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Location: 30 km S from Ennis, Montana, United States
7951 E Maplewood Avenue Suite 120Greenwood VillageColorado, United States80111
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Talc deposits of the Yellowstone Mine occur in an area of folded Precambrian (Archean) dolomitic marble, along the east limb of a large, southwest-plunging fold. The dolomitic marble occurs over a zone about 1.5 miles wide and 3.5 miles long. To the southeast and northwest these rocks are in contact with older foldedmetamorphic schist and gneiss, to the southwest the marble is in contact with younger Paleozoicsediments, and to the east the marble is unconformably overlain by Quaternary gravel. Tertiary volcanic rocks unconformably overlie the marble along the axes of structural grabens and elsewhere in paleo topographic depressions.The talc deposits at the Yellowstone Mine were formed in the early Proterozoic by hydrothermal alteration and replacement of the Archean dolomitic marble. The talc is massive and predominantly light green to light gray in color. Replacement of the dolomite by talc is complete; thus, the talc is easily separated from the host dolomite waste during ore sorting. The talc occurs mainly as tabular veins, but locally, pods and lenses are also found. Most talc veins are parallel or sub-parallel to the metamorphic structural foliation developed in the surrounding dolomite. This is because crosscutting foliation cleavage planes and nearly parallel fault zones provided pathways for silicarich ore forming fluids to penetrate and interact with the carbonate host rock to produce the calc-silicate talc replacement deposits. The dominant structure associated with the deposit is a major north south trending fault called the Growth Fault. This fault is over 100 feet wide in places and is locally associated with some karst features.Yellowstone Mine talc is free of tremolite and other asbestiform minerals and contains only trace amounts of other impurities (iron and graphite). The talc ore bodies of the Yellowstone Mine are among the largest and mineralogically purest of their kind in the world. The presence of iron oxide impurities in a talc ore causes a yellow coloration or “warm” tints in the talc powder after milling. This affects optical properties by lowering the brightness and increasing the yellow index values. Both the brightness and yellow index are used as measurements of talc grade. It is because of these properties that Yellowstone Mine talc is used worldwide in processes designed to incorporate these unique characteristics.
