283 Critical Mineral Resources and Recovery in the Americas: Emerging Methods in Exploration and Sustainable Extraction, Part II

This session explores the role of critical minerals in technology and the low-carbon economy, focusing on geological discoveries, sustainable extraction methods, and recycling to support secure, environmentally responsible mineral supply chains in the Americas.

Wednesday, 22 October 2025
1:30 PM - 5:30 PM
HBGCC, 217A

Using 87Sr/86Sr and Rb/Sr in Soils as Proxies for LCT Pegmatites

Paper number: 283-8
Program: Technical Sessions
Time: 3:25 PM – 3:40 PM

Authors

Abstract

Lithium-cesium-tantalum (LCT) pegmatites are rich in lithium-bearing minerals such as spodumene, petalite, and lepidolite and account for the majority of global hard-rock lithium production, along with most tantalum and cesium production. They occur as dikes and sills intruded into metamorphic country rocks but they can be difficult to locate and distinguish from barren pegmatites when covered by soil. LCT pegmatites are characteristically enriched in incompatible elements, such as Li, Cs, Ta, and Rb. Recent work has focused on searching for these elements in soils overlying pegmatites, however, concentrations of some elements in soils can be enriched or depleted during bedrock weathering processes. During magmatic fractionation, LCT pegmatites become enriched in Rb and depleted in Sr, resulting in distinctly high Rb concentrations and high Rb/Sr ratios (typically ~10-500) relative to country rock (~0.26 for the average upper continental crust). Additionally, most pegmatites are old (typically > than 300 Ma), meaning there is ample time for the natural radioactive decay of 87Rb to 87Sr. Therefore, Rb-rich pegmatites also develop unusually high proportions of 87Sr, resulting in 87Sr/86Sr values far exceeding those of most rocks. While typical granites and host rocks rarely contain 87Sr/86Sr greater than about 0.750, pegmatites often yield values greater than 1 (this work measures even higher 87Sr/86Sr ratios between ~0.9-120.9). Since soils primarily inherit the 87Sr/86Sr values of their parent rocks and the LCT pegmatites have distinctly elevated 87Sr/86Sr and Rb/Sr ratios, we propose using both Sr isotopes (87Sr/86Sr) and elemental ratios (Rb/Sr) in soils as exploration indicators for the occurrence of subsurface pegmatite dikes. This study proposes that laboratory (acid digestion followed by mass-spectrometry) and field (x-ray fluorescence spectroscopy) analyses of these ratios in soils overlying pegmatite dikes may serve as a useful prospecting tool. Data collected from two LCT pegmatites and their overlying soils, one from the Late Paleozoic Tin-Spodumene Belt in North Carolina and the other from the Proterozoic Animikie Red Ace pegmatite in Wisconsin provide promising support for this concept, demonstrated by extremely elevated 87Sr/86Sr (~1.0-6.8) and Rb/Sr (~10-120) in the overlying topsoils, which can be easily distinguished from background soils (87Sr/86Sr ~0.74-0.77, Rb/Sr ~0.76-3.9).

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412858

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