Geology, geochemistry, selected physical properties, and genesis of the Cretaceous Clay Spur Bentonite in Wyoming and Montana

Ash erupted from volcanoes associated with the emplacement of the Boulder Batholith and deposited in the Mowry Sea, which occupied a portion of the Western Interior Basin from Late Albian to Early Cenomanian time, altered to form the Clay Spur bentonite. The Clay Spur bentonite, which is mined primarily for drilling mud, foundry sand, and taconite pelletizing, has unique physiochemical properties that may be attributed to its mineralogy. The purpose of this study is to determine, using a variety of techniques, the role that the parent ash, the environment of deposition, diagenesis and weathering have played in determining the mineralogy and chemistry of this deposit.;The sodium smectite exhibits regional as well as local variations in major element chemistry, trace element chemistry, exchangeable cation chemistry, organic carbon content and consequently physical properties. Bentonite rheological properties such as plastic viscosity and filter cake permeability are controlled, among other things, by the surface chemistry of the clay which is a function of the structural chemistry of the clay. Weathering reactions have most significantly affected the surface chemistry of the clay through the oxidation of octahedrally bound iron which causes a decrease in the net negative surface charge. Lowering of the surface charge combined with a decrease in pH, and a decrease in the abundance of exchangeable Ca has resulted in an enhancement of bentonite properties. In contrast, diagenesis has had a negative effect on bentonite quality as a result of the precipitation of silica along the edges of smectite flakes which causes a significant decrease in viscosity.;The chondrite-normalized rare earth element patterns of sodium smectite separated from the bentonite indicate that the parent ash was rhyolitic in composition as evidenced by pronounced negative Eu anomalies, a decrease in heavy rare earth element abundances relative to light rare earth elements, and relatively high overall rare earth element abundances. Oxygen isotope values indicate that alteration probably occurred in an ash-dominated system and under closed hydrologic conditions in contact with pore fluids derived from Mowry seawater that was stratified with respect to salinity and degree of oxygenation.