Summary:
The Upper Rhine Valley Brine Field (URVBF) is part of the Upper Rhine Graben (URG). The roughly 020° orientated Cenozoic Upper Rhine URG in west-central Europe forms part of the European Cenozoic Rift System (ECRIS). The URG can be subdivided into southern (Basel – Strasbourg), central (Strasbourg – Speyer) and northern (Speyer – Frankfurt) segments, each approximately 100 km long.
The Phase One of the Zero Carbon Lithium™ Project area is in the Upper Rhine Valley Brine Field (URVBF), a sub-surface geothermal-lithium brine reservoir. The focus of the Project in the URG is on aquifers associated with the Permo-carboniferous Rotliegend Group sandstone, the Triassic Buntsandstein Group sandstone, and the Middle Triassic Muschelkalk Formation, which is composed of carbonate sediments, collectively the ‘Permo-Triassic strata. The Permo-Triassic strata underly all Vulcan Property licences and are characterized as a laterally heterogeneous sandstone unit within a structurally complex rift basin. The Middle Triassic Muschelkalk succession, however, is only present from the Taro licence area towards the south in the URG. The lithium mineralisation at the URVBF is situated within confined, subsurface aquifers associated with the Permo-carboniferous Rotliegend Group, the Lower Triassic Buntsandstein Group, and the Middle Triassic Muschelkalk Group (collectively, the Permo-Triassic strata) sandstone aquifers and carbonates situated within the URVBF at depths of between 2,165 and 4,004 m below surface.
The Permo-Triassic strata are comprised predominantly of terrigenous sand facies, with minor shales, carbonates, and anhydrites, deposited in arid to semi-arid conditions in fluvial, sandflat, lacustrine and eolian sedimentary environments. The various facies exert controls on the porosity (1% to 27%) and permeability (<1 to >100 mD) of sandstone sub-units. Within the Permo-Triassic strata, porosity, permeability, and fluid flow rates are dependent on the fault, fracture and micro-fracture zones that are targeted by geothermal companies in the URVBF.
The Rotliegend Group within the URG formed during the late stage of the Variscan Orogeny with local extension already happening. The Variscan Orogeny was accompanied by volcanism that led to the deposition of intrusive deposits into the basement, which is underlying the URG. Those intrusive deposits are believed to form an essential part of the lithium system. The actual rifting of the URG occurred during Cenozoic times. Hence, the fault system is comparably young. The Lower Rotliegend is comprised of alluvial-fan/fan-delta to fluvial-dominated Carboniferous and Permian sedimentary rocks. The basin infill subsequently transitioned from fluvial dominated to alluvial and eolian depositional environments during Upper Buntsandstein times.
The Lower Triassic Buntsandstein Group is subdivided into the Lower, Middle and Upper Buntsandstein subgroups as defined by distinct progradational and retrogradational fluvial sedimentary cycles. The Buntsandstein Group aquifer domain is defined as a confined sandstone aquifer that occurs between the fine grained Upper Buntsandstein Group and the coarse-grained base of the Lower Buntsandstein. The Middle Triassic Muschelkalk represents the marine sedimentation that succeeds the fluvial deposition of the Buntsandstein. It consists of argillaceous dolomites and limestones as it represents a marine transgression. Towards the top of the Muschelkalk, evaporitic sediments dominate. The Upper Triassic Keuper is dominated by pelitic sediments and represents a marine regression which provides a top seal for the reservoirs of interest together with the pelitic dominated Tertiary overburden.
Brine aquifers within the Rotliegend Group and Buntsandstein Group are considered to have some degree of hydrogeological communication. This is particularly evident in zones with a high degree of faulting and fracturing in which fluid brine can flow throughout the Permo-Triassic strata and can also penetrate the underlying faulted, fractured and altered granitic basement and the overlying Muschelkalk zone. These fault/fracture zones generally contain hot brine and exhibit high fluid flow rates. Consequently, they are a prime target for geothermal development. Lithium mineralisation occurs in the brine that is occupying the Permo-Triassic aquifer pore space.
With respect to a deposit model, the lithium chemical signature of the brine is believed to be controlled by geothermal fluid-rock geochemical interactions. With increasing depth, total dissolved solids (TDS) increase in NaCl-dominated brine. Lithium enrichment associated with these deep brines is related to interaction with hot crystalline basement fluids and/or dissolution of micaceous materials at higher temperatures. Historical and Vulcan-conducted recent geochemical analysis of the aquifer brine from the Permo-Triassic strata shows the brine is enriched with lithium, which is very consistent both temporally and spatially within the reservoir. The geothermal wells represent potentially cost-effective access points to acquire deep, geothermally heated, lithium-enriched brine associated with the Permo-Triassic aquifers overlying the crystalline basement.
For the Phase One licences, the average lithium content from brine collected by Vulcan from six geothermal wells (including its 100%-owned Insheim geothermal plant) located throughout the URVBF and within or proximal to its licences was used as the representative grade for Resource Estimation. This grade was 181 mg/L Lithium (n=13 total metal analyses by ICP-OES). In addition, a detailed assessment of Permo-Triassic aquifer brine at the Insheim resource area production well yielded 181 mg/L Lithium (n=26 analyses). This grade was used as the regional lithium brine value for previous resource estimates and for the current update. These brine geochemical results demonstrate that the Permo-Triassic brine in the URG has a relatively homogeneous lithium chemical composition in the vicinity of the Phase One licences, both temporally and spatially.
Dimensions
The geometry of the Permo-Triassic strata in the URV has a gentle northward dip at the southern end of the field (i.e., at the Ortenau licence area) which transitions to a south-east dip further northwards at the Taro licence area. The top and base surface elevations of the Buntsandstein Group under the URV licences are approximately from 2000 m (south) to 3800 m (north) subsea (m SS) with an average thickness range of 310 m in the north and 380 m in the south, up to 475 m thick locally. The top and base surface elevations of the Rotliegend Group under the URV licences south of the Taro licence are approximately from 2200 m SS to 3300 m SS with an average thickness range of 120 m to 310 m, across the URV.