Highly saline lithium-rich hydrothermal fluids occur in the deep calcareous Muschelkalk aquifer of the northern Alpine foreland basin. We have combined geologic, hydraulic, hydrochemical, and stress field data of the Triassic Muschelkalk aquifer beneath sediments of the Molasse basin of SW-Germany for a synthesis to constrain the origin and development of these brines. In contrast to the regional southeast plunge of Mesozoic and Cenozoic strata, low gradient groundwater flow in the Upper Muschelkalk aquifer is to the north, induced by regional recharge from west, south, and east. North trending maximum horizontal stress axes might provide development of fracture permeability in the competent carbonates of the Upper Muschelkalk aquifer for northward flow. The highest lithium concentrations and total dissolved solids (TDS) can be found in the southeastern parts of the Muschelkalk aquifer, close to the Vindelician High, whereas during northward transport TDS and lithium concentrations are increasingly diluted. We argue that the highly saline lithium-rich fluids originate from fluid-rock interaction of meteoric water with Variscan crystalline basement rocks and entered the Muschelkalk aquifer by permeable faults and fractures. The marginal calcareous sand-rich facies of the Muschelkalk enables the inflow of brines from crystalline basement faults and fractures into the aquifer. We thus argue for an external origin of these brines into the aquifer. Potentials are considered as 100±25 t Li/yr per well.