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.

Open pit operations at the Yellowstone Mine use conventional mining methods including drilling, blasting, and loading and hauling using trucks and shovels. Mining is based upon a standard height between benches of 25 feet. Benches vary from 15 to 50 feet in width (toe to crest) depending upon wall rock competency and requirements of future mining plans. Overburden consisting primarily of dolomite is drilled on a 15 by 15-foot grid to a depth of 28 feet and blasted using ANFO (a mixture of ammonium nitrate and fuel oil). The overall stripping ratio of overburden to ore is approximately 5:1. Overburden is loaded into trucks using hydraulic shovels and hauled to one of the permitted overburden disposal sites. Once exposed, most of the talc is mined using hydraulic shovels, with no blasting required. Run-of-mine talc ore is transported to the ore sorting facility for processing. Haul roads within the pit are a minimum 60 feet wide with a 4-foot-high berm along the outside edge and a maximum grade of 10 percent.

Overburden stripping and talc mining is permitted for 7 days per week year-round operations, using one 10-hour shift per day. At the present time, mining operations are conducted 4 to 7 days per week, depending upon market demand.

Talc is visually graded at the mine face before being loaded onto trucks for transport to designated stockpiles adjacent to the processing facility. The existing ore processing facilities are located in the Sorter Area and in the Optical Sorter.

Ore is sized and classified by passing it through an ore sorter that uses a friction technology to separate ore into four sizes: oversize, coarse,small particle, and fines. Oversize material is periodically crushed and reclassified for plant feed. Coarse feed, small particles, and fines are sampled and graded.

Mechanical and visual sorting techniques account for approximately 70 percent of the marketable talc product processed. Sorted talc is graded and stockpiled for shipment either directly to customers or to an offsite mill facility. An additional 20 percent of the talc is stockpiled as low-grade talc, and the remaining material is discarded with the overburden.