineralization has formed from interaction between multiple fluids with distinct physicochemical conditions. The proposed formational model for the Young-Davidson deposit is of fluid mixing between magmatic and metamorphic fluids during the deformation events that generated the extensive veining systems at Young-Davidson.