Illegal mining spreads rare earths, uranium, and thorium in Amazon soils

Illegal mining in a remote corner of the Brazilian Amazon is leaving behind more than deforested riverbanks and mercury-tainted water. New research shows that garimpo operations extracting tin and monazite in the Taboca district of São Félix do Xingu, in the state of Pará, are concentrating rare earth elements, uranium and thorium in soils and mining residues—while simultaneously stripping the soil of the very properties that would keep those elements locked in place. The study, published in Environmental Geochemistry and Health, is the first to establish a source-rock-constrained geochemical baseline for soils in this part of the southeastern Amazon, and its findings carry a stark warning for the children and miners who live and work amid the tailings.

The team, led by Antonia Andreyna Pereira Rodrigues and José Tasso Felix Guimarães of the Instituto Tecnológico Vale, collected 130 surface samples from the region: 34 native soils developed on the granites of the Antônio Vicente batholith and 96 tailing residues left behind by illegal alluvial cassiterite and monazite mining. What makes this landscape so challenging—and so dangerous—is that the rocks beneath it are naturally loaded with the very elements that mining concentrates. The Antônio Vicente Granite belongs to the Paleoproterozoic Velho Guilherme Intrusive Suite, a family of evolved A-type granites crystallized roughly 1.88 to 1.86 billion years ago. These tin-fertile magmas carried accessory minerals that are, in effect, natural warehouses for strategic elements: monazite, a phosphate that hosts light rare earth elements and thorium; xenotime, which stores heavy rare earth elements and yttrium; and zircon, which takes up uranium, thorium, hafnium and the heavy rare earths.

Because these minerals resist weathering, they accumulate in residual and alluvial soils even without human intervention. Illegal gravity-based mining, which washes and sorts alluvial sediments to recover dense cassiterite, further concentrates the same heavy minerals—and with them the rare earths, thorium and uranium. The result is a geochemical assessment nightmare: without knowing what the natural background looks like, it is nearly impossible to tell mining-derived contamination from inherited geological enrichment.

To disentangle the two, the researchers deployed a battery of complementary techniques. They used chondrite-normalized rare earth element patterns to trace mineral provenance; mass-transfer coefficients referenced to a composite niobium–tantalum denominator to quantify element gains and losses between native soils and residues; EDTA extraction to estimate potentially mobile fractions; and redundancy analysis to link geochemistry to soil properties. Contamination was evaluated with a geoaccumulation index built on locally derived, median-based thresholds rather than generic international guidelines—a critical choice in a region where the natural baseline is itself extraordinary.

The mineral fingerprints in the native soils were unambiguous. Light rare earth elements varied coherently with thorium and phosphorus pentoxide, the signature of detrital monazite, while heavy rare earths and yttrium tracked hafnium and zircon, marking zircon and xenotime as their hosts. Europium anomalies were strongly negative throughout, recording the feldspar-fractionated evolution of the parental granites. Cerium anomalies above one pointed to oxidation of cerium from its trivalent to tetravalent state and its scavenging by iron and manganese oxides. In short, the soils faithfully preserve the mineral imprint of the granite beneath them.

The tailings tell a different story—one of selective enrichment paired with chemical degradation. Mass-transfer calculations show the residues gain titanium-rich phases such as ilmenite and rutile, retain phosphate-bearing monazite with attendant light rare earths and thorium, and lose most of their fine, reactive material. Aluminum silicate fines, kaolinite, gibbsite and feldspar are preferentially washed away. The consequences for soil function are dramatic. Organic matter collapses from a median of 2.15 percent in native soils to 0.54 percent in tailings. Effective cation exchange capacity falls from 3.23 to 1.29 centimoles of charge per cubic decimeter, and aluminum saturation—already a stressor in acidic tropical soils—jumps from 2.85 to 16.35 percent. The residues behave as coarse, sandy, low-retention substrates that cannot buffer acidity or hold nutrients, let alone bind trace metals.

