Exploration for Platinum-group elements (PGE) in various geotectonic settings of Greece

Maria Economou-Eliopoulos, George Tsoupas, and George Kiousis

The platinum-group elements (Os, Ir, Ru, Rh, Pd and Pt) or PGEs, which are the most valuable elements, