Talc production was first reported from the eastern Mojave in 1910, although the presence of large talc bodies had been noted as early as the late 1800's. From 1910 until 1978 California was a significant talc producer, accounting for 10-20% of the annual U.S. total. Unfortunately, many of the mines were located in Death Valley National Monument and pressure from the Park Service resulted in their closure significantly reducing California talc production.
Talc deposits occur in the Crystal Spring Formation of Late Proterozoic (Precambrian) age. The belt of deposits, encompassing 50 mines and prospects, stretches south eastward from the Paniment Range to the Kingston Range, a distance of 100 kilometers. The most productive areas were within the National Monument in the adjacent Warm Spring and Galena canyons.
Talc is a hydrous magnesium silicate containing 63.5% SiO2, 31.7% MgO and about 5% water. In nature, talc rarely occurs in pure form, but rather is intergrown with other silicates such as tremolite, chlorite, epidote and diopside. This results in commercial talc of widely varying composition, but in general the compositional range does not deter from the marketability of the talc.
Commercial talc falls into four categories:
Most commercial talc products are a blend of two or more types
specifically designed to meet the specifications of the buyer.
For example, the ceramic industry prefers hard, massive, white
talc while the paint and plastics industries prefer soft, foliated
talc. Mojave talc, because of its purity has commanded a premium
price and has been much in demand for specialized applications
which require higher purity, such as cosmetics.
Talc occurs only in the Crystal Spring Formation, the stratigraphically lowest member of the Pahrump group of Late Proterozoic age. The Crystal Spring Formation is 1000 to 1400 meters thick and consists characteristically of:
The most persistent of the diabase sills lies immediately below
the middle carbonate unit of the Crystal Spring. intrusion of
the diabase sills has produced the talc mineralization as well
as widespread silicification of the Crystal Spring Formation.
Typically, zones of silicification range from a few centimeters
to 100 meters in thickness. The persistent diabase sill at the
base of the middle carbonate unit, mentioned above, has produced
the best talcose alteration and most of the mines have exploited
The Excelsior/Snow White mine has operated nearly continuously until the past decade. It lies in the northeastern part of the Kingston Range and consists of a series of patented claims along the south flank of a north-trending ridge. Most of the mine development was underground, however, the Snow White claim block was mined by open pit methods.
The Snow White deposit strikes west-northwest and dips moderately to the north. The talc horizon is truncated by a northeast-trending fault which has moved the talc-bearing beds 400 meters to the northeast along strike. Thus, the Snow White claims represent a faulted extension of the Excelsior mine. To the east the talc beds terminate against another northeast-trending fault with brings stratigraphically younger Kingston Peak Formation in contact with talc.
The Snow White segment of the talc-bearing zone is 15 to 30 meters
thick, 100 meters wide and 400 meters in length. It is underlain
by a diabase sill, by locally the talc is separated from the diabase
by a layer of dolomite. The talc is overlain by quartzite and
dolomite of the upper Crystal Spring Formation. Well foliated
talc-schist is the most abundant rock type, but green "alteration
rock" is common as are manganese and iron oxides. Unaltered
carbonate blocks are also a common feature. You can also note
the appearance of brown silicate alteration high on the pit walls.
Geologic map of the Excelsior/Snow White Mine