That degradation matters directly for element mobility. EDTA extraction revealed that potentially extractable fractions of the heavy rare earths and yttrium climb above 24 percent in tailings—ytterbium at 24.6 percent and lutetium at 24.3 percent—with maximum values exceeding 70 percent for several elements. Uranium shows a particularly concerning shift: its EDTA-extractable fraction rises more than threefold, from 2.8 percent in native soils to 9.4 percent in residues. The researchers attribute this to oxidative dissolution of tetravalent uranium phases during alluvial reworking, followed by partial adsorption onto freshly precipitated iron oxyhydroxides under acidic conditions. Thorium, by contrast, remains largely locked in monazite that survived the mining process, showing the opposite trend. This contrast makes uranium, rather than thorium, the priority actinide for downstream monitoring—even though thorium pseudo-total concentrations are higher in absolute terms.

The contamination picture that emerges from the geoaccumulation index is severe. At the median, tailing residues fall into the moderately contaminated class for middle and heavy rare earth elements, yttrium and praseodymium, with index values between 1.19 and 2.02. At the most enriched sample sites, the index crosses into the extremely contaminated class—above 5—for lanthanum, praseodymium, samarium, europium and gadolinium. Individual tailing samples reach heavily contaminated levels for uranium and thorium as well. A robustness check using an independent anomaly-threshold method flagged essentially the same elements and samples, confirming the pattern does not depend on the choice of statistical reference.

The human health screening is the most sobering part of the study. Using standard US Environmental Protection Agency exposure frameworks, the team estimated average daily doses from soil ingestion, dust inhalation and dermal contact for two scenarios: an adult artisanal miner and a child resident. For adult miners, the cumulative hazard index for non-radiological exposure crosses the safety threshold of 1, ranging from 1.0 at the median to 3.7 at the 95th percentile. For child residents, the index reaches 18 at the median and a staggering 67 at the 95th percentile—even native soils yield a median hazard index of about 10 for children. Thorium and cerium dominate the hazard contributions in tailings, followed by neodymium, lanthanum and yttrium. The child scenario is the more sensitive of the two because children ingest more soil per unit of body weight and are exposed nearly year-round rather than seasonally.

The aquatic compartment shows pressure as well. By coupling EDTA-extractable concentrations to literature-based soil–water partition coefficients and comparing predicted environmental concentrations against predicted no-effect concentrations for freshwater, the researchers found ratios exceeding one under the worst-case tailing scenario for lanthanum, cerium, neodymium, thorium, yttrium, ytterbium and uranium. Native soils only marginally exceed the threshold for cerium and thorium, confirming that the residues, not the unaltered substrate, are the dominant potential source of aquatic hazard in the affected catchments. The authors caution that these are screening-level indicators, not regulatory determinations—partition coefficients carry roughly an order of magnitude of uncertainty in acidic tropical systems—but the direction of the signal is clear.

The broader significance of the work extends well beyond the Taboca district. Brazil’s environmental legislation contains no specific guidelines for rare earth elements, and in geologically enriched provinces like the Carajás Mineral Region, generic contamination thresholds would misread naturally extreme backgrounds as anomalies—or, conversely, dismiss genuine mining contamination as geological noise. Only the joint use of provenance normalization, mass-transfer analysis and selective extraction, the authors argue, can separate the mining-added fraction from the inherited geogenic signal. The framework they propose is transferable to other tropical tin–rare earth provinces where artisanal mining coexists with naturally heterogeneous geochemistry.

For Brazilian regulators, the practical recommendations are concrete. First, adopt lithology-aware baselines when setting reference values for soil, sediment and water in the Iriri-Xingu domain. Second, prioritize uranium, yttrium and ytterbium in monitoring programs, since these combine comparatively high operational mobility with predicted-to-no-effect concentration ratios above one; the other extractable heavy rare earths, with extractable fractions well above 20 percent, also warrant attention. Third, extend the exposure assessment beyond soils to surface waters, sediments and biota—including fish and cattle along Xingu tributaries—to close the soil–water–organism exposure pathway. It is the children living in mining-impacted areas, the authors emphasize, for whom that extension matters most.