216-5 Syn-mineralization quartz-molybdenite vein shear zones in the Butte, Montana porphyry Cu-Mo deposit

Session: Geochronology of Critical Mineral Deposits with Special Reference to U-Th-Pb Dating of Common-Pb-Rich Minerals

Presenting Author:

Marisa Acosta

Authors:

Acosta, Marisa D. ¹, Descutner, Kade², Hofmann, Florian³, Talbot, Indie⁴, Eastman, Kyle⁵, Griffith, Amanda⁶, Reed, Mark H.⁷

(1) Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, USA; Department of Geosciences, University of Alaska Fairbanks, Fairbanks, Alaska, USA, (2) Department of Geosciences, University of Alaska Fairbanks, Fairbanks, Alaska, USA, (3) Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, USA, (4) Department of Geosciences, University of Alaska Fairbanks, Fairbanks, Alaska, USA, (5) Montana Bureau of Mines and Geology, Butte, Montana, USA; Department of Geological Engineering, Montana Technological University, Butte, Montana, USA, (6) Montana Resources, Butte, Montana, USA, (7) Department of Earth Sciences, University of Oregon, Eugene, Oregon, USA,

Abstract:

At the base of the Butte porphyry Cu-Mo deposit, a barren quartz stockwork centered on quartz porphyry dikes grades upwards into a network of quartz-molybdenite veins. In the footwall of the Continental Fault, a basin-and-range style normal fault with ~1.3 km of vertical displacement on it, there are 5–30 cm wide shear zones with margins demarcated by molybdenite-rich bands within quartz-molybdenite veins. Shear zone interiors consist of banded quartz veins intercalated with ductilely deformed Butte Granite. Within the shear zones, magmatic biotite has been recrystallized to a different composition (as measured with electron microprobe analyses) and texturally defines a foliation texture. Magmatic K-feldspar was sericitized during hydrothermal alteration but converted back to K-feldspar during shearing, as recorded by petrographic textures, in which partly sericitized feldspar cores are mantled by neocrysts of K-feldspar. We interpret the replacement textures, which are only observed within the shear zones and not elsewhere in the deposit, to reflect changes in the activities of potassium, hydrogen, and aqueous silica. Electron backscatter diffraction data show that although ductile shear deformation in a water-rich environment is recorded by a biotite foliation, quartz and K-feldspar have annealed at high temperature and do not preserve a deformation fabric. ⁴⁰Ar/³⁹Ar geochronology of biotite and K-feldspar from within shear zones yield ages that correspond to the age of the porphyry system itself. We had originally interpreted these shear zones to have formed in the ductile damage zone of the post-mineral Continental Fault, but in light of the new geochronology data, we conclude that the shear zones must be a primary feature of the Butte porphyry system. The Butte deposit is deeper than other porphyry Cu deposits, having formed between 7 and 9 km depth rather than the typical 2 and 4 km, and so has a more extensive high-temperature deformation history than other systems. Our working interpretation is that the quartz-molybdenite shear zones formed while the magmatic-hydrothermal system that created the porphyry deposit was active.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10064

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.