Paleomagnetic documentation of the early, high-temperature zone of mineralization at Butte, Montana

Abstract

Butte quartz monzonite, host rock to mineralization in the Butte district, Montana, was pervasively altered to a high-temperature K-silicate alteration assemblage during pre-Main Stage, porphyry-type mineralization within the deep levels of the district. Magnetization directions of steep, negative inclination (reverse polarity) are characteristic of Butte quartz monzonite within the zone of pervasive pre-Main Stage alteration and mineralization. Butte quartz monzonite in surrounding, less altered parts of the district and farther from Butte contains only steep, positive inclination characteristic directions (normal polarity). Magnetic, petrographic, and radiometric evidence together with the exact correlation of reversed polarity directions with high-temperature pre-Main Stage alteration assemblages suggest that, in the deep, central portion of the Butte district, the original normal polarity thermoremanent magnetization was completely unblocked by heating to temperatures in excess of 578 degrees C. A secondary thermoremanent magnetization was acquired in subsequent cooling in a field of reversed polarity prior to Main Stage, fracture-controlled mineralization. The detection of reversed polarity directions in a portion of the Butte quartz monzonite pluton serves as a method for delineating the extent of a high-temperature (greater than 578 degrees C) thermal perturbation during mineralization at Butte.