Paleomagnetism, Rock Magnetism, and Aspects of Structural Deformation of the Butte Mining District, Butte, Montana

Abstract

Previously, field relations in the Butte District, Montana, have been interpreted to indicate a regional, 40-60° or greater westward tilting of the Late Cretaceous Butte quartz monzonite (BQM), the host rock to mineralization. The tilting was inferred to be Eocene or younger in age, based upon structural features in the overlying Eocene Lowland Creek Volcanics. Extensive paleomagnetic data reveal a more complicated picture. For surface exposure BQM, the in situ mean direction of characteristic magnetization of 13 site means is D = 1.5°, I = 73.3° (K = 27.6, $$\alpha_{95} = 2.6^{\circ}$$). Characteristic directions from BQM affected by early mineralization and high-temperature potassic alteration (antipodes) and two sets of quartz porphyry dikes, which pre-and post-date this mineralization, coincide with results from the surface and Berkeley Pit exposures of BQM. Studies of the magnetic oxides, along with the results of alternating field and thermal demagnetization, indicate a primary thermoremanent origin for the characteristic directions from all units except the high-temperature altered BQM, where a secondary thermoremanent magnetization was imparted. District-wide tilting of the magnitude visualized in the past is precluded by the fact that characteristic magnetization directions of Late Cretaceous units in the Butte district correspond closely to those of time-equivalent units elsewhere in North America. Systematic regional or non-systematic local tilting of as much as 15° could have occurred, but movements of 40-60° or more are ruled out for the BQM units analyzed. The Eocene volcanics overlying the BQM present an interesting tectonic enigma. Immediately northwest of Butte, the units dip steeply to the west or northwest in a graben-like structure; farther west or northwestward, the dips become more shallow. When compared with the predicted Early Tertiary field direction, the data for these units suggest that they have indeed been rotated westward by 30-50° or more about an approximately north-south axis. The immediately underlying BQM is unexposed, but inliers (probably horsts) of BQM surrounded by the volcanics within the graben give characteristic directions identical to those of untilted BQM elsewhere in the district. The discrepancy must relate to movements within the Eocene volcanic units after the magnetic carriers had acquired a stable thermoremanent magnetization, possibly during differential compaction and slumping. The study supports long-standing interpretations that the zonal patterns of mineralization and hydrothermal alteration in BQM have not been extensively reoriented by tectonic tilting.