Díaz-Acha, and 12 co-authors, reported in 2020 that The essential role of Critical Elements (CE) in 21st century economy has led to an increasing demand of these metals and promotes the exploration of non-conventional deposits such as weathering profiles. The present work is focused on the study of a weathering profile located at the Archaeological Park of the Gavà Neolithic Mines, Barcelona, Catalonia, Spain. In the Gavà deposit, acid and oxidising meteoric fluids generated intense weathering during the early Pleistocene, affecting series of Llandoverian black shales and associated syn-sedimentary phosphates. The circulation of these acid fluids at deeper levels of the profile generated supergene vein-like mineralization comprised of secondary phosphates (e.g., variscite, perhamite, crandallite, phosphosiderite) and sulphates (e.g., jarosite, alunite).
This supergene mineralisation is significantly enriched in certain CE (e.g., Ga, Sc, REE, In, Co and Sb) that were mobilized from host rock components and later hosted in the crystal lattice of supergene minerals. Weathering processes and corresponding supergene enrichment of CE at the Gavà deposit could be used as an example to determine exploration guidelines of CE in weathering profiles and associated supergene phosphates worldwide.
The authors concluded that:
1. In the Gavà area, significant weathering processes involving acid and oxidizing meteoric fluids occurred during the early Pleistocene affecting a series of Llandoverian black shales and associated syn-sedimentary phosphates.
2. Acid fluids would promote the alteration of phyllosilicates and disseminated framboidal pyrite in Llandoverian shales as well as associated beds of syn-sedimentary phosphates, producing a general enrichment of fluids in sulphate, Fe, Al and phosphate, respectively.
3. The deposits of the Gavà area include supergene mineralisation distributed among veins that are hosted in weathered and bleached Llandoverian shales. They are mainly comprised of supergene phosphates (e.g., variscite, perhamite, crandallite, phosphosiderite), sulphates (e.g., jarosite, alunite) and associated clay minerals (e.g., halloysite and smectite).
4. The supergene mineralisation is significantly enriched in certain CE. Such anomalous contents are related to the action of these acid and oxidised fluids associated with weathering.
5. Precipitation of supergene vein-like mineralisation is related to an increase of pH conditions in deeper parts of the profile. CE are hosted in the crystal lattice of supergene minerals.
6. Weathering processes and corresponding enrichment of CE in supergene mineralisation at the Gavà deposit can be used as an example to explore CE in primary phosphates affected by weathering worldwide.
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