How hydrothermal alteration upgrades lithium claystones

01

Why this matters

Volcano-sedimentary lithium (Li) deposits combine shallow, low-carbon footprint and potentially large tonnage with a difficult genetic question: why do some caldera-lake claystones contain modest Li grades while some intervals or subdomains reach substantially higher concentrations? The answer matters for exploration because simply locating a rhyolitic basin or Li-bearing smectite does not identify the processes that produced the richest ore.

Benson et al. (2023) address this problem at McDermitt caldera, where Li-bearing smectite hosted within lacustrine sediments extend across much of the basin but the exceptionally high-grade illite horizon is restricted to the southern caldera at Thacker Pass. The study tests whether the high grades reflect primary lake-sediment formation alone or a later hydrothermal event. Its key contribution is a two-stage metallogenic model in which early basin-scale Li capture was locally upgraded by later fluid-rock interaction.

02

What the study found

The McDermitt caldera formed during eruption of Li-rich peralkaline rhyolite at approximately 16.4 Ma. Thick intracaldera ignimbrite created the floor of a closed lake in which more than 200 m of tuffaceous lacustrine sediment accumulated. Smectite-rich intervals occur widely across the caldera and generally contain 1,000–4,000 ppm Li in whole-rock assays. At Thacker Pass, however, illite-rich intervals contain 4,000–8,000 ppm Li, with maximum assays near 9,000 ppm.

The authors interpret the widespread Mg-rich smectite as authigenic rather than detrital or produced mainly by alteration of interbedded glass. After eruption, Li and other solutes were leached from volcanic glass and supplied by cooling and degassing intracaldera tuff. The closed, alkaline lake concentrated these solutes through evaporation until Li-bearing magnesian smectite precipitated within the sediments.

A younger hydrothermal event then produced the highest grades. Magmatic resurgence uplifted the southern caldera and generated fractures that channelled hot fluids upward. These fluids likely acquired additional Li by leaching the already Li-rich lacustrine sequence, then migrated laterally through permeable tephra and carbonate beds. At approximately 180–230°C, smectite was converted to mixed illite-smectite; at about 230–300°C, it was replaced by illite. The same alteration dissolved primary carbonates, formed fluorite, replaced analcime with K-feldspar and precipitated pyrite. The hottest, most intensely altered zone is therefore also the most Li-rich.

03

Our interpretation

The strongest aspect of the paper is its distinction between lithium accumulation and lithium upgrading. The first stage created a large, regionally extensive inventory by coupling a Li-bearing volcanic source with prolonged leaching, a closed basin and evaporative concentration. The second stage redistributed and further concentrated part of that inventory into an illite-rich hydrothermal zone. Neither process alone explains both McDermitt’s tonnage and Thacker Pass’s exceptional grade.

This framework also explains the spatial asymmetry of the deposit. Smectite occurs throughout the caldera because lake processes operated at basin scale. High-grade illite is concentrated near southern resurgence structures because effective illitization required sufficiently hot and voluminous hydrothermal fluids. Cooler hydrothermal systems along the northern caldera margin, generally at or below approximately 200°C, did not generate the same widespread conversion to Li-illite.

The interpretation is compelling but should not be reduced to a universal “illite equals high grade” rule. Illite can form in many Li-poor hydrothermal systems. Its exploration significance depends on demonstrating an initially fertile volcanic-basin system, the correct alteration assemblage and a spatial relationship to resurgence-related fluid pathways.