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Magnetite deposits comprise a very small volume of a serpentinized peridotite body that constitutes the Eretria chrome mine of the East Othris ophiolite in central Greece. Magnetite deposits have been examined to aid determination of their mode of occurrence, mineralogy and origin. Research attention has been focused on an ore body that consists of a ~30 cm-thick pod of semi-massive to massive magnetite (~75%–85% modal) hosted in a serpentinite shear zone. The silicate matrix of magnetitite (~10%–15% modal) is composed of unstrained serpentine, chlorite and subordinate amounts of andradite and actinolite. Base metal sulfides (BMS; ~5%–10% modal) and phosphates are dispersed between the magnetite grains. Variably altered Cr-spinel crystals are poikilitically enclosed within magnetite. The investigated ore samples show great variations in the Cu (890–9530 ppm), Ni (190–3260 ppm), Co (120–2180 ppm) and Zn (20–1240 ppm) contents. The δ18ΟSMOW value for magnetite from a magnetitite sample is −2.2‰. The undeformed nature of the silicate minerals in magnetitite points toward crystal growth from a post-magmatic fluid. The low Ni, Mn, Mg and high Si contents in magnetite suggest that it was not derived from fluids produced concurrent to serpentinization. The presence of phosphates, BMS and hydrosilicates suggest that the ore forming fluids were acidic and halogen-bearing. Our preferred hypothesis for the formation of magnetitite is that mixing of upwelling, metal-rich brines along the dilational segments of the host shear zone caused saturation of the ensuing fluids with Fe, which eventually prompted magnetite precipitation. The T estimates deduced from mineralogy, with an uppermost limit of ~460 °C required, and cessation of mineralization at ~150–100 °C, are consistent with the ambient T present within a cooling oceanic lithospheric slab. In analogy with some modern serpentinite-hosted BMS deposits at rifted settings, the BMS-bearing magnetitite of Eretria may represent part of an ancient seafloor hydrothermal system. Our study provides important insights into the initiation and episodicity of hydrothermal activity in oceanic settings. © 2020 Elsevier B.V.

The lithospheric architecture of Africa consists of several Archean cratons and smaller cratonic fragments, stitched together and flanked by polycyclic fold belts. Here we investigate the structure and chemistry of spinels from lithospheric mantle xenoliths from distinct tectonic settings, i.e. from the the Saharan metacraton in Libya (Waw-En-Namus) which could show archaic chemical features, Cameroon (Barombi Koto and Nyos Lakes) where the Sub Continental Lithospheric Mantle was modified during the Pan-African event and fluxed by asthenospheric melts of the Tertiary Cameroon Volcanic Line and Morocco (Tafraoute, Bou-Ibalrhatene maars) in the Middle Atlas where different metasomatic events have been recorded. From a structural point of view it is to notice that the Libyan spinels can be divided into two groups having different oxygen positional parameter (u > 0.2632 and u < 0.2627, respectively), while those from Cameroon are in between those values as the Moroccan ones already studied by other authors. The intracrystalline closure temperature (Tc) of the here studied spinels is different among the different samples with one Libyan group (LB I) showing Tc in the range 490-640°C and the other 680-950°C (LB II). Cameroon and Morocco spinels show a Tc in the range 630-760°C. About 150 different spinels have been studied for their trace element content and it can be seen that many of them are related to Cr content, while Zn and Co are not and clearly distinguish the occurrences. Differences in the trace element chemistry, in the structural parameters and in the intracrystalline closure temperatures suggest that a different history should be considered for Cameroon, Morocco and LB I and LB II spinels. Even if it was not considered for this purpose, we tentatively used the Fe2+/Fe3+ vs. TiO2 diagram that discriminate between peridotitic and the so-called “magmatic” spinels, i.e. spinel crystallized from melts. LB I and LB II spinels plot in the peridotitic field while Cameroon and Morocco spinels fall in the magmatic one. Consequently, the xenoliths sampled from a probably juvenile SCLM at the edge of the most important lithospheric roots (i.e. Cameroon and Morocco) apparently have spinels possibly fractionated in situ from percolating melts and do not represent a real spinel-peridotite facies. On the contrary mantle xenoliths from Libya exhibit spinels with peridotitic features compatible with a slow ascent of a mantle diapir (plume).

