INVESTIGATION OF THE BOULDER PORPHYRY PROSPECT JEFFERSON COUNTY, MONTANA

With increasing demand for domestic sources of base, precious, and critical metals, previously underexplored mineral systems in the United States are gaining attention. The Boulder Porphyry project (BPP), located 15 miles north of Butte, in Jefferson County, Montana, represents a concealed and underexplored copper-molybdenum porphyry system that shares a similar geologic setting with the world-class mining district in Butte, Montana. This thesis applies a combination of traditional and modern exploration techniques to better understand the nature of hydrothermal mineralization at the BPP. Short-wave infrared (SWIR) analysis of drill core, coupled with bulk rock geochemistry, is used to characterize hydrothermal alteration assemblages and their possible correlations with copper grades down hole. These data are supplemented by reflected light microscopy, sulfur isotope studies, and radiometric age dating to compare the BPP with the well-studied Butte deposit. Due to time and monetary constraints, only one drill hole was examined in detail in this study. The top several hundred feet of this hole were drilled through Eocene Lowland Creek Volcanics that unconformably overly the late Cretaceous Butte Granite and that hosts porphyry-style mineralization at the BPP. The most abundant sulfide minerals found were pyrite and chalcopyrite, although molybdenite was seen towards the bottom of the hole. Scanning electron microscopy documented the presence of small amounts of bismuth, copper, arsenic, silver, lead-sulfide minerals in two samples of mineralized vein material. The SWIR analysis and whole rock chemical trends shows that sericitic (phyllic) alteration commonly occurs along centimeter-scale quartz-pyrite-chalcopyrite veins. Relatively limited evidence for strong potassic or advanced argillic alteration was found in this study for the one drill hole investigated, although potassic alteration has been documented by company geologists. Sulfur isotopes of sulfide minerals (pyrite, chalcopyrite, molybdenite) separated from different depths in the drill core fell in a narrow range of +3.1 to +5.3 ‰. These values are typical of hydrothermal mineral deposits in the Boulder Batholith, and overlap with the isotopic composition of sulfide minerals from Butte. An in-situ rhenium-osmium age date of 75.0±1.5 Ma was obtained for molybdenite in a quartz vein from near the bottom of the hole examined. This result indicates that porphyry-style mineralization began shortly after emplacement of the host Butte Granite. However, the single age determination does not preclude the presence of overprinting hydrothermal events at the BPP.