Structural control of lode gold deposits in the Pony mining district, Tobacco Root Mountains, Montana

Abstract

Lode gold production in the Pony mining district has come principally from veins emplaced along faults and shear zones that strike northwest, west to west-northwest, northeast, and north. Host rocks are Archcan gneisses and amphibolites, Proterozoic diabase dikes, and Late Cretaceous granitic rocks of the Tobacco Root batholith. The faults and shear zones are interpreted to be related to east-west crustal shortening during the Late Cretaceous to early Tertiary Laramide orogeny that resulted in oblique left slip on the northwest-trending Mammoth fault, the principal zone of strain in the district. The west- to west-northwest-striking fractures are mostly characterized by reverse and left slip, and are interpreted as synthetic oblique-slip faults with respect to left slip on the Mammoth fault. Northeast-striking fractures are inferred to be antithetic strike-slip faults and the north-trending fractures are interpreted to be reverse and thrust faults. The vast majority of the gold production has been from the Boss Tweed, Clipper, and Mammoth mines that were developed on west- to west-northwest-trending veins in shear zones hosted by the metamorphic rocks.Initial movement on the Mammoth fault, and probably on the westerly to west-north-westerly trending shear zones, was prior to emplacement of Middle Proterozoic dikes. Early movement on the west- to west-northwest-striking shears resulted mainly in production of fine-grained cataclasite with minor amounts of mylonite. The shear zones were silicified, pyritized, and multiply reactivated. The youngest major episode of deformation, inferred to have occurred during the Laramide orogeny, resulted in brecciation of the silicified cataclasite and mylonite and provided permeability for mineralizing solutions.The most productive veins, also the longest and widest in the district, are nearly parallel to the direction of inferred maximum horizontal compressive stress during the Laramide and thus may be partly extensional fractures as well as having undergone contemporaneous oblique slip. Local undulations in the fault surfaces also affected the mineralization inasmuch as the ore zones are thickest where the hanging-wall dip is low and/or where the strike bends to the left. In addition, the largest stopes are found on the south sides of well-developed, easterly striking fractures that form a trailing extensional imbricate fan within the Boss Tweed fault zone.Deposition of chalcopyrite, galena, sphalerite, mackinawite, pyrrhotite, hessite, tetradymite, silver telluride, and gold is interpreted to have taken place in the Late Cretaceous during or shortly after emplacement of the Tobacco Root batholith and its satellite plutons. Ore minerals are found principally as replacements and in fractures in pyrite and thus appear to be dependent on the presence of pyrite and open fractures to provide permeability for mineralizing solutions. The source of the gold is not known, but it could have come from the igneous rocks or it might have been scavenged from the metamorphic rocks.