Textural features and chemical characteristics of chromites in ultramafic rocks,Chalkidiki Complex (Northeastern Greece)

Summary The lower ultramafic part of the Chalkidiki Ophiolite Complex consists of a successive alternation of chromitite, dunite, and harzburgite, which is repeated periodically in a cyclic manner. This cyclic pattern is consistent with an origin as magmatic cumulates. However, the tectonic fabric superimposed on the cumulate texture of this sequence suggest a solid-state deformation at considerable depth.In the present study, the textural and chemical features of chromite are discussed. The interrelationships between the compositions of chromite, its mode of occurrence as well as the associated silicates are emphasized. It is found that the investigated chromites have properties in common with both podiforn and stratiform chromites. From a geometrical and structural point of view, they are of podiform type. On the other hand, their appreciably high iron content, the strong interdependence between the composition of chromite and the coexisting silicates together with the cryptic layering exhibited by the two mineral groups contrast markedly with podiform chromites.An alternative hypothesis is proposed involving fractional crystallization of an ultrabasic melt introduced to the magma chamber as periodic inflows of essentially the same composition. An upwelling upper-mantle that is subjected to stepwise partial fusion may fulfill these conditions.

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Chemismus und Texturen von Chromiten in ultramafischen Gesteinen des Chalkidiki Komplexes, Nordost-Griechenland

Zusammenfassung Der untere, ultramafische Teil des Chalkidiki Komplexes besteht aus einer wechselnden Abfolge von Chromititen, Duniten und Harzburgiten, die sich zyklisch wiederholt. Diese zyklische Abfolge ist in guter Übereinstimmung mit einer Entstehung als magmatische Kumulate. Das tektonische Gefüge, das auf die Kumulat-Texturen überprägt wurde, weist jedoch auf Deformationen in festem Zustand in beträchtlicher Tiefe hin.Die vorliegende Untersuchung befaßt sich mit dem Chemismus und den Texturen der Chromite. Besonderes Interesse wendet sich dabei den Wechselbeziehungen zwischen den Zusammensetzungen, der Art des Vorkommens und den assoziierten Silikaten zu. Dabei zeigt sich, daß die untersuchten Chromite in vieler Hinsicht Ähnlichkeiten sowohl mit podiformen wie mit stratiformen Vorkommen erkennen lassen. Geometrisch und texturell gesehen gehören sie dem podiformen Typ an. Nicht in Einklang damit stehen jedoch die beträchtlichen Eisengehalte sowie die deutlichen Wechselbeziehungen zwischen der Zusammensetzung von Chromit und der koexistierenden Silikate, und schließlich auch der kryptische Lagenbau, den die zwel Mineralgruppen erkennen lassen. Ein genetisches Modell wird vorgeschlagen, das fraktionierte Kristallisation einer ultrabasischen Schmelze von im wesentlichen gleichmäßiger Zusammensetzung vorsieht, die in periodischen Abständen der Magmenkammer zugeführt wurde. Eine Aufwölbung des oberen Mantels, die schrittweiser, partieller Aufschmelzung ausgesetzt ist, könnte diesen Bedingungen entsprechen.

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With 11 Figures     

Diverse contributing sources to chromitite petrogenesis in the Shebenik Ophiolitic Complex, Albania: evidence from new PGE- and Os-isotope data

Summary Chromitites sampled from four different pseudostratigraphic levels of the Mesozoic Shebenik Ophiolite Complex, Albania, have low PGE totals <1 μg/g but show different types of PGE enrichment (Burgath et al., 2003) as well as differing mineral chemistry, PGM mineralogy and Os isotopic signatures. To circumvent analytical problems with low PGE abundances, representative samples were analyzed using HPA-digestion followed by isotope dilution ICP-MS. Osmium isotopes were determined by ICP-QMS and N-TIMS techniques. Podiform chromitites exposed in the mantle (Group I) and tabular chromitites exposed in the upper mantle (Group II) are Os-Ir-Ru-Rh enriched. In the upper mantle to mantle-crust transition zone, schlieren type chromitites (Group III) are enriched in Ru-Rh with low Os-Ir and low Pt-Pd. Within the mantle-crust transition zone disseminated chromitites in dunite are variably enriched in Ru-Rh-Pt, however, Os, Ir and Pd are low. IPGE rich chromites contain abundant small laurite inclusions whilst Rh and Pt are located in sulfarsenides marginally attached to transition zone chromites (see also Burgath et al., 2003). High Cr/Al ratios (>0.75) and low titanium contents of chromites throughout the sample suite are consistent with chromitite petrogenesis in a SSZ environment. Shebenik mantle chromitites with low ¹⁸⁷Re/¹⁸⁸Os ratios have an average, slightly suprachondritic initial osmium isotopic composition of 0.1285 ± 0.0022 (2s). Towards higher pseudostratigraphic levels, ¹⁸⁷Re/¹⁸⁸Os ratios increase and initial Os isotopies are very heterogeneous. Distinctly suprachondritic Os signatures require input of radiogenic source components, whereas subchondritic samples require assimilation of long term Re-depleted PGM.     

Unconventional PGE occurrences and PGE mineralization in the Great Dyke: metallogenic and economic aspects

Platinum group elements (PGE) are strategic materials because 96–99% of the world production is derived from just five mining districts and because they cannot be replaced as catalysts in many chemical processes. In order to lessen the strategic character of PGE, both conventional deposits and unconventional PGE mineralizations were investigated in an attempt to locate viable deposits which would diversify the supplier base. In the Great Dyke, conventional PGE mineralization occurs in the form of pristine sulfide ores mined underground and oxidic surface ores. New observations such as bimodal distributions of the PGE in the Main Sulfide Zone (MSZ), elevated Pt/Pd ratios in the oxidized MSZ compared to the sulfidic part and distinct differences between the platinum group mineral (PGM) assemblages of the MSZ and stream sediments of adjacent rivers emphasize the fact that even though the Great Dyke seems to be the second or third largest PGE occurrence in the world, the complicated PGE distributions and supergene redistributions should be kept in mind during planning and mine operation. Investigations of unconventional PGE occurrences in ophiolites, Alaskan-type intrusions, porphyry copper deposits and in the Kupferschiefer show that economically exploitable PGE concentrations can be expected in a broader variety of host rocks than considered favourable in the past. In the Albanian Mirdita ophiolite average contents of 860 ppb Pt and 60 ppb Pd were detected. Flotation concentrates of porphyry copper deposits, for example from Mamut, Malaysia, Santo Tomas, Philippines, Elacite, Bulgaria, and Ok Tedi, Papua New Guinea, contain between 827 and 1860 ppb Pd + Pt. In selected profiles of the Polish Kupferschiefer average contents of 255 ppb Pt, 94 ppb Pd, 2.4 ppm Au and 13.0 ppm Ag were analysed. The distribution of the PGE resources in the world and the annual production rates, however, underline the fact that the chances for a significant change in the supplier base are relatively low. The Bushveld Complex will remain the largest producer, followed by Noril'sk-Talnakh, Sudbury and Stillwater. If the operations in the Great Dyke reach their planned capacities, the Great Dyke will rank in third place among the PGE-producing deposits in the world. Received: 12 September 1998 / Accepted: 7 December 1999