Ioneer Ltd is the 100% owner of the Rhyolite Ridge Lithium Boron Project.
Sibanye-Stillwater announced in September 2021 that it had reached agreement with ioneer to establish a joint venture company with respect to Rhyolite Ridge. On February 26, 2025, Sibanye-Stillwater has made a decision not to proceed with the Rhyolite Ridge Lithium-Boron Project under the joint venture agreement with ioneer Ltd.
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Summary:
The HiB-Li and LoB-Li mineralization at Rhyolite Ridge occurs in two separate Miocene sedimentary basins; the North Basin and the South Basin, located within the Silver Peak Range in the Basin and Range terrain of Nevada, USA.
The South Basin stratigraphy comprises lacustrine sedimentary rocks of the Cave Spring Formation overlaying volcanic flows and volcaniclastic rocks of the Rhyolite Ridge Volcanic unit. The Rhyolite Ridge Volcanic unit is dated at approximately 6 megaannum (Ma) and comprises rhyolite tuffs, tuff breccias and flows. The Rhyolite Ridge Volcanic rocks are underlain by sedimentary rocks of the Silver Peak Formation.
The Cave Spring Formation comprises a series of 11 sedimentary units deposited in a lacustrine environment, as shown in the following table. Within the study area the Cave Spring Formation can reach total thickness in excess of 400 m. Age dating of overlying units outside of the area and dates for the underlying Rhyolite Ridge Volcanic unit bracket deposition of the Cave Spring Formation between 4-6 Ma; this relatively young geological age indicates limited time for deep burial and compaction of the units. The Cave Spring Formation units are generally laterally continuous over several miles across the extent of the South Basin; however, thickness of the units can vary due to both primary depositional and secondary structural features. The sedimentary sequence generally fines upwards, from coarse clastic units at the base of the formation, upwards through siltstones, marls and carbonate units towards the top of the sequence.
The key mineralized units are in the Cave Spring Formation and are, from top to bottom, the M5 (high-grade Li, low- to moderate- grade B bearing carbonate-clay rich marl), the B5 (high-grade B, moderate-grade Li marl), the S5 (low- to high Li, very low B) and the L6 (broad zone of laterally discontinuous low- to high- grade Li and B mineralized horizons within a larger low-grade to barren sequence of siltstone-claystone). The sequence is marked by a series of four thin (generally on the scale of several meters or less) coarse gritstone layers (G4 through G7); these units are interpreted to be pyroclastic deposits that blanketed the area. The lateral continuity across the South Basin along with the distinctive visual appearance of the gritstone layers relative to the less distinguishable sequence of siltstone-claystone-marl that comprise the bulk of the Cave Spring Formation make the four grit stone units good marker horizons within the stratigraphic sequence.
The Cave Springs Formation is unconformably overlain by a unit of poorly sorted alluvium, ranging from 0 to 40 m (mean of 20 m) within the Study Area. The alluvium is unconsolidated and comprises sand through cobble sized clasts (with isolated occurrences of large boulder sized clasts) of the Rhyolite Ridge Volcanic Rocks and other nearby volcanic units.
Structurally, the South Basin is bounded along its western and eastern margins by regional scale high angle faults of unknown displacement, while localized steeply dipping normal, reverse and strike-slip faults transect the Cave Spring formation throughout the the basin. Displacement on these faults is generally poorly known but most appear to be on the order of tens of meters of displacement although several located along the edge of the basin may have displacements greater than 30 m. Major fault structures within the basin tend to have a series of minor faults associated with them. These tend to have smaller offset than the parent fault structure. Along the western side, South Basin is folded into a broad, open syncline with the sub-horizontal fold axis oriented approximately north-south. The syncline is asymmetric, moderate to locally steep dips along the western limb. The stratigraphy is further folded, including a significant southeas HiB-Li and LoB-Li mineralization is interpreted to have been emplaced by hydrothermal/epithermal fluids travelling up the basin bounding faults; based on HiB-Li and LoB-Li grade distribution and continuity it is believed the primary fluid pathway was along the western bounding fault. Differential mineralogical and permeability characteristics of the various units within the Cave Spring Formation resulted in the preferential emplacement of HiB-Li bearing minerals in the B5 and L6 units and LoB-Li bearing minerals in the M5, S5 and L6 units. HiB-Li mineralization occurs in isolated locations in some of the other units in the sequence, but with nowhere near the grade and continuity observed in the aforementioned units.
Dimensions
The Mineral Resource evaluation presented in this area of approximately 458 Ha within the South Basin of Rhyolite Ridge. The Mineral Resource plan dimensions, defined by the spatial extent of the B5 unit Inferred classification limits, are approximately 3,650 m North-South by 1,400 m East-West. The upper and lower limits of the Mineral Resource span from surface, where the mineralized units outcrop locally, through to a maximum depth of 420 m below surface for the base of the lower mineralized zone (L6 unit).
Variability of the Mineral Resource is associated primarily with the petrophysical and geochemical properties of the individual geological units in the Cave Spring Formation. These properties played a key role in determining units that were favourable for hosting HiB-Li and LoB-Li mineralization versus those thatwere not.
South Basin and The Shelf Zone
Rhyolite Ridge is a sediment-hosted lithium and boron deposit located in the Silver Peak Range of southwestern Nevada. Sedimentary layers containing lithium and boron were deposited into a lake bed approximately six million years ago.
Today, the sedimentary rocks are up to 300m in thickness, can be subdivided into 11 separate units and are almost entirely concealed beneath a 20 m thick layer of unconsolidated alluvium (gravel).
At least four sub-basins have been identified within the South Basin. The sub-basins are separated by faults, flexures and fold axis that have either uplifted or down-dropped the sedimentary layers that host lithium and boron mineralisation. An uplifted block in the southeast portion of the South Basin that is herein referred to as “The Shelf Zone” was the primary focus of the most recent drilling.
The Shelf Zone represents a highly prospective area due to the shallow depth of the mineralized units, the sediments sub-crop beneath unconsolidated gravel, lithium grades are consistently higher than the Resource average, sediments are relatively flat lying and the entire area lies outside of Tiehm’s buckwheat critical habitat. The Shelf Zone measures approximately 1500 x 750 m and until recently, was largely undrilled.
Within the area of The Shelf Zone, mineralised units lie within 30 metres of the surface and are covered by unconsolidated gravel. The mineralised units dip to the east at shallow angles which is likely to prove favourable for geotechnical stability of pit walls. Previously, these units were thought to dip to the west – toward the centre of the basin. The uplifted block that separates The Shelf from the deeper mineralised units to the west is well defined by gravity and magnetic data and has been confirmed with multiple drill hole intersections.