Understanding the Science Behind the Splinter Rock Clay-Hosted REE System
- Noel Ong
- 1 day ago
- 10 min read
Based on the multidisciplinary study undertaken by CSIRO and OD6 Metals
A Landscape Built for Rare Earth Element Discovery
The Splinter Rock clay-hosted REE prospect, located northeast of Esperance within the east Albany–Fraser Orogen in Western Australia, has emerged as one of the most scientifically compelling regolith-hosted REE systems in the country (Figure 1). A multidisciplinary study led by CSIRO has produced the first integrated geological, geophysical, mineralogical, and geochemical characterisation of this emerging clay-hosted REE district.
The research addresses the fundamental question:
1. How do REEs concentrate within the deep weathering profiles of Southern WA
2. What does this mean for future economic extraction?
Figure 1: Simplified geology map of the east Albany–Fraser Orogen showing the Splinter Rock project area (source: Ore Geology Reviews Volume 186, November 2025, 106929)
What stands out is the scientific rigour placed on decoding the regolith architecture, clay mineral distribution, palaeo-valley development, and REE-hosting mineral phases—all necessary precursors to understanding true clay-hosted REE potential.
The Business of the Prospect: Why Splinter Rock Matters In the Clay-Hosted REE System
Splinter Rock sits within a highly weathered terrain shaped by long-term tectonic uplift, prolonged weathering, and episodic marine transgressions. The geological context presents a unique combination of:
REE-fertile Booanya Suite source granites
Deep, clay-rich palaeo-valleys acting as natural traps for weathered materials
Well-developed saprolite horizons enriched in kaolinite and secondary REE minerals
A regolith profile thick enough to host meaningful accumulations of TREO
This regolith-dominated system is fundamentally different from hard-rock REE systems. Its behaviour, mineralogy, and extraction implications require a precise understanding of the weathering sequence, mineral hosts, and mobility of REEs over geological time.
Figure 2 demonstrates the basement geological framework, showing Splinter Rock aligned with Booanya Suite intrusions and major shear zones of the east Albany–Fraser Orogen—a structural and lithological setting well suited for developing REE-enriched weathering profiles.
Figure 2: Basement geology, structural setting, and project location (source: Ore Geology Reviews Volume 186, November 2025, 106929)
Highlights: What the Multidisciplinary Study Reveals (Key Findings)
1. A Well-Defined Four-Layer Regolith Model
The study identifies a consistent, mappable vertical sequence across the project:
Layer 1: Upper transported cover — sands, clay, calcrete
Layer 2: High-conductivity transported clays, including the black clay unit, where present
Layer 3: Saprolite — the primary REE-rich zone
Layer 4: Saprock — less weathered basement transitioning to fresh granite
This model comes directly from integrating AEM conductivity layering, downhole geochemistry, and hyperspectral mineralogy, and is clearly illustrated in the schematic regolith stratigraphy (Figure 3).
Figure 3: Regolith Layers and Landforms at Splinter Rock (source: Ore Geology Reviews Volume 186, November 2025, 106929)
2. AEM Geophysics Maps the Palaeo-Valleys Controlling REE Distribution
Electromagnetic (AEM) data clearly delineate deep palaeo-valleys containing thick conductive clay sequences—key hosts of REE-enriched saprolite:
The thickest clay packages occur to the south-east.
The conductivity layering correlates strongly with regolith stratigraphy.
Layer 3 (saprolite) maps precisely to where TREOY grades peak (Figure 4).
Figure 4: Thickness of the saprolite (Layer 3) interpreted from AEM (source: Ore Geology Reviews Volume 186, November 2025, 106929)
The highest TREOY concentrations consistently occur within the saprolite, just above the saprock. To see how this looks in a section, Figure 5 takes a slice through the Centre prospect. The AEM conductivity profile maps the layered regolith architecture from transported cover down into saprolite and saprock, with the strongest REE grades sitting neatly in the saprolite horizon just above the basement.
