PRELIMINARY SCANNING ELECTRON MICROSCOPY INVESTIGATION OF TALC ORE FROM WILLOW CREEK MINE, MONTANA IDENTIFIES THE OCCURRENCE OF MERCURY SELENIDE (TIEMANNITE) INCLUSIONS IN GRAPHITE AND OTHER PARTICLES OF CONCERN

Talc has been extensively mined and used for a wide variety of products such as seed coatings, paints, plastics, and cosmetics. Many historic mines were comparatively small in scale, operated over limited windows of time, and appear to not have their mineralogy investigated in detail. Thus, a lack of mineralogical data exists on these historic mines, and this is critical to understanding 1) the geologic and geochemical evolution of these deposits and 2) providing context for environmental properties of derived products. The Willow Creek Mine is one example of a small talc mine that had a limited operating life in southwest Montana. Six samples of ore of the Willow Creek Mine were acquired from the geological repository of the Montana Bureau of Mines and Geology in October of 2019 for study. Although COVID-19 interrupted analytical work, preliminary hand sample study and preliminary scanning electron microscopy (SEM) data were acquired. Preliminary sample inspection indicates that graphite and pyrite are common, and in some material they are abundant. Talc-rich material has a range of color from white to medium gray. Using back-scatter electron detection and spot energy dispersive spectroscopy analyses, SEM data indicate mineral particles with elements of environmental concern (e.g., mercury, selenium, zinc, and thorium) occur. Mercury and selenium co-occur as mineral inclusions in graphite. This phase is interpreted as a mercury selenide and is tentatively identified as tiemannite (HgSe) as this is the only known natural mercury selenide. The association of graphite and tiemannite appears to be a new mineralogical association. The observation of tiemannite is of health concern as this mineral may have been included in talc product produced from Willow Creek. Tiemannite is black to dark grey, being similar in color to graphite and would not be able to have been effectively removed by handpicking or other techniques used by industry. Mercury is well recognized as toxic and causes extensive human disease (e.g., ATSDR). The nature of mercury and other contaminants in these samples and any historical product should be investigated further to assess potential human exposure. More detailed work on samples is planned using diffraction with synchrotron radiation and transmission electron microscopy (TEM).