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Allanite-bearing black coastal sands of Kavala (N. Greece) were studied using a combination of single-crystal XRD, EMPA, μ-XRF, bulk ICP-MS, LA-ICP-MS, μ-XANES and γ-ray spectrometry. The sands are rich in REE (ΣREE + Y: 4010 to 10,810 mg/kg), Th (236–1205 mg/kg) and other critical metals such as Nb, Ta and Co. The main REE- and Th-hosts are allanite and titanite. The allanite unit cell parameters were calculated whereas its formula was found to be (REE 0.470 Ca 1.499 Th 0.031) (Ti 0.031 Fe+3 1.089 Al 1.880) (Si 2.906 Al 0.094 O 12) (OH). The μ-XANES spectra showed that LREE are present in trivalent oxidation state. Analyses of the non-magnetic sand fractions showed higher LREE (12,470 mg/kg) due to accumulation of allanite. The materials showed elevated radioactivity ranging from 885 ± 13 to 3467 ± 20 Bq/kg. The obtained results provide new insights on the provenance of the sands, the abundance of REE, Th, and other immobile elements, and offer new clues for potential exploration and exploitation. [ABSTRACT FROM AUTHOR]

The meta-ultramafic bodies of Gomati and Nea Roda, e. Chalkidiki, are part of the Serbomacedonian Massif, which is considered an extension of the Rhodope Massif which also includes meta-ultramafic and mafic occurrences. Podiform chromitites in the East Chalkidiki have undergone metamorphism and their host rocks are pervasively serpentinized. Chromitites are widely used as petrogenetic tools to unravel the settings under which they have formed and which have subsequently affected the mantle section. Likewise, various alternating/metasomatic processes can also be studied focusing on chromitites while focusing on their mineralogy and geochemistry. Under this scope, it is important to specify primary and secondary mineralogical and chemical features in order to answer to the following questions: a) under what settings have the East Chalkidiki chromitites were generated and b) under what settings and conditions have they been modified. The answer in those questions will offer insights in the general geotectonic evolution of the Serbomacedonian Massif. Electron microprobe (Eugen F. Stumpfl Laboratory of the Leoben University, Austria, using a Superpobe Jeol JXA 8200) and LA-ICP-MS analyses (system at the NAWI Graz Central Lab for Water, Minerals and Rocks) were conducted mainly upon spinel-group minerals. Focus was given in secondary mineral phases such as chlorite, diopside, garnet, platinum-group minerals (PGM) to further assist the interpretations. The mantle section of e. Chalkidiki hosts both Al-rich and Cr-rich chromitites. Using the normalized trace element patterns of the spinel-group minerals, the primary core was distinguished from the metamorphic rims. The main chemical differences include increase in Ti, Zn, Co, Mn during metamorphism, and those abundances point to a peak of amphibolite facies. In the pristine cores, the included PGM are primary, whereas desulphurized and zoned PGM are included in modified spinel. Garnet and chlorite were crystallized during greenschist facies, as their chemistry implies. Secondary diopside hosted in chromitite and diopsidite demonstrate subduction features (LA-ICP-MS) and this is in agreement with the serpentinite geochemistry (ICP-MS). The platinum-group element (PGE) abundances (Instrumental Neutron Activation Analysis) of chromitites have been preserved and increase in Pd/Ir ratios in some cases are attributed to fractionation, with the main mechanism for chromitite formation being the partial melting that the mantle source has underwent. The spinel grains of the Al-rich chromitites demonstrate flat normalized trace element profiles typical of back-arc chromitites, whereas chromite from the Cr-rich chromitites demonstrate patterns related with supra subduction zone (SSZ) settings. Similar environments have been described for the chromitite occurrences of the Rhodope Massif (Colás et al., 2014; González-Jiménez et al., 2015). After their formation the ultramafic section along with the chromitites were introduced into a subduction zoned reaching amphibolite facies that were later-on overprinted by greenschist facies. These events have also been recorded upon the meta-pellitic rocks of the Serbomacedonian Massif (Kydonakis et al., 2014; Siron et al., 2018). These results point to common settings being responsible for the formation and modification of both the chromitites of Serbomacedonian and the Rhodope Massifs. [ABSTRACT FROM AUTHOR]

Iron formations (IF) represent an iron-rich rock type that typifies many Archaean and Proterozoic supracrustal successions and are chemical archives of Precambrian seawater chemistry and post-depositional iron cycling. Given that IF accumulated on the seafloor for over two billion years of Earth's early history, changes in their chemical, mineralogical, and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving Earth. Perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere. Linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes, the supply of nutrients to the oceans, and, ultimately, diversification of the biosphere and complex life forms. Many of the key recent innovations in understanding IF genesis are linked to geobiology, since biologically assisted Fe(II) oxidation, either directly through photoferrotrophy, or indirectly through oxygenic photosynthesis, provides a process for IF deposition from mineral precursors. The abundance and isotope composition of Fe(II)-bearing minerals in IF additionally suggests microbial Fe(III) reduction, a metabolism that is deeply rooted in the Archaea and Bacteria. Linkages among geobiology, hydrothermal systems, and deposition of IF have been traditionally overlooked, but now form a coherent model for this unique rock type. This paper reviews the defining features of IF and their distribution through the Neoarchaean and Palaeoproterozoic. This paper is an update of previous reviews by Bekker et al. (2010, 2014) that will improve the quantitative framework we use to interpret IF deposition. In this work, we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans. [ABSTRACT FROM AUTHOR]