MINERALOGY AND TEXTURES OF TALC ORE FROM THE WILLOW CREEK MINE, MONTANA: AN INITIAL INVESTIGATION USING SCANNING ELECTRON MICROSCOPY

The Willow Creek Mine in southwestern Montana is a former talc mine that operated for approximately 10 years, largely in the 1970s. The mine closed when a slope failure occurred that buried the deposit. Historical samples of the mine were acquired for study as the Willow Creek Mine is relatively unstudied compared to other talc mines globally. Recent work has found the occurrence of mercury, selenium, nickel, chromium, and thorium bearing minerals. A limited number of thin sections of talc ore from the mine were recently analyzed with SEM-EDS. Talc and chlorite are major minerals observed thus far and minor minerals include pyrite, apatite, graphite, calcite, Ti-oxide, and thorianite. A minor REE-rich phase is also present. Several examples of the talc matrix are slightly ferrous and highly aluminous. Examples of talc in one sample are approximately equigranular, with no obvious preferred orientation. The talc texture in one sample with higher pyrite content is less obviously granular, appearing more reminiscent of an alteration product. This sample also contains major calcite. Chlorite is also present in clusters, sometimes in discrete books but often interleaved with talc. The talc matrix often appears to grade into the chlorite grains along the foliation. Some examples of chlorite have a significant Fe component, with trace Mn. Much of the graphite is present as books or as anastomosing sheets. This initial SEM-EDS study opens major questions regarding the nature and evolution of the talc deposit. Whether the talc ore is only a carbonate replacement, if there are multiple generations of talc, or if there are extensive chlorite-talc reactions are open questions. Transmission electron microscopy work and study of the occurrence of the diverse set of minerals previously not recognized will provide more detailed constraints on the geologic history of this ore unit.