The Lower Devonian plant-bearing Rhynie Chert, Aberdeenshire, UK, consists of sinters deposited by a hot spring system. Like many modern hot springs, the Rhynie geothermal system was lithium-rich, and its silica deposits are richer in lithium than other current or fossil sinters. Twenty samples of Rhynie sinter have a mean content of 255 ppm lithium. The high values imply exceptional lithium contents in the spring waters. Together with pegmatites and granites in the same region, the chert is related to a lithium-rich Late Caledonian magmatic suite, of which it is a surface expression. The measurements suggest that ancient hot spring deposits could provide valuable data during the exploration for lithium.