The Cantung (W-Cu-Au) skarn deposit, Northwest Territories, Canada, is one of the most significant high-grade W deposits in the world. The deposit occurs at the contact between Cambrian limestone (Sekwi Formation) of the eastern Selwyn Basin and the Cretaceous Mine Stock Pluton of the Tungsten-Tombstone magmatic belt in the northern Canadian Cordillera. The Mine Stock pluton consists of a sub-alkaline biotite monzogranite with coeval aplite, pegmatite, and lamprophyre dykes. Reduced W skarn deposits, such as Cantung, form through the interaction of W-rich, low salinity fluids with limestone country rock, producing a zoned array of calc-silicate skarn endowed in scheelite (calcium-tungstate). It has been postulated that the fluids are magmatic in origin and exsolve late in the crystallization of strongly fractionated granitoids. The greatest potential for mineralization occurs when the source magma crystallizes in a deep setting. Slow cooling allows the magma to undergo extensive fractional crystallization and produce an immiscible fluid that is rich in incompatible elements such as W. Mineralizing fluids for Cantung were either derived from the Mine Stock Pluton, or a related magmatic-hydrothermal system at depth, which is now preserved as the late-stage dykes and quartz veins. This study characterizes the apatite-hosted melt inclusions within the Mine Stock, in order to test whether the Mine Stock Pluton is the source of tungsten in the Cantung deposit.
The melt inclusions occur predominately as crystallized (i.e., multi-phase), colorless and transparent inclusions, exhibiting a negative crystal shape, suggesting primary origin. Trace element concentrations of un-homogenized melt inclusions have been determined via laser ablation induced coupled plasma mass spectrometry (LA-ICP-MS). The inclusions have variable compositions, some of which are highly fractionated (Ti/Zr = 1 to 23; Zr/Hf = 1 to 29; n = 60), similar to previously reported whole-rock data for the late stage aplite dykes (Ti/Zr = 3 to 13; Zr/Hf = 8 to 17). The inclusions contain high and variable abundances of incompatible elements such as W (2 to 40 ppm), Sn (27 to 121 ppm), B (100 to 10271 ppm), Cs (10 to 1448 ppm), and Bi (11 to 399 ppm). The W content is an order of magnitude higher than continental crust (~ 1 ppm). Both Au and Cu are below detection limits. Continued work includes homogenization experiments and electron probe microanalysis to quantify major elements abundances and volatiles in the melt. Whole-rock major and trace element abundances of the Mine Stock will also be determined via X-ray fluorescence and ICP-MS. This data will be used to model melt evolution and W-enrichment and determine the crystallinity of the melt at the point of fluid saturation.