CHEMISTRY AND MINERALOGY OF THE LITHIUM-BORON DEPOSIT AT RHYOLITE RIDGE, ESMERALDA COUNTY, NV, USA

The late Miocene Cave Spring Formation, a 1500 ft, fine-grained volcanogenic lacustrine sequence, hosts the Rhyolite Ridge lithium-boron (Li-B) resource in the ­central Silver Peak Range, NV, USA. The Li-B resource lies in unit 2, the carbonate-rich middle 100 ft of Cave Spring strata. The unit’s upper half contains Li mineralization in Mg-clay and its lower half contains Li-rich, Al-bearing Mg-clay with searlesite (NaBSi₂O₅(OH)₂). Ar-Ar age dates for euhedral sanidine in tuffs above and below unit 2 constrain the period of its sedimentation to 700 Ky.

Twelve subdivisions compose Unit 2 based upon characteristic textures identified in multiple drill holes. One core hole provided samples collected from each subdivision. Bulk material and centrifuged clay separates (<0.5 μm) were analyzed for chemistry and mineralogy along with bulk material. X-ray diffraction (XRD) revealed mineral phases using random packed mounts of bulk and clay-sized material, while oriented mounts informed clay mineralogy. Optical and electron microscopy assisted with phase identification and paragenetic association. Inductively coupled plasma (ICP) analyses, using 4-acid digestion, established Li and B concentrations.

The upper half of Unit 2, composed of 62% clay (determined by sedimentation analysis), averages 2668 ppm Li in bulk material and 5450 ppm Li in clays while the lower half averages 2601 and 5612 ppm Li, respectively, and is 50% clay. The highest Li concentration in clay, 6600 ppm, represents the oldest subunit and could represent the early stages of mineralization. Illite-smectite (I/S) mixed layer clays (d-spacings of 12-14Å for smectite and 9-10Å for illites) comprise the clays in the lower half while smectites dominate the upper half. Boron averages 14,000 to 16,000 ppm in bulk assays in the lower half but averages 825 ppm above the transition zone. Searlesite hosts B mineralization, but ulexite (NaCa[B₅O₆(OH)₆] · 5H₂O) occurs in the transition zone between the two halves of the unit.

Compared to smectite, I/S phases show enhanced Li concentration when bulk Li is normalized by clay content. We hypothesize that precipitation of searlesite in the Cave Spring closed basin controls the equilibrium activity of SiO_2(aq) and Na⁺ in pore waters and affects the saturation states of clays during diagenesis, leading to I/S transformation.