Rhyolite Ridge project covers two separate lithium-boron deposits (North Basin and South Basin) located 4km apart. The mineralisation occurs in flat lying sedimentary rocks as two or more stacked layers or lenses. The sedimentary rocks are up to 300m thick and the mineralised layers within are 20-70m thick. The mineralised layers contain lithium only (clay-rich) and lithium-boron (clay-poor) mineralisation.
The lithium-only mineralisation typically contains over 2,000ppm lithium, less than 0.02% boron and occurs in clay-rich layers. The lithium-boron mineralisation typically contains 1,500-2,000ppm lithium and greater than 1% boron, is higher in silica, sodium and potassium and lower in calcium and magnesium and occurs in 20m to 70m thick layers containing abundant searlesite (20-40%) and low in clay. Searlesite is a sodium-boron-silicate mineral.
There are at least two separate layers of lithium-boron mineralisation (upper zone and lower zone) separated by 30-50m of barren sediments. The upper zone outcrops along the western margin of South Basin over a strike length of approximately 3km. Both types of mineralisation are very consistent laterally over at least several square kilometres.
The host rocks are dominated by the minerals searlesite (boron-bearing), sepiolite (lithium-bearing), K-feldspar, calcite and dolomite. Unlike most other sedimentary-type lithium deposits, the lithium-boron mineralisation at Rhyolite Ridge has low clay content.
Both basins have not been significantly structurally disturbed since deposition and the strata/mineralisation are very consistent laterally.
High-grade searlesite lithium-boron mineralisation in outcrop at South Basin
Rhyolite Ridge is one of the very few lithium-boron deposits globally. This mineralisation style is different to the brine and pegmatite deposits that are the source of nearly all the lithium mined today. The unique combination of the boron-bearing mineral searlesite and the lithium-bearing mineral sepiolite in a low-clay host rock has provided attractive processing routes with high recoveries of both metals.