Karst bauxites as a non-conventional source of Critical Raw Materials: insights from Mediterranean and Dominican Republic deposits

Karst bauxites are traditionally exploited as the principal source of aluminium, but recent studies have highlighted their growing importance as non-conventional resources for critical raw materials (CRMs), particularly rare earth elements and yttrium (REY), as well as gallium (Ga) and scandium (Sc). This contribution provides a comparative overview of the REY-hyper-enriched karst bauxites of the Sierra de Bahoruco, southwestern Dominican Republic, and Mediterranean karst bauxites, with emphasis on differences in geochemistry and mineralogy.

The REY-hyper-enriched bauxites of the Sierra de Bahoruco display highly variable REY contents, ranging from several hundred ppm to nearly 3 wt%, with median values two to three times higher than those of Mediterranean karst bauxites. In the Bahoruco samples with highest REY contents, the hosting mineralogy is dominated by discrete REY-bearing phosphates and/or carbonates, including monazite, xenotime, rhabdophane, churchite, and bastnäsite-group minerals. By contrast, in bauxites with moderate REY contents, Al-hydroxides, mostly gibbsite, represent the principal REY host, with REY occurring largely as adsorbed species. Mediterranean karst bauxites, mainly of Mesozoic age, typically exhibit only moderate to low REY contents, generally on the order of a few hundred ppm, and are hosted by Al-oxyhydroxides, predominantly boehmite.

Gallium and scandium are consistently present in both Mediterranean and Dominican Republic karst bauxites at levels of potential economic interest. Consequently, Ga and Sc are regarded as promising by-products of bauxite and alumina processing, regardless of geographic setting, whereas REY enrichment and mineralogical controls vary significantly between Mediterranean and Dominican karst bauxite systems.