Origins of lithium in geothermal fluids in a continental rift setting –Insights from the Upper Rhine Graben and regional Vosgian analogues, France

The Upper Rhine Graben (URG) is a continental rift where lithium-rich geothermal fluids have been exploited in Hercynian fractured crystalline basement and overlying Buntsandstein sandstones to produce heat and electricity since 2007. A combined mineralogical and geochemical study of Hercynian granites and Permian and Buntsandstein sandstones, sampled from geothermal wells and analogue outcrops was carried out to investigate lithium sources and lithium enrichment processes that lead to the present lithium concentrations in brines. Whole-rock lithium contents in sandstones (11–164 ppm) and in granites (53–398 ppm) are generally low. Micas are the dominant lithium carriers in both reservoir rocks. Magmatic biotite and muscovite in granites contain up to ∼2000 and 570 ppm Li respectively, whilst detrital muscovite in sandstones contains up to 2800 ppm Li. Petrological observations indicate that alteration of biotite and muscovite into illite is a key process controlling lithium mobilisation from crystalline and sedimentary reservoirs into geothermal fluids. Present-day URG brines are close to equilibrium with Buntsandstein sandstone reservoirs but not with crystalline basement rocks, highlighting the importance of sediment-basement transition in lithium enrichment. Lithium mobilisation likely occurred during multiple geological stages: Dismantling of Hercynian chain that originated Permian–Triassic deposits, hydrothermal activity in deep basin setting and continental rifting. Argillic hydrothermal stages in granite significantly redistributed lithium, with hydrothermal quartz acting as lithium sink. Lithium mass balance estimates confirm its mobilisation by geothermal fluids circulating through the fractured network of granitic reservoirs.