On the diversity of the PGE content in chromitites hosted in ophiolites and in porphyry-Cu systems: Controlling factors

Economou-Eliopoulos, Maria; Eliopoulos, Demetrios G.; Tsoupas, George

doi:10.1016/j.oregeorev.2017.04.030

Cited by 20 publications

(6 citation statements)

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Section: Geochemical Characteristicssupporting

confidence: 51%

“…The bulk rock analysis of carbon-bearing chromitites showed that the trace elements Ni, Co, Zn, and V, and platinum-group elements (Os, Ir, Ru, Rh, Pt, and Pd) ( Table 2) fall in the range of previously published data [41]. More specifically, the total PGE content of chromitites from the Othrys complex was lower than 100 ppb, as in the majority of the chromite deposits and occurrences associated with ophiolites [41]. Although the PGE content is generally low, there is a significant PGE enrichment in compatible (Os, Ir, Ru) or IPGE at Achladones in Skyros, Milia in Pindos, and Veria chromitites, while the Pd/Ir ratio, which is considered to reflect the fractionation degree of the parent magma [42] is relatively low in these chromitites.…”

Section: Geochemical Characteristicssupporting

confidence: 51%

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Occurrence of Graphite-Like Carbon in Podiform Chromitites of Greece and Its Genetic Significance

2019

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The role of post-magmatic processes in the composition of chromitites hosted in ophiolite complexes, the origin of super-reduced phases, and factors controlling the carbon recycling in a supra-subduction zone environment are still unclear. The present contribution compiles the first scanning electron microscope/energy-dispersive (SEM/EDS) data on graphite-like amorphous carbon, with geochemical and mineral chemistry data, from chromitites of the Skyros, Othrys, Pindos, and Veria ophiolites (Greece). The aim of this study was the delineation of potential relationships between the modified composition of chromite and the role of redox conditions, during the long-term evolution of chromitites in a supra-subduction zone environment. Chromitites are characterized by a strong brittle (cataclastic) texture and the presence of phases indicative of super-reducing phases, such as Fe–Ni–Cr-alloys, awaruite (Ni3Fe), and heazlewoodite (Ni3S2). Carbon-bearing assemblages are better revealed on Au-coated unpolished sections. Graphite occurs in association with hydrous silicates (chlorite, serpentine) and Fe2+-chromite, as inclusions in chromite, filling cracks within chromite, or as nodule-like graphite aggregates. X-ray spectra of graphite–silicate aggregates showed the presence of C, Si, Mg, Al, O in variable proportions, and occasionally K and Ca. The extremely low fO2 during serpentinization facilitated the occurrence of methane in microfractures of chromitites, the precipitation of super-reducing phases (metal alloys, awaruite, heazlewoodite), and graphite. In addition, although the origin of Fe–Cu–Ni-sulfides in ultramafic parts of ophiolite complexes is still unclear, in the case of the Othrys chromitites, potential reduction-induced sulfide and/or carbon saturation may drive formation of sulfide ores and graphite-bearing chromitites. The presented data on chromitites covering a wide range in platinum-group element (PGE) content, from less than 100 ppb in the Othrys to 25 ppm ΣPGE in the Veria ores, showed similarity in the abundance of graphite-like carbon. The lack of any relationship between graphite (and probably methane) and the PGE content may be related to the occurrence of the (Ru–Os–Ir) minerals in chromitites, which occur mostly as oxides/hydroxides, and to lesser amounts of laurite, with pure Ru instead activating the stable CO2 molecule and reducing it to methane (experimental data from literature).

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Section: Geochemical Characteristicssupporting

confidence: 51%

Section: Geochemical Characteristicssupporting

confidence: 51%

Occurrence of Graphite-Like Carbon in Podiform Chromitites of Greece and Its Genetic Significance

2019

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“…Terrestrial PGE nuggets are commonly found in ore bodies that when eroded, can become concentrated in riverine placer deposits, including those of the Jordan River floodplain. To compare Fe–Ir–Pt relationships among the TeH nuggets, we compiled data from nearby placer deposits in Greece 109 , Turkey 110 , 111 , and Iraq 112 , along with distant placers in Russia 113 – 115 , Canada 116 , and Alaska, USA 117 , 118 . The compilation of 109 Pt-dominant placer nuggets indicates that the average Fe concentration is 8.2 wt.%, Ir = 2.9 wt.%, and Pt = 80.3 wt.%.…”