Figure 5: AEM conductivity cross-section across the Centre prospect, showing layered regolith and TREOY distribution (source: Ore Geology Reviews Volume 186, November 2025, 106929)
3. Mineralogical Work Confirms REE-Hosting Phases, Not Ion-Adsorbed Clays
Contrary to Chinese-style ion-adsorption deposits, Splinter Rock (Figure 6) shows:
REE-phosphate minerals (rhabdophane/monazite) increasing with depth
Al-REE phosphates (e.g., Ce-florencite) in upper saprolite
Minor REE-fluorocarbonates (e.g., parisite/synchysite)
No strong evidence of significant ion-adsorbed REEs
This has major implications for leaching behaviour, processing routes, and economic potential.
Figure 6: SEM images of REE phosphates and fluorocarbonates (source: Ore Geology Reviews Volume 186, November 2025, 106929)
4. Geochemistry Confirms Saprolite as the True REE Host
Key chemical signatures include:
TREOY up to 6600 ppm directly above saprock
Strong correlation between kaolinite crystallinity and REE enrichment
Potassium spikes marking the saprolite–saprock boundary
Black clay layers possessing very high TOC and S, indicating a transported, estuarine influence (Figure 7)
Together, these create a strong decision-making framework for differentiating economic saprolite vs. non-economic transported cover.
Figures 7: Drill hole SRAC0220 intersected the black clay layer at the Centre prospect (source: Ore Geology Reviews Volume 186, November 2025, 106929)
In areas where the black clay horizon is missing (Figure 8), everything below the sandy cover transitions straight into a clay-dominated profile, with colours shifting from orange–red through to darker grey tones.
Figures 8: Drill hole SRAC0226 in the Centre prospect without the black clay layer (source: Ore Geology Reviews Volume 186, November 2025, 106929)
5. A Complete Regolith Evolution Model for Exploration Targeting
The study reconstructs the geological weathering and palaeo-environmental history (Mesoproterozoic → present), showing:
Granite emplacement
Deep weathering in Mesozoic humid conditions
Eocene valley incision
Marine transgression and organic-rich sedimentation
Miocene aridification and saline groundwater development
In simple terms, the work shows how REEs have moved through the landscape and ended up concentrated in the saprolite. The block model (Figure 9) and AEM data back this up by mapping out the old valleys, where the cover gets thicker in the lowlands, and the geology lines up with what the drill holes are actually hitting in the ground.
Figure 9: Block diagram of erosional vs. depositional regimes (source: Ore Geology Reviews Volume 186, November 2025, 106929)
Samso Take: Interpreting the Science Behind Splinter Rock
This work gives us much more confidence that the Splinter Rock REE story is not guesswork but grounded in solid science. The system is now described in a way that explorers and investors can actually hang their hat on. The study outlines a project where:
• The source rocks are clearly defined
• The key mineral hosts are identified
• The regolith processes are mapped out
• The palaeo-valleys hang together structurally
• The saprolite is confirmed as the main enrichment horizon
For an emerging clay-hosted REE province in Western Australia, this level of detail is not common. It puts OD6 and its scientific partners a step ahead of the usual early-stage regolith REE stories.
About The Splinter Rock clay-hosted REE Prospect (Western Australia)
The Splinter Rock prospect sits in a deeply weathered granitic terrain where the regolith has been stripped, reworked, and infilled over a long period of time. The profile steps down from thin or absent cover on the higher granitic areas into thicker clay-dominated sequences developed along palaeo-valleys.
Figure 3 illustrates the schematic regolith stratigraphy along a SE–NW transect at Splinter Rock, showing how the transported cover, saprolite, and saprock stack up across the granitic terrain and where the key REE-hosting horizons sit in that profile.
The saprolite consistently delivers the highest TREOY grades, making it the primary exploration and processing focus. Figure 10 helps put this into a map view, showing how the different soil types and regolith regimes are distributed across the project area and where the key prospects sit along those trends. The simplified regolith-regime map highlights the transition from erosional highs into depositional lowlands, which is where the thicker clay packages — and the better-developed saprolite — are most likely to occur.
Figure 10: Soil classification and simplified regolith-regime map for the Splinter Rock project area (source: Ore Geology Reviews Volume 186, November 2025, 106929)
Near-Term Milestones to Watch
Based on this work, the next phase is less about “finding” Splinter Rock and more about proving how it can work in practice. The value now sits in tightening up the mineralogy, the leach response, and the scale of the system.
Mineralogical refinement
o Pin down the balance between monazite and rhabdophane in the REE phosphates
o Work out how much of the REE inventory sits in primary vs. secondary phases
Leachability testing
o Test how these REE-hosting minerals actually respond to leaching
o Use those results to see which processing routes are realistic and which are not
Regolith model expansion
o Push the AEM + geochemistry framework out across more of the tenement
o Check that the four-layer regolith model and saprolite focus hold up at scale
Palaeo-valley targeting
o Drill deeper into the SE palaeo-valley zones highlighted as thicker and more prospective
o Confirm continuity of saprolite-hosted TREOY along those valley trends
These steps will ultimately decide whether Splinter Rock can move from being a technically impressive clay-hosted REE system on paper to a project that is scalable and economically defensible in the real world.
How Samso Understands the Investment Memo for Splinter Rock
From where I sit, Splinter Rock looks less like a blue-sky story and more like a technically advanced regolith-hosted REE system that has already done a lot of the hard geological work. It is backed by credible, multi-disciplinary science rather than just a few good drill intervals and a slide deck.
The project benefits from:
Confirmed REE mineral hosts – we know where the REEs sit and in what minerals, not just that they are “in the clays”.
Strong geological controls – the link to Booanya Suite granites, palaeo-valleys, and saprolite development is now well defined.
Saprolite-based enrichment rather than guesswork in the cover – the highest TREOY sits in a clear, mappable horizon.
Clear exploration vectors – AEM, regolith mapping, and geochemistry all point to how and where to grow the system.
A depth of geological data that is well ahead of most REE juniors – particularly in the regolith-hosted space.
For me, the question at Splinter Rock is no longer “Is there a system here?” – the geology says there is. The real task now is to show that the REE mineral hosts can be leached and processed in a way that stacks up economically and environmentally.
Samso Concluding Comments
For me, one of the most important outcomes of this work at Splinter Rock is that it takes a lot of the guesswork out of the clay-hosted REE story. Many regolith REE narratives are built on limited data and broad assumptions, but here the integration of AEM, hyperspectral logging, SEM, XRD, and detailed geochemistry creates a coherent picture of how the system works from surface down to saprock. That moves Splinter Rock out of the realm of “interesting anomaly” and into the category of a system that is genuinely understood in three dimensions.
The confirmation that REEs are largely hosted in secondary phosphates and minor fluorocarbonates, rather than being dominantly ion-adsorbed on clays, is a critical distinction. Western Australia does not share the same climatic and weathering history as the classic Chinese ion-adsorption districts, and this study reinforces that point. The long-lived weathering history, changes in groundwater, and evolving redox conditions all appear to have pushed the system towards more stable mineral hosts over time. That is a different processing challenge, but at least the mineralogical reality is now on the table rather than assumed.
The palaeo-valley framework and regolith architecture emerging from the AEM and mapping work also give Splinter Rock a strong geomorphic and structural context. Instead of isolated high-grade intervals with uncertain continuity, the project is now framed within an interpreted network of incised, granite-sourced valleys infilled with transported and residual clays. That kind of model is what allows explorers to think about scale, repeatability, and where to place the next dollar of drilling with more confidence, rather than chasing spotty geochemical highs.
In the end, this study should be seen as a starting platform, not a final answer. Science has done its job in defining the architecture, the mineral hosts, and the key controls on REE distribution. The focus now has to shift towards metallurgical testwork, leachability studies, and an honest assessment of deleterious elements and processing complexity.
If the REE-bearing phases at Splinter Rock respond favourably to leaching and the project can demonstrate a technically and environmentally defensible flowsheet, then this body of work will have laid the groundwork for a credible development story. If not, it will still stand as a reference model for how to properly characterise a clay-hosted REE system in Western Australia—something the broader industry can learn from.
References:
Reid, N., Thorne, R. L., Iglesias-Martinez, M., Lampinen, H. M., Davis, A., Bamforth, T. G., Spaggiari, C., Fan, R., Webster, N. A., Pinchand, T., Cribb, B., & Hazelden, B. (2025). Multi-disciplinary characterisation of the splinter rock clay-hosted REE prospect, Western Australia. Ore Geology Reviews, 186, 106929
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