Use of near-surface waters in identifying elemental associations with geothermal-sourced Li

Unprecedented demand for lithium (Li) is being driven by its use in electric vehicle batteries. Currently, the majority of Li comes from pegmatite mining and salar brines, however, new sources such as geothermal brines will be required to meet future demand. The North Pennines, Northern England has been found to host brines with lithium concentrations exceeding 90 mg/L from 411 to 995 m. However, deep subsurface water chemistry for the region is limited to a single abandoned borehole, necessitating the use of other techniques in assessing the resource potential of these brines. This work investigated the potential of surface and near-surface water samples from abandoned mine workings to expand the known geographic extent of the underlying Li brine resource. Li concentrations were 1.9 to 784 μg/L at the 44 locations sampled. Principal component, cluster, and covariate analyses identified two distinct water chemistry clusters mostly related to dimension 1 of the PCA (22.5% of variance) and included alkalinity, Ca, Cd, Cl, F, K, Li, Mg, Na, Se, and SO 4 2- ; the “near surface” or potentially orebody related group which included Al, As, Cu, Eu, Fe, P, Pb, V, and Y. Two smaller clusters are present on the positive and negative axis of dimension two (14.1%); on the positive is B, Ba, Br, Cr, pH, and Si, and on the negative, Co, Mn, Ni, Sc, Sr, and Zn. The Cambokeels Mine, 0.5 km from the original borehole, had the highest Li concentration of 78.4 mg/L. However, the deep brine signature and Li enrichment were also found at a cluster of mines 15 km away, significantly expanding the geographical extent of the North Pennine Li brine resource. These findings show that relatively low-cost elemental analysis and statistical analyses could be a promising exploration tool for regions where there is limited data. Developing tools using geochemical finger printing of near-surface waters to identify Li resources in deeper geothermal brines will be essential for the cost-effective development of critical minerals. • Near-surface measurements may be useful in assessing geothermal resources. • North Pennine geothermal Li is likely sourced from the Weardale Granite. • Li associated with alkalinity, Ca, Cd, Cl, F, K, Mg, Na, Se, and SO 4 in mine water. • Statistical analysis is a low cost first step before expensive drilling.