SHRIMP U-Pb and 40Ar/39Ar age constraints for relating plutonism and mineralization in the Boulder batholith region, Montana

The composite Boulder batholith, Montana, hosts a variety of mineral deposit types, including important silver-rich polymetallic quartz vein districts in the northern part of the batholith and the giant Butte porphyry copper-molybdenum pre-Main Stage system and crosscutting copper-rich Main Stage vein system in the southern part of the batholith. Previous dating studies have identified ambiguous relationships among igneous and mineralizing events. Mineralizing hydrothermal fluids for these types of deposits and magma for quartz porphyry dikes at Butte have all been considered to be late-stage differentiates of the Boulder batholith. However, previous dating studies indicated that the Boulder batholith plutons cooled from about 78 to 72 Ma, whereas copper-rich Main Stage veins at Butte were dated at about 61 Ma. Recent efforts to date the porphyry copper-molybdenum pre-Main Stage deposits at Butte resulted in conflicting estimates of both 64 and 76 Ma for the mineralizing events. Silver-rich polymetallic quartz vein deposits elsewhere in the batholith have not been dated previously.

To resolve this controversy, we used the U.S. Geological Survey, Stanford, SHRIMP RG ion microprobe to date single-age domains within zircons from plutonic rock samples and ⁴⁰Ar/³⁹Ar geochronology to date white mica, biotite, and K-feldspar from mineral deposits. U-Pb zircon ages are Rader Creek Granodiorite, 80.4 ± 1.2 Ma; Unionville Granodiorite, 78.2 ± 0.8 Ma; Pulpit Rock granite, 76.5 ± 0.8 Ma; Butte Granite, 74.5 ± 0.9 Ma; altered Steward-type quartz porphyry dike (I-15 roadcut), 66.5 ± 1.0 Ma; altered Steward-type quartz porphyry dike (Continental pit), 65.7 ± 0.9 Ma; and quartz monzodiorite of Boulder Baldy (Big Belt Mountains), 66.2 ± 0.9 Ma. Zircons from Rader Creek Granodiorite and quartz porphyry dike samples contain Archean inheritance. The ⁴⁰Ar/³⁹Ar ages are muscovite, silver-rich polymetallic quartz vein (Basin district), 74.4 ± 0.3 Ma; muscovite, silver-rich polymetallic quartz vein (Boulder district), 74.4 ± 1.2 Ma; muscovite, early dark micaceous vein (Continental pit), 63.6 ± 0.2 Ma; biotite, early dark micaceous vein (Continental pit), 63.6 ± 0.2 Ma; potassium feldspar, early dark micaceous vein (Continental pit), 63 to 59 Ma; and biotite, biotite breccia dike (Continental pit), 63.6 ± 0.2 Ma.

Outlying silver-rich polymetallic quartz veins of the Basin and Boulder mining districts probably are directly related to the 74.5 Ma Butte Granite, whereas Steward-type east-west quartz porphyry dikes and Butte pre-Main Stage deposits are parts of a 66 to 64 Ma magmatic-mineralization system unrelated to emplacement of the Boulder batholith. The age of the crosscutting Main Stage veins may be about 61 Ma as originally reported but can only be bracketed as younger than the 64 Ma pre-Main Stage mineralization and older than the about 50 Ma Eocene Lowland Creek intrusions.

The 66 Ma age for the quartz monzodiorite of Boulder Baldy and consideration of previous dating studies in the region indicate that small ca. 66 Ma plutonic systems may be common in the Boulder batholith region and especially to the east. The approximately 64 Ma porphyry copper systems at Butte and gold mineralization at Miller Mountain are indicative of regionally important mineralizing systems of this age in the Boulder batholith region. Resolution of the age and probable magmatic source of the Butte pre-Main Stage porphyry copper-molybdenum system and of the silver-rich polymetallic quartz vein systems in the northern part of the Boulder batholith documents that these deposits formed from two discrete periods of hydrothermal mineralization related to two discrete magmatic events.