Distribution of REE-bearing minerals in felsic magmatic rocks and paleosols from Gran Canaria, Spain: Intraplate oceanic islands as a new example of potential, non-conventional sources of rare-earth elements

Abstract

Gran Canaria is a hotspot-derived, intraplate, oceanic island, comprising a variety of alkaline felsic magmatic rocks (i.e. phonolites, trachytes, rhyolites and syenites). These rocks are enriched in rare-earth elements (REE) in relation to the mean concentration in the Earth's crust and they are subsequently mobilised and redistributed in the soil profile. From a set of 57 samples of felsic rocks and 12 samples from three paleosol profiles, we assess the concentration and mobility of REE. In the saprolite that developed over the rhyolites, we identified REE-bearing minerals such as primary monazite-(Ce), as well as secondary phases associated with the edaphic weathering, such as rhabdophane-(Ce) and LREE oxides. The averaged concentration of REE in the alkaline bedrock varies from trachytes (449 mg kg−1), to rhyolites (588 mg kg−1) and to phonolites (1036 mg kg−1). REE are slightly enriched in saprolites developed on trachyte (498 mg kg−1), rhyolite (601 mg kg−1) and phonolite (1171 mg kg−1) bedrocks. However, B-horizons of paleosols from trachytes and phonolites showed REE depletion (436 and 994 mg kg−1, respectively), whereas a marked enrichment was found in soils developed on rhyolites (1584 mg kg−1). According to our results, REE resources on Gran Canaria are significant, especially in Miocene alkaline felsic magmatic rocks (declining stage) and their associated paleosols. We estimate a total material volume of approximately 1000 km3 with REE concentrations of 672 ± 296 mg kg−1, yttrium contents of 57 ± 30 mg kg−1, and light and heavy REE ratios (LREE/HREE) of 17 ± 6. This mineralisation can be considered as bulk tonnage and low-grade ore REE deposits but it remains necessary to develop detailed mineral exploration on selected insular zones in the future, without undermining environmental and socioeconomic interests.