REVISED 40AR/39AR AGES OF THE WHITE CLOUD MOLYBDENUM DEPOSIT, CUSTER COUNTY, IDAHO

With the exception of specific, focused investigations (i.e., Butte and Thompson Creek), the temporal and basement controls on molybdenum (Mo) and copper-Mo deposits in the northeast trending Idaho-Montana porphyry belt (Armstrong et al., 1978; Rostad, 1978) are largely unknown. As part of a broad-scope investigation of this porphyry belt, we are using high precision geochronology and isotope tracers to help construct a regional framework for the formation of Mo-bearing deposits. We present preliminary data from the White Cloud (Little Boulder Creek) high-grade Mo (~0.2 %) deposit in Custer county, Idaho.

Three replicate analyses on sericite from Mo-bearing quartz veins, hosted in the Little Boulder Creek satellite pluton, yield indistinguishable plateau ages within the limits of precision and provide a mean age of 84.65 Ma. We also report a plateau age of 84.80 ± 0.30 Ma on sericite from a similarly mineralized quartz vein in hornfelsed Paleozoic sediments adjacent to the Little Boulder Creek stock. These ages clearly date the Mo-mineralization event at 84 m.y., which is at odds with prior conventional K-Ar estimates of 61.5 m.y. (Armstrong et al., 1978). This new data suggest that the Little Boulder Creek stock and the associated White Cloud Mo deposit are older and perhaps more closely related to the proximal ~83 Ma White Cloud quartz monzonite of the Idaho batholith than previously thought.