The source of lithium in connate fluids: Evidence from the geothermal reservoir at Soultz-sous-Forêts, Upper Rhine Graben, France

Geothermal brines in the Upper Rhine Graben have been used as a spa or for salt production since Roman times.
Heat and power are generated in geothermal power plants since 2007. Recently, their elevated Li-content has
additionally attracted economic interest. This increased interest is in contrast with our understanding of the
geological-hydrothermal evolution. We use petrology, major and trace element mineral chemistry and mass
balance calculation from drill cores that intersect granitic geothermal reservoir rocks at Soultz-sous-Forˆets be-
tween Strasbourg and Karlsruhe to shed light on fluid-rock interaction in a reservoir that is actively used for heat
and power generation. The alkali feldspar and the two-mica granite in the reservoir have a typical plagioclase, K-
feldspar, quartz, biotite and muscovite assemblage with some accessories of titanite, apatite and zircon. Two
hydrothermal alteration events are distinguished: (1) albitization of the feldspars; (2) distal replacement of
feldspars by sericite and calcite, of biotite by chlorite and titanite; and proximal to hydrothermal veins
replacement of the feldspars by sericite and kaolinite. ... mehrEvent 2 feldspar alteration quantitatively releases Pb and
Ba to the fluid, whereas Rb, Cs, Sr and Zn show different behaviour depending on whole-rock and mineral
composition. Event 2 biotite-chlorite alteration releases Li, Rb, Cs, Sr, Ba, Zn and Pb to the fluid. Mass balance
calculation indicates that Si, Fe, Ca, K, Rb, Sr, Zn and Pb contents of the Soultz-sous-Forˆets geothermal brine may
be explained by fluid-rock interaction in the reservoir. However, the reservoir rock volume that needs to be
leached in order to reach recent brine composition varies by several orders of magnitude between the different
elements. Many of the elements may be leached during hydrothermal alteration, however in particular Li and Cs
require unrealistic fluid-rock ratios of >1/300. These considerations indicate that Na, Ca, Li, Cs and Ba need an
additional external source. Based on this, we propose a model where Middle Triassic bittern brines already
enriched in Li, Rb, and Cs reacted with the reservoir rocks during hydrothermal event 2 and subsequently mixed
with Jurassic-Cretaceous marine water that dissolved evaporites during downward migration. This agrees with
Jurassic-Cretaceous illite ages from various sites in the Black Forest and indicates a complex ~150 m.y. hy-
drothermal evolution for the brines. There is likely no single source of Li, and it is likely derived from complex
fluid-rock interaction with the sedimentary (evaporite) and, less importantly, the crystalline strata of the Upper
Rhine Graben. Critical for Li-resource development is the complex hydrothermal history of connate fluids that
interacted with sedimentary strata and the preservation in deep-seated reservoirs.