The Rhyolite Ridge Lithium-Boron Project covers two separate lithium-boron deposits (North Basin and South Basin) located 4km apart.
From the 1850s through the 1890s 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:
- 1980s - 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.
Rhyolite Ridge is a geologically unique lithium-boron deposit that occurs within lacustrine sedimentary rocks of the South Basin, peripheral to the Silver Peak Caldera. The South Basin within the Project boundaries measures 4 miles by 1 mile and covers an area of just under 2,000 acres.
The photo below shows the high-grade lithium-boron (Searlesite) mineralisation at the South Basin of Rhyolite Ridge:
Regional geology. The Rhyolite Ridge Project site is situated in the Silver Peak Range, part of the larger geo-physiographic Basin and Range Province of western Nevada. Horst and graben normal faulting is the dominant characteristic of the Basin and Range Province, which is believed to have occurred in conjunction with large-scale deformation due to lateral shear stress. This is evidenced in the disruption of large-scale topographic features throughout the area. The project area sits within the Walker Lane Fault System, a northwest trending belt of right-lateral strike slip faults.
The regional geology is characterized by relatively young Tertiary volcanic rocks thought to be extruded from the Silver Peak Caldera, which date to approximately 6.1 million to 4.8 million years old. The northern edge of the Silver Peak Caldera is exposed approximately 2 miles to the south of the South Basin area and is roughly 4 miles by 8 miles in size. The Tertiary rocks are characterized by a series of interlayered sedimentary and volcanic rocks, which were deposited throughout west-central Nevada. These rocks unconformably overlie folded and faulted metasedimentary basement rocks that range from the Precambrian through Paleozoic periods.
Local geology. Rhyolite Ridge is one of only two major lithium-boron deposits globally and the only known deposit associated with the boron mineral searlesite. This mineralization style is different to the brine and pegmatite deposits that are the source of nearly all the lithium produced today.
The lacustrine (lake) beds that host the mineralization lie within the Cave Spring Formation and overlie the 6-million-year-old Rhyolite Ridge Tuff and Argentite Canyon volcanic rocks. The lacustrine section that measures up to 1,500 feet thick is composed of 3 members, divided by marker beds of 'gritstone' comprised of airfall debris with abundant pumice lapilli. The middle member, which is bounded top and bottom by distinctive gritstones, is dominantly marl, composing nearly 200 feet of section, and bears anomalous lithium in its upper half. About 60 feet of this section contains high concentrations of boron - contained in the sodium borosilicate mineral, searlesite (up to 30,000 parts per million [ppm] boron) - as well as lithium in mixed illite-smectite layers (about 1,500 to 2,500 ppm lithium). This marl is composed of very fine grained, intimately mingled searlesite, smectite, illite, potassium feldspar, and carbonate. The searlesite zone is capped by about 40 feet of smectite-rich marl with relatively high lithium values (commonly 2,000 to 2,500 ppm). The grade and thickness of this middle member are laterally uniform and continuous over a distance of at least 2 miles north to south.
The stratigraphic column for the South Basin is shown below in Figure 1-14 and is summarized as follows:
- Alluvium: 70-foot overburden layer to be prestripped.
Cave Springs Formation: The Cave Spring Formation hosts the ore zones of interest for the project, designated as follows:
- M5 - 42-foot-thick lithium claystone layer to be removed and separately stockpiled for possible future processing.
- B5 - 60-foot-thick upper ore zone of high-grade lithium-boron ore making up most of the ore planned for extraction.
- L6 - 130-foot-thick lower ore zone of lower-grade lithium-boron ore zone.
- Rhyolite Ridge Volcanics: Underlying sedimentary and volcanic rocks of the Silver Peak Formation.
Project Stratigraphic Column (Mineralized Units Shaded Green): The exploration and drilling information supporting the mineral resource model stems from work performed by ioneer in 2016-2019. Approximately 112 drill holes totaling 80,000 feet have been drilled, testing approximately one third of the total area of the South Basin, and were integrated into the geological model. The resource remains open in three directions for potential quarry expansion. Future exploration drilling in the South Basin will initially target the extensions of high-grade ore to the south, where it is expected to be increasingly shallow with positive impact on the mine plan. The northern limits of the deposit could be mined subject to additional drilling and the success of environmental management plans.