Abstract

The MAX porphyry Mo deposit is located at the northern end of the Kootenay arc in southeastern British Columbia, Canada. Molybdenum mineralization is genetically related to the variably altered Trout Lake granodiorite stock, which has lithogeochemical affinities to the low fluorine, calc-alkaline, granodiorite-hosted porphyry Mo deposit type. Three molybdenite samples from early and late Mo-bearing veins yield Re-Os dates that overlap within analytical error, with a weighted average age of 80.3 ± 0.2 Ma. These dates are in close agreement with two 206Pb/238U weighted average zircon ages for phases of the Trout Lake stock at 80.3 ± 1.6 and 79.2 ± 1.0 Ma, indicating that crystallization of the stock and hydrothermal molybdenite formation were coeval and likely cogenetic. 40Ar/39Ar plateau ages for primary biotite in igneous dikes, secondary biotite in early potassic alteration, and muscovite in late phyllic alteration range from 80 to 76 Ma. The younger 40Ar/39Ar plateau ages may reflect unusually slow cooling of the magmatic-hydrothermal system or a late thermal disturbance.

Pb-Zn-Ag vein deposits are common throughout the Kootenay arc and several such veins are located within and proximal to the MAX porphyry Mo deposit. The presence of rare molybdenite associated with sphalerite and argentiferous galena in some of these Pb-Zn-Ag veins suggests that they represent a late-stage event related to the MAX magmatic-hydrothermal system. Fluid inclusions in quartz from Mo and Pb-Zn-Ag veins are similar and can be divided into two compositional groups: type 1, aqueous liquid-rich fluid inclusions; and type 2, aqueous-carbonic liquid-rich fluid inclusions. Pressure-corrected (1.4–1.7 kbar) trapping temperatures of typical type 1 inclusions from Mo-bearing veins range from 430° to 460°C, and for Pb-Zn-Ag veins, from 335° to 365°C. The ranges of salinity (0–12 wt % NaCl equiv) in fluid inclusions from both vein types are almost identical, and type 2 inclusions from both vein types contain similar amounts of CO2 (avg XCO2 = 0.08) with traces of CH4. The similarity of chemistry in the two vein types suggests a genetic link between porphyry Mo mineralization and adjacent Pb-Zn-Ag deposits. However, muscovite from the alteration halo around a Pb-Zn-Ag vein within the MAX deposit yielded a 40Ar/39Ar plateau age of 72.2 ± 0.5 Ma. This date is significantly younger than the ~80 Ma age of porphyry magmatism and Mo mineralization, suggesting either that this event is in fact unrelated to porphyry Mo mineralization at 80 Ma, or that the K-Ar isotope system has been disturbed by a later thermal event.

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