Date of Award
Master of Science (MS)
The Heddleston porphyry Cu-Mo deposit is located in Lewis and Clark County, Montana, near the headwaters of the Blackfoot River. It is immediately west of the historic Mike Horse mine, an important producer of Pb-Zn from polymetallic veins and lodes. The Heddleston property was explored extensively by the Anaconda Company in the 1960s and 1970s, but was never mined. Specimens of polished drill core from the deposit are archived in the Anaconda Research Collection at Montana Tech campus. The purpose of this research project was to use the archived samples to examine the geochemistry and mineralogy of the Heddleston and Mike Horse deposits using modern methods of ore deposit research, including portable X-ray fluorescence (pXRF), short-wave infrared (SWIR) mineral analysis, fluid inclusions, scanning electron microscopy (SEM) and sulfur isotope analysis.
The Heddleston deposit is centered on the Mike Horse stock, an Eocene (44.5 Ma) quartz monzonite porphyry, which has intruded into argillite of the mid-Proterozoic Spokane Formation and a thick diorite sill, also Precambrian in age. Drill core examined in this study was from DH 265-161, completed near the center of the district but just outside the mapped limit of the Number 3 Tunnel ore body. Several generations of quartz veins are present, including early quartz-chalcopyrite-pyrite veins with narrow potassic alteration envelopes, quartz-molybdenite veins with no alteration, and quartz-pyrite-chalcopyrite veins with phyllic alteration. Some of the late veins also contain galena, sphalerite, and Ag-bearing tetrahedrite-tennantite. Based on SWIR data, the most common alteration minerals in the altered porphyry host rock are muscovite (sericite), K-illite, kaolinite, and halloysite. Most of the kaolinite is well crystalline and is probably hypogene, while some is poorly crystalline and may have formed during weathering. Fluid inclusions from quartz-molybdenite and quartz-pyrite veins homogenized between 350 and 450˚C and have widely varying liquid/vapor ratios. Many inclusions contain halite daughter minerals, with sylvite and/or chalcopyrite daughter minerals also sometimes being present. This information suggests that boiling of a primary magmatic fluid occurred in the temperature range of 400 to 450˚C. Stable isotopes of S (d34S) in pyrite from Heddleston range from 3.5 to 5.2‰, and overlap with d34S values for pyrite, sphalerite, and galena in two samples from Mike Horse. These data also overlap with d34S data for hypogene sulfides in the world-class Butte porphyry-lode deposit. This suggests that the two porphyry systems may have inherited their sulfur from a common source. However, the Heddleston deposit differs from Butte in many ways, including its smaller size, its younger age (Eocene vs. late Cretaceous), its host rocks (Precambrian metasediments vs. Butte Granite), a lack of copper-rich “Main Stage” veins in the center of the district, and a shallower depth of emplacement.
Schubert, Ben, "Mineralogy and Geochemistry of the Heddleston Porphyry Cu-Mo Deposit, Montana" (2017). Graduate Theses & Non-Theses. 114.