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

Radiometric dating of ore deposits containing critical minerals. Studies discussing analytical aspects of methods such as U-Th-Pb, Ar-Ar, Rb-Sr, Sm-Nd, Re-Os and/or using radiometric ages for ore genesis research or critical mineral resource evaluation.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 304C

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

Paper number: 216-5
Program: Technical Sessions
Time: 2:40 PM – 2:55 PM

Authors

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 Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414168

© Copyright 2025 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.