RE-OS AND U-PB AGES FOR THE BUTTE COPPER DISTRICT, MONTANA: A SHORT- OR LONG-LIVED HYDROTHERMAL SYSTEM?

The Butte District formed in three hydrothermal events. Pre-Main Stage porphyry Cu-Mo mineralization was followed by development of a large barren zone characterized by pyrite and gray-sericitic alteration, which is cut by Cu-rich Main Stage veins. The Butte Quartz Monzonite hosts the deposits, and six zircon analyses yielded a concordant TIMS age of 76.3 ± 0.1 Ma. The ⁴⁰Ar/³⁹Ar data for micas have cooling ages (~325°C) of ~64-61 Ma for Pre-Main Stage ores and gray-sericitic-pyrite zones, and ~63-62.5 Ma for Main Stage ores (Snee et al., 1999).

Using both single and double Os spike methods, we determined new Re-Os ages for four molybdenites from Pre-Main Stage quartz-molybdenite veins of 66.44 ± 0.24, 66.03 ± 0.21, 65.33 ± 0.21, and 64.80 ± 0.23 Ma. The 66.0 Ma molybdenite is intensely overprinted by younger gray-sericitic alteration, and shows no age disturbance, consistent with Re-Os systematics in molybdenite elsewhere (e.g., Stein et al. 2001). The positive correlation of Re-Os ages with sample elevation suggests shallower molybdenites were deposited later as the hydrothermal system thermally prograded upward over a 1-2 m.y. interval. These ages are consistent with the 66 ± 1 Ma U-Pb age for quartz porphyry dikes synchronous with Cu-Mo ores (Lund et al., 2002), and are older than the ⁴⁰Ar/³⁹Ar cooling ages of contemporaneous biotite.

U-Pb zircon and monazite TIMS analyses from younger porphyries suffer from inheritance and reverse discordance, respectively. The Modoc quartz porphyry cuts Pre-Main Stage veins, is cut by Main Stage veins, and yielded two TIMS monazite analyses with ²⁰⁷Pb/²³⁵U dates of 63.8 ± 0.7 and 65.9 ± 2.5 Ma. Three zircons from this sample yielded a weighted mean SHRIMP ²⁰⁶Pb/²³⁸U age of 64.1 ± 3.2 Ma (n=4). U-Pb zircon and monazite systematics (TIMS) for two samples of a rhyodacite porphyry dike that cuts Main Stage veins suffer from inheritance. Seven monazite analyses yielded reversely discordant ²⁰⁷Pb/²³⁵U dates that range from 65 to 58 Ma, whereas a single concordant zircon yielded an age of 60.4 ± 0.3 Ma that is interpreted as the maximum age of the dike.

Re-Os and U-Pb data suggest a duration of about 1.5 m.y. (~66.4 to 64.8 Ma) for Pre-Main Stage porphyry Cu-Mo ore, whereas the duration of Main Stage ore is bracketed between about 65 Ma and the 62.5 Ma ⁴⁰Ar/³⁹Ar cooling age.