FRANKLIN WELLS HECTORITE DEPOSIT, AMARGOSA VALLEY, INYO COUNTY, CALIFORNIA

Hectorite, the rare lithium smectite mineral, is mined at only four locations on earth. One of these is near Franklin Wells in the western Amargosa Valley of California, on the eastern flank of the Funeral Mountains Wilderness Study Area. The mineral occurs in lake beds as a hydrothermal alteration product of volcanic ash along a fault zone. The surface geology consists of alluvial fans and pediment surfaces. Most of the surface above the deposit is desert pavement. The deposit is in the Amargosa Valley near the Amargosa Valley Fault. To the southwest are Paleozoic sediments of the Funeral Mountains and to the southeast are Pre-Cambrian and Cambrian units of the Shadow Mountains. The clays in the area include extensive deposits of sepiolite and saponite—a magnesium rich smectite which matches some properties of sodium bentonite as well as hectorite. There is extensive evidence of hydrothermal activity along the surface expression of the Franklin Wells fault, including massive sinter deposits southeast of the current mine area. At the Franklin Wells Hectorite Deposit, the valuable minerals are hectorite and some stevensite which makes up 5 to 35% (averaging 13%) of the ore. Gangue consists of other associated clay minerals, carbonate and silica. The ore body is tabular, 3 km (1.9 mi) long, 0.6 km (0.4 mi) wide, and, locally at least 6 m (20 ft) thick. The ore body is defined by a Lithium/Fluorine geochemical anomaly, which clearly delineates the position of the Franklin Wells fault zone. At the Franklin Wells deposit, the hectorite is an alteration product of volcanic ash in the carbonate breccia and formed from hot spring activity along a fault More specifically, hectorite mineralization occurred as the replacement of Li for Mg in the montmorillonite crystal lattice of volcanic (dacite) ash grains during hydrothermal (spring) activity. This alteration is most pronounced along a fault zone, which we name the Franklin Wells Fault. The three controlling factors in hectorite genesis are the presence of dacitic ash beds, a fault conduit system and lithium-rich hydrothermal activity. The fortuitous overlapping in space and time of these conditions make hectorite a rare and valuable specialty mineral. Regional mapping of faults and lineaments identify targets for hectorite when they cut through the Quaternary section.