Section: High-temperature Melted Mineralsmentioning

confidence: 99%

A Tunguska sized airburst destroyed Tall el-Hammam a Middle Bronze Age city in the Jordan Valley near the Dead Sea

Bunch

LeCompte

Adedeji

et al. 2021

Sci Rep

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We present evidence that in ~ 1650 BCE (~ 3600 years ago), a cosmic airburst destroyed Tall el-Hammam, a Middle-Bronze-Age city in the southern Jordan Valley northeast of the Dead Sea. The proposed airburst was larger than the 1908 explosion over Tunguska, Russia, where a ~ 50-m-wide bolide detonated with ~ 1000× more energy than the Hiroshima atomic bomb. A city-wide ~ 1.5-m-thick carbon-and-ash-rich destruction layer contains peak concentrations of shocked quartz (~ 5–10 GPa); melted pottery and mudbricks; diamond-like carbon; soot; Fe- and Si-rich spherules; CaCO3 spherules from melted plaster; and melted platinum, iridium, nickel, gold, silver, zircon, chromite, and quartz. Heating experiments indicate temperatures exceeded 2000 °C. Amid city-side devastation, the airburst demolished 12+ m of the 4-to-5-story palace complex and the massive 4-m-thick mudbrick rampart, while causing extreme disarticulation and skeletal fragmentation in nearby humans. An airburst-related influx of salt (~ 4 wt.%) produced hypersalinity, inhibited agriculture, and caused a ~ 300–600-year-long abandonment of ~ 120 regional settlements within a > 25-km radius. Tall el-Hammam may be the second oldest city/town destroyed by a cosmic airburst/impact, after Abu Hureyra, Syria, and possibly the earliest site with an oral tradition that was written down (Genesis). Tunguska-scale airbursts can devastate entire cities/regions and thus, pose a severe modern-day hazard.

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“…The ophiolite complexes form an important component of the Tethyan metallogenetic belt, extending from the Mirdita zone in Albania to the Vourinos, Pindos and Othrys complexes of the south zone. These ophiolite complexes host large chromite deposits and smaller chromite occurrences, characterized by high-Cr and high-Al, low-and high-IPGE and PPGE compositions, and by PGE-patterns with both negative and positive slopes [10,14,15,31]. Such a compositional diversity in chromitites may provide an opportunity for in detail studies aimed at a better understanding of the role of magmatic processes on the Cr isotope signatures of the respective chromites [21][22][23][24].…”

Section: Introductionmentioning

confidence: 99%

Factors Controlling the Chromium Isotope Compositions in Podiform Chromitites

Economou-Eliopoulos

Mitsis

2019

Minerals

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The application of Cr isotope compositions to the investigation of magmatic and post-magmatic effects on chromitites is unexplored. This study presents and compiles the first Cr stable isotope data (δ 53 Cr values) with major and trace element, contents from the Balkan Peninsula, aiming to provide an overview of the compositional variations of δ 53 Cr values in ophiolite-hosted chromitites and to delineate geochemical constraints controlling the composition of chromitites. The studied chromitites exhibit δ 53 Cr values ranging from −0.184% to +0.159% , falling in the range of so-called "igneous Earth" or "Earth's mantle inventory" with values −0.12 ± 0.11% to 0.079 ± 0.129% (2sd). A characteristic feature is the slightly positively fractionated δ 53 Cr values of all chromitite samples from Othrys (+0.043 ± 0.03% ), and the occurrence of a wide range of δ 53 Cr values spanning from positively, slightly negatively to the most negatively fractionated signatures (Pindos, δ 53 Cr = −0.147 to +0.009% ; Skyros, δ 53 Cr = −0.078 to +0.159% ). The observed negative trend between δ 53 Cr values and Cr/(Cr + Al) ratios may reflect a decrease in the δ 53 Cr values of chromitites with increasing partial melting degree. Alternatively, it may point to processes related to magmatic differentiation, as can be seen in our data from Mikrokleisoura (Vourinos).

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On the diversity of the PGE content in chromitites hosted in ophiolites and in porphyry-Cu systems: Controlling factors

Cited by 20 publications

References 97 publications

Occurrence of Graphite-Like Carbon in Podiform Chromitites of Greece and Its Genetic Significance

Occurrence of Graphite-Like Carbon in Podiform Chromitites of Greece and Its Genetic Significance

A Tunguska sized airburst destroyed Tall el-Hammam a Middle Bronze Age city in the Jordan Valley near the Dead Sea

Factors Controlling the Chromium Isotope Compositions in Podiform Chromitites

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