Tectonic origin of fractures for fissure vein emplacement in the Boulder Batholith and adjacent rocks, Montana

Abstract

In the Boulder batholith and adjacent rocks, the dominant and most productive metalliferous fissure veins trend approximately east and dip steeply. Other steep veins, generally narrower, trend northwest, northeast, and rarely north. Fissure veins also occur locally in thrust faults in volcanic rocks along the west side of the batholith. Fractures along which these veins were emplaced were probably initiated by a regional compressional stress field during Laramide (Late Cretaceous-early Tertiary) time. The batholith was emplaced during the Late Cretaceous and is synorogenic with respect to the surrounding Laramide thrust and fold belt, which is characterized by east-west compression. Postorogenic rocks (Eocene and younger) do not host major fissure vein deposits.Fractures utilized by east-trending metalliferous veins are parallel to the axis of maximum compressional stress and thus appear to be extensional; this may be the reason these veins are widest and longest. Northeast- and northwest-trending veins were probably emplaced along conjugate shears; these veins are narrower than east-trending veins. Veins that trend north are less common and probably formed along release fractures perpendicular to the principal axis of compressional stress. These veins are narrower than many of those that trend east.Mechanical behavior of various rock units during deformation had an important effect on ore deposition. The longest, widest, and most productive fissure veins occur in competent, brittle rocks (Precambrian crystalline basement, batholith intrusions, and Elkhorn Mountains Volcanics). Ductile rocks (Mesozoic shales and Belt Supergroup argillites) do not commonly contain major fissure veins, except where these rocks have been contact metamorphosed and have become more brittle. Paleozoic strata, mainly carbonates, are of intermediate ductility and are hosts to fissure veins in only a few districts adjacent to the batholith.