Strontium isotopes and Rb/Sr tracers in surface soils for locating subsurface lithium pegmatites

Lithium-cesium-tantalum (LCT) pegmatites are a major source of global Li production. They typically occur as dike swarms intruded into metamorphic country rocks. The subsurface locations of these dikes can be difficult to identify from the surface. One common approach is to investigate geochemical anomalies in overlying soils and use these as indicators for the occurrence of subsurface pegmatite dikes. However, even with these methods, economically valuable pegmatites can be difficult to identify, and additional techniques can be beneficial for optimizing geological exploration. Here we present a new methodology for detecting subsurface pegmatites through the analysis of Rb/Sr and strontium isotope (⁸⁷Sr/⁸⁶Sr) variations in relatively shallow soils overlying LCT pegmatites. During their formation, pegmatites become enriched in Rb and depleted in Sr, resulting in distinctly high Rb/Sr ratios (typically ≫10), and with time, the decay of ⁸⁷Rb leads to high ⁸⁷Sr/⁸⁶Sr ratios (typically ≫1.0) far exceeding those of common regional host rocks (typically with Rb/Sr < 10 and ⁸⁷Sr/⁸⁶Sr < ∼0.75). Since soils primarily inherit the ⁸⁷Sr/⁸⁶Sr values of their underlying parent rocks, we propose using these distinctive geochemical fingerprints in site-specific soil geochemical surveys to detect subsurface LCT pegmatites. We demonstrate the potential and utility of this methodology using surface soils around and overlying buried LCT pegmatites in the Late Paleozoic Tin Spodumene Belt in North Carolina and the Proterozoic Animikie Red Ace in Wisconsin, USA. Soils directly overlying the LCT pegmatites inherit distinctly elevated Rb/Sr (∼10–120) and ⁸⁷Sr/⁸⁶Sr (∼1.0–6.8) ratios compared to background soils away from the buried LCT pegmatites (⁸⁷Sr/⁸⁶Sr ∼0.74–0.77, Rb/Sr ∼0.76–3.9), which can be used to identify and reconstruct the location of subsurface LCT pegmatite dikes. The common and uniquely elevated Rb/Sr and ⁸⁷Sr/⁸⁶Sr signatures of global pegmatites suggest that this method can be widely and globally applied to soils derived from LCT pegmatites.

Duke Scholars

Author Daniel D. Richter Earth and Climate Sciences

Author Gordon Williams  

Author Avner Vengosh Earth and Climate Sciences