Critical Elements in Supergene Phosphates: The Example of the Weathering Profile at the Gavà Neolithic Mines, Catalonia, Spain

Abstract

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 mineralisations 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 mobilised 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.

Colours of Gemmy Phosphates from the Gavà Neolithic Mines (Catalonia, Spain): Origin and Archaeological Significance

“…Supergene mineralization is only hosted in the Silurian series of the Paleozoic basement. These correspond to sulphide-rich black shales of Llandoverian age and interbedded with layers of a few centimeter-thick and nodules of fluorapatite and chert [20,[43][44][45][46]54]. These rocks are pale grayish in the surface outcrop and, below the Quaternary unconformity, they become progressively darker downwards, reaching the original black colour at a depth of 15 m. The variation in colour of the shales has been attributed to the oxidation of the contained organic matter during the interaction with oxidizing acid fluids.…”

“…In the latter, its characterization and the corresponding study of colours can provide significant data about the source area of archaeological remains and ornaments, among other reasons (e.g., development of exchange networks and construction of cultural identities). Moreover, changes in variscite colour were attributed to the grade of Fe oxidation [20] and could therefore provide information about the level of finding in a single deposit [21]. First works focused on the study of variscite colouration indicated that a typical greenish colour was probably produced by the occurrence of chromophores such as Cr 3+ [22].…”

“…It is estimated that several hectares in the Gavà area were affected by mining activity but only a few tens of mines have been totally or partially excavated among the more than 100 shafts or galleries identified in the area [42]. Mining structures vary in complexity and size from short (2-3 m depth), vertical, or sub-vertical shafts, which evolved to complex mining structures reaching a depth of up to 15 m, to chambers and horizontal tunnel-shaped galleries at different levels [20,[43][44][45][46].…”

Sandstone-Hosted Uranium Deposits as a Possible Source for Critical Elements: The Eureka Mine Case, Castell-Estaó, Catalonia

“…Transition metals (and Ba) are effectively transported as chloride complexes in diagenetic and hydrothermal fluids [78], whereas sulphur is commonly transported as sulphate (SO 4 2− ) complexes during the formation of sandstone-type uranium deposits [58] and many low-temperature hydrothermal deposits [79]. Sulphate complexes can also be effective in the transport of uranium and other critical elements [80,81]. The arrival of hydrothermal fluids to the coal-rich reducing domains triggered a massive precipitation of Cu-V-U-rich minerals, including silicates, sulphides, arsenides and selenides in Eureka.…”

Distribution of Uranium and Rare Elements in Radioactive Phosphate-Bearing Anomalies in Southeast Mongolia

“…The demand for uranium, rare earth and related rare elements (REE) has increased in recent times to meet new technological requirements and this has increased interest in exploration activity for these elements [6][7][8]. More than 1100 uranium and radioactive anomalies and~300 such zones were discovered during earlier Soviet exploration and more recent exploration undertaken by Western companies.…”