The Rhyolite Ridge Lithium-Boron Project covers two separate lithium-boron deposits (North Basin and South Basin) located 4km apart.
From the 1850’s through the 1890’s, boron was extracted from several small mines in the district. Small-scale historical mining for boron took place in the South Basin.
In 1966, Albemarle’s Silver Peak Mine began extracting lithium from brines in Clayton Valley. Located approximately 25km east of Rhyolite Ridge, Silver Peak is currently the only producing lithium mine in the USA.
Two campaigns of modern mineral exploration are known to have been undertaken within ioneer’s claims:
- 1980's - US Borax (subsidiary of Rio Tinto Group) explored primarily for boron over both the South and North Basins; and
- 2010 to 2011 - American Lithium Minerals Inc and Japan Oil, Gas and Metals National Corporation (“JOGMEC”) explored for lithium in the South Basin.
An option over Rhyolite Ridge was acquired by ioneer in June 2016 and the purchase of a 100% interest in the project was completed in July 2017.
In the South Basin, the lithium-boron mineralisation occurs in flat lying sedimentary rocks as two or more stacked layers. The sedimentary rocks are up to 300m thick and the mineralised layers within are 20-70m thick. Separate layers host lithium-boron (Searlesite) mineralisation and lithium-only (clay-rich) mineralisation. The lithium-boron (Searlesite) mineralisation has a low clay content making it significantly easier and lower cost to process.
The photo below shows the high-grade Searlesite lithium-boron mineralisation in outcrop at South Basin:
The lithium-boron mineralisation typically contains 1,500-2,000ppm lithium and greater than 1% boron. The lithium-only mineralisation typically contains over 2,000ppm lithium, less than 0.02% boron and occurs in clay-rich layers.
There are at least two separate layers of lithium-boron (Searlesite) mineralisation (upper zone and lower zone) ranging from 20m to 70m thick and separated by 30-50m of barren sediments. The upper zone outcrops and the lower zone is shallow along the western margin of South Basin. 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 low clay content of the lithium-boron (Searlesite) mineralisation makes it simpler to process than lithium-clay deposits.
The South Basin has not been significantly structurally disturbed since deposition.
Rhyolite Ridge is one of only two major lithium-boron deposits globally. This mineralisation style is different to the brine and pegmatite deposits that are the source of nearly all the lithium produced today.
This unique combination of the boron-bearing mineral searlesite and the lithium-bearing mineral sepiolite in a low-clay host rock enables simple acid leaching to recover > 80% of both metals.