The occurrence and timing of gold mineralization at the Red Pine Mine, western Tobacco Root Mountains, southwestern Montana

Abstract

The Red Pine mine is located in Archean metasediments in the Sheridan mining district of the western Tobacco Root Mountains. The mine is developed in a fissure vein that occurs in marble and along the contact between marble and quartzofeldspathic gneiss. The present study broadens the knowledge of gold mineralization in the range. It describes ore and gangue minerals and establishes their paragenetic sequence. The study also interprets the conditions and timing of ore emplacement and the sources of ore metals and fluids. Two principal periods of mineralization and a minor period of secondary alteration are evident in the deposit. Moreover, textures show that multiple episodes of fracturing and brecciation accompanied the mineralization. Gold is present primarily in the first period of mineralization as discrete grains in fractures in pyrite or on the borders of brecciated pyrite. It is also found as minute inclusions and open-space fillings within pyrite. Gold is also associated with milky quartz, bismuthinite, and, less commonly, with carbonate, chalcopyrite, and tetrahedrite. Period I mineralization, recognized by milky quartz, contains pyrite, bismuthinite, chalcopyrite, sphalerite, and silver-bearing tetrahedrite. Trace amounts of pyrrhotite have been noted with pyrite and chalcopyrite. A major episode of brecciation separates Period 1 from Period 2. Gangue mineralization predominates in Period 2 with the deposition of graphite, white mica, dark gray microcrystalline quartz, calcite, and minor amounts of pyrite and chalcedony. Some gold is found along fractures in milky quartz that are filled with graphite and mica. Gold is also attached to graphite in these fractures. Goethite, djurleite, malachite, and minor covellite occur ' as sulfide alteration products. Interpretations of timing and pressure-temperature conditions of ore emplacement are made from mineralogy, textures, paragenetic sequence, and geologic relations observed at the mine. A mesothermal to hypothermal temperature (200° to 500° C) of deposition at pressures no greater than 2 kilobars is indicated by field relations and sulfide paragenesis. Ore deposition is interpreted as hydrothermal and as related to the Tobacco Root batholith or an associated igneous body. This relationship suggests a Late Cretaceous to early Tertiary age for the deposit.