Copper content of biotites from the Boulder Batholith, Montana

Abstract

Copper content (by instrumental neutron activation analysis) of sixty-one biotites from rocks of the Late Cretaceous, composite Boulder batholith ranges from 6 to 4,395 ppm and averages 700 ppm. High copper values for the samples studied can generally be accounted for by the presence of minute inclusions of sulfide minerals in the biotite, rather than to extensive isomorphic substitution of copper in the crystal structure.Although the distribution pattern is variable, the highest copper contents in biotite are observed for samples from the Butte Quartz Monzonite, the largest single pluton of the batholith and the host rock for the rich deposits at Butte and many other major ore deposits of the region. Plutons older or younger than the Butte Quartz Monzonite have less copper (fewer or no sulfide inclusions) in their biotites. Within the Butte unit itself, higher values of copper in biotite tend to be found for rocks from a broadly defined, NNE-SSW-trending belt, which parallels the long dimension of the mass and contains the Butte district in the south and the Wickes and Clancy districts in the north.