Li-rich Claystone in the McDermitt Caldera, Nevada: Characteristics and
Possible Origin
Abstract
The Lithium Nevada Corp. Thacker Pass Project has a measured resource of
242M tonnes at 0.29% Li (~0.7M tonnes Li). The deposit
is the southernmost of five known lenses hosted within intracaldera
tuffaceous sediments along the west side of the McDermitt caldera, which
formed by eruption of 16.4 Ma peralkaline-metaluminous tuff. Analyses of
9880 core samples (~1.5-m thick) give a mean of
~ 2500 ppm Li, with some >8800 ppm. Entire
sedimentary sections in the southern and western parts of the caldera
basin have high Li (1500+ ppm). Pre-caldera rocks, the McDermitt Tuff,
late caldera volcanics, and unaltered tuffaceous sediments have
relatively low Li (5 -140 ppm). Sediment deposition was mainly
subaqueous in the closed caldera basin. Li-rich fill may have been
deposited from airborne sources. Claystone comprises most of the Thacker
Pass deposit and dominates Li-rich intervals. The rest is mostly
feldspar-rich ash. X-ray diffraction (XRD) data from six core holes
across the deposit show consistent lateral mineral zoning. A Li-rich
zone (mean >4,000 ppm) mainly contains illitic clay and
underlies a zone of lower Li with smectite + analcime. Based on XRD peak
locations and chemistry, the closest analogs to the illitic clay are Li
micas (e.g. tainiolite). Montmorillonitic clay and a basal silicified
zone underlie the illitic zone. Some transitional rock of mixed-layer
clay between the illite and smectite zones also has high Li. All clay
has similar optical properties but SEM images show illite forms tiny
hexagonal plates and smectite has no resolvable crystallinity. Calcite,
albite, K-feldspar, and quartz occur in all zones, dolomite and fluorite
near the top of the illite zone, and pyrite and bitumen in unoxidized
rock. The Li-rich zone has relatively high K, Rb, Mo, As, and Sb.
Secondary Kspar in the Li-rich zone gave an 40Ar/39Ar age of 14.87±0.05
Ma. No origin fully explains known characteristics. The presence of
illite suggests a moderate temperature hydrothermal system. Release of
Li from tuffaceous sediments and transport by alkaline groundwater are
consistent with most mineralogy (characteristic of closed hydrologic
system diagenesis) and with the Kspar date that long postdates caldera
magmatism. Release of 10 ppm Li from the likely 50-200 km3 of
intracaldera sediments would contribute ~1-4M tonnes Li.