FIELD, PETROGRAPHIC, & GEOCHEMICAL CHARACTERISTICS OF THE PRICE CREEK VOLCANICS, SOUTHEAST BLACKTAIL MOUNTAINS, MONTANA

Geologic mapping in the SE portion of the Blacktail Mountains has revealed a more diversified stratigraphic sequence than previously documented (Muller & Krol, 2004). Prior mapping of this area has shown it as a single volcanic unit composed of a maroon rhyolite tuff, possibly erupted from the Eocene Dillon volcanic center located ~50 km to the northwest. Our mapping allowed us to further subdivide this unit, which we term the Price Creek unit (PCu), into two distinct lithologies based on mesoscopic characteristics; 1) a basal volcanic breccia and 2) a rhyolitic tuff. This study focuses on petrographic and geochemical analyses of the PCu in an effort to better define and elucidate its petrogenesis. The lowermost unit of the PCu is a maroon, coarse-grained, matrix-supported breccia that unconformably overlies Archean gneiss. It contains subangular clasts (1-100 cm) of predominately granitic gneiss with crystal fragments of quartz and feldspar. The contact between the basal breccia and the gneiss is sharp and highly irregular. Locally, maroon, aphanitic veinlets cross-cut and intrude parallel to the gneissic foliation in basement outcrops. The breccia is overlain by an aphanitic, maroon rhyolitic tuff. Petrographic analysis reveals angular to subrounded lithic and dominantly quartz crystal fragments with rare euhedral quartz phenocrysts set in a microcrystalline groundmass. Preliminary XRF analysis of several PCu tuff samples shows a very high SiO₂ content (84-87%) and a severe depletion of all other major element oxides with the exception of aluminum and iron (<1 wt% K₂0, Na₂0, Ca0, Mg0). The extreme enrichment of silica, depletion in other elements, and hematitic staining strongly suggests major geochemical alteration and modification following the formation of the rhyolite. The precise timing and nature of this alteration event is poorly constrained but may be related to the development and hydrothermal activity associated with post-Laramide normal movement on the Jake Canyon fault. The Jake Canyon fault, which forms the range front at Price Creek, is highly silicified with large masses of vein quartz (locally 10s meters in thickness) found along its trace. Hydrothermal fluids permeated footwall and hanging wall rocks causing hydrothermal alteration 100s of meters from the fault (Tysdal et al., 1990).