The chrome ores of the abandoned Eretria mine of the East Othris ophiolite occur within a pervasively serpentinized and sheared harzburgite body. They consist of massive chromitites with mylonitic fabric in imbricate shaped pods. Modal analyses of these ores average at about 90–95% chromian spinel (Cr-spinel) and 5–10% secondary silicates. Chromian spinel compositions vary in Cr# [Cr/(Cr + Al) × 100] and Mg# [Mg/(Mg + Fe2+) × 100] from 44 to 62 and from 59 to 81, respectively. Trace element (Ti, Ni, V, Mn, Zn, Sc, Co and Ga) contents in Cr-spinel do not show significant variations from grain cores to grain boundaries. However, Cr-spinel compositions show depletions in Ti, Zn and Sc when compared to the composition of accessory Cr-spinel from typical mid-ocean ridge basalts (MORB). Mineral inclusions hosted in Cr-spinel comprise a range of (hydrous and anhydrous) silicate and base metal (BM) minerals occasionally intergrown with phosphate minerals and rare intermetallic compounds. A number of these inclusions have Cr-spinel rims with higher Cr# (63–68) than those of the enclosing Cr-spinel grains. The absence of dunite sheaths around chromitites is interpreted as an artifact of dunite structural obliteration during prolonged ductile shearing within harzburgite. The microtextural characteristics of a number of inclusions in Cr-spinel imply that they were initially fully molten. Furthermore, primary hydrosilicate (amphibole, phlogopite) inclusions in Cr-spinel indicate that chromitites crystallized from a water-bearing melt. Chromian spinel rims around silicate inclusions probably represent early crystals generated from a primitive magma produced by melting of a depleted mantle source. Geochemical calculations demonstrate that the parental melts of chromitites had intermediate affinity between MORB and arc-related magmas. Our preferred hypothesis for the genesis of the Eretria chromitites is that they were formed from a melt originated within the hydrated mantle wedge beneath a nascent forearc basin during subduction initiation. © 2018 Elsevier GmbH

A group of Bronze Age whetstones from Protohistoric hill forts, locally called Castellieri, of eastern Friuli Venezia Giulia (north eastern Italy) has been studied using different techniques, including non destructive methods such as X-ray computed micro-tomography and portable X-ray fluorescence, in order to characterize the raw material and define its origin. The obtained results suggest that small pebbles of reddish subarkose and quartz arenites collected from the gravel deposits of river Isonzo, perhaps deriving from Val Gardena Formation outcrops, were exploited for the production of the studied artefacts during the Bronze Age. These data complement our knowledge about the lithic raw materials exploitation strategies during the ancient phase of Castellieri culture, almost exclusively based on local rock types

Serpentinite is one of the most frequently raw materials used to produce polished stone artefacts in prehistory. Several conventional analytical techniques can be applied to identify the serpentine minerals but their application generally requires a powdered sample. This implies that the artefacts to be analysed must be damaged, grinded and the possibility to analyze a mixture of different serpentine polymorphs is high. The use of spatially resolved techniques is therefore a necessity to overcome this problem. Several thin sections of serpentinitic rocks and prehistoric axes have been analyzed by Synchrotron Fourier Transform InfraRed Micro-Spectroscopy while some spectra were acquired directly on specific points of polished stone artefacts and allowed recognizing the different polymorphs of serpentine minerals without causing any damage to the objects. The results comfort the infrared micro-spectroscopy technique as a performing tool for the characterization of archaeological lithic material.

The Sambron–Kamenica Zone is situated on the northern margin of the Levoc¡ske´ vrchy mountains and Sarisska` vrchovina Highland, where the Central Carpathian Paleogene joins the Pieniny Klippen Belt. Sandstone outcrops in this area. From Cretaceous to Late Oligocene in age, these sediments suggest transport directions from S and SE. The heavy mineral assemblages of this sandstone include Cr-spinel grains, mainly displaying types II and III alpine-peridotite affinities, and are representative of Ocean Island Basalt volcanism. A sample from Upper Eocene sediments at Vit’az shows a clear change in Cr-spinel composition, which turns out to have types I and II peridotite affinities, and to derive from arc and Middle Ocean Ridge Basalt volcanism, with sediment transport directions from SW and WSW. These data indicate major variations in the Upper Eocene tectonic setting, giving constraints to paleogeographic reconstruction of the Slovak Central Carpathians.

[Abstract] In 1999 two borings were performedin the area of Pocala Cave, one outside the cave and the other inside in the cave. In particular four samples of the first boring were studied.It seems that mineralogy is strictly connected with sedimentology where quartz and plagioclase are present. It could be a consequence of a periodical floods of flysch sediments during interglacial period