The story begins some 310 million years ago, when the late- to post-tectonic Variscan granites of the Castelo Branco Massif crystallized deep in the crust. The massif, which outcrops over more than 390 square kilometers in the Central Iberian Zone, is composed of several concentric S-type granitic facies, ranging from fine-grained muscovite-biotite granite in the core to porphyritic monzonitic granites toward the middle. Crucially, geochemical studies have shown that these middle facies are enriched in rare earth elements relative to the core and border granites. Within these rocks, tiny accessory minerals—monazite, xenotime, apatite, and zircon—host the bulk of the rare earth elements, thorium, and uranium, while ilmenite, the most abundant opaque mineral, carries titanium. Over millions of years, erosion dismantled these granites, and rivers sorted and concentrated the dense, weathering-resistant heavy minerals into alluvial deposits, the same natural process that has formed economically valuable placer deposits around the world.

The research team, led by Rute Salgueiro, collected twelve alluvial samples from granitic terrains near the eastern edge of the massif, deliberately targeting areas around late northeast-southwest tectonic faults where earlier geochemical surveys had identified anomalous rare earth, thorium, and uranium concentrations in stream sediments. The samples were sieved to less than three millimeters, panned in the field, and then treated in the laboratory with heavy liquids of density 2.89 grams per cubic centimeter to isolate the heavy mineral fraction. Magnetic separation then split each concentrate into two fractions: a magnetic and paramagnetic fraction dominated by minerals such as ilmenite, tourmaline, monazite, and biotite, and a diamagnetic non-magnetic fraction containing andalusite, apatite, anatase, zircon, rutile, and other light-colored minerals.

What the researchers found under the binocular microscope was striking. The magnetic fraction accounted for roughly 85 percent of the total heavy mineral volume, and within it, ilmenite reigned supreme with an average abundance of just over 50 percent of the entire sample. Titanium minerals as a group—ilmenite, rutile, anatase, brookite, sphene, and leucoxene—made up an average of 54.28 percent of the sample volume. The rare earth element minerals, though accessory, summed to about 12 percent, with monazite the most abundant at 4.97 percent, followed by apatite at 4.14 percent, zircon at 2.58 percent, and xenotime at 0.53 percent. Monazite and xenotime appeared as fine, hyaline, mostly subhedral grains with rounded shapes, their light yellow and green colors betraying their identity, while zircon displayed a range of crystal habits from colorless hyaline prisms to semi-translucent orange-yellow grains studded with dark red inclusions.

The chemical analysis, performed with a portable X-ray fluorescence spectrometer equipped with a rhodium tube and silicon drift detector, confirmed and quantified what the mineralogy suggested. Iron reached up to 31.09 percent and titanium up to 28.88 percent in the concentrates, reflecting the dominance of ilmenite. The non-magnetic fraction, dominated by zircon, showed astonishing zirconium contents of up to 207,018 milligrams per kilogram. Most importantly for the critical raw materials debate, the magnetic fraction yielded light rare earth element sums—cerium, lanthanum, neodymium, and praseodymium combined—reaching 4,557 milligrams per kilogram, alongside thorium contents of up to 1,969 milligrams per kilogram and uranium up to 244 milligrams per kilogram. The non-magnetic fraction, containing apatite and zircon, paradoxically hosted even higher uranium contents, reaching 448 milligrams per kilogram.

These numbers are not merely high; they are anomalous in a geologically meaningful way. Compared with the average composition of the granitic source rocks, the alluvial heavy mineral concentrates show enrichments of an order of magnitude or more for most elements, a result of natural sorting that concentrates dense, resistant grains in river sediments, further enhanced by laboratory separation. Compared with the North American Shale Composite, a standard geochemical reference, the samples are enriched in titanium, vanadium, yttrium, niobium, antimony, lanthanum, cerium, tantalum, tungsten, lead, thorium, and uranium by factors of ten or more, with zirconium in the non-magnetic fraction reaching a staggering thousand-fold enrichment. The light rare earth contents even exceed those of the anomalous stream sediments identified in earlier surveys, confirming that detrital monazite is the principal carrier of the light rare earth signal in the region.

The geochemical fingerprint also tells a clear story of provenance. Parallel elemental profiles between the alluvial concentrates and the parent granites—for example, elevated chromium, vanadium, zirconium, rubidium, cobalt, cerium, lanthanum, and titanium in both—point to a direct genetic link between the sediments and the Castelo Branco granitic rocks. The mineral assemblage incorporates the main heavy minerals identified in petrographic studies of the massif, including ilmenite, tourmaline, monazite, zircon, apatite, andalusite, and sillimanite. Interestingly, some minerals found in the alluvial samples, such as xenotime, garnet, brookite, anatase, and cinnabar, had not previously been recorded in the granitic rocks, likely because their low abundance in bedrock escaped detection at the scale of conventional petrography. The researchers also noted that biotite, abundant in the source granites, was underrepresented in the concentrates, probably because its lower density allowed grains to float during heavy liquid separation and its larger grain size was partly rejected during sieving.

Beyond the headline minerals, the study documented a suite of elements with industrial applicability that could add future value to any exploitation. Niobium reached up to 1,339.5 milligrams per kilogram and tantalum up to 270.75 milligrams per kilogram, hosted in titanium oxide polymorphs, ilmenite, biotite, and possibly cassiterite. Tin contents in the non-magnetic fraction climbed to 5,122 milligrams per kilogram in one sample, hinting at cassiterite grains derived from the tin-tungsten mineralized quartz veins that cut the granite along late Variscan faults. Scarce scheelite, identified under ultraviolet light, accounted for detectable tungsten, while traces of cinnabar explained the mercury signal. These accessory phases mean that any future mining of the titanium and rare earth minerals could recover additional commodities as by-products, improving the economic case.

Perhaps the most compelling historical echo in the study is the region’s own mining past. Between 1980 and 1988, the Lardosa alluvial placer deposits on the Castelo Branco Massif were commercially exploited for cassiterite and ilmenite, a direct precedent that titanium-rich placers sourced from these granites can form and be mined. The new data indicate that the same natural concentration machinery that once produced economic tin and titanium placers is also enriching monazite and ilmenite across a potentially wider area. Worldwide, placer deposits have historically supplied rare earth elements through monazite and, more rarely, xenotime, typically recovered as coproducts of ilmenite extraction—a model that fits the Castelo Branco situation remarkably well, given ilmenite’s overwhelming abundance in these sediments.

The authors are careful to note that the potential for placer-type deposit formation depends on mineral grades, quality, and deposit volumes being sufficient for economic extraction, questions that require follow-up investigation. Nevertheless, the study demonstrates that two simple, rapid, and relatively inexpensive methodologies—semiquantitative binocular microscopy and portable X-ray fluorescence—can efficiently characterize heavy mineral assemblages and their geochemical fingerprints, offering a practical exploration tool for critical raw materials. As the European Union’s Raw Materials Act pushes for diversified and secure supply chains to meet its 2030 climate and digital targets, the unassuming river sands of Vale da Silveira stand as a reminder that the raw materials of the future may lie not in exotic remote deposits, but in the patient, million-year work of water and weathering acting on ordinary granite, quietly concentrating the elements of the green transition one grain at a time.

Subject of Research: Rare earth element and critical raw mineral characterization of alluvial heavy mineral deposits derived from the Castelo Branco granitic massif in central Portugal

Article Title: Insights into rare earth elements and other critical raw materials from Castelo Branco massif alluvial deposits (Portugal)

Article References: Salgueiro, R., Silva, T. P., Martín-Méndez, I., de Oliveira, D., Batista, M. J., & Inverno, C. (2026). Insights into rare earth elements and other critical raw materials from Castelo Branco massif alluvial deposits (Portugal). Frontiers in Geochemistry, 4, Article 1766699. https://doi.org/10.3389/fgeoc.2026.1766699

Image Credits: AI Generated

DOI: 10.3389/fgeoc.2026.1766699

Keywords: rare earth elements, critical raw materials, monazite, ilmenite, alluvial deposits, placer deposits, Castelo Branco Massif, Portugal, heavy minerals, portable X-ray fluorescence, granite geochemistry, mineral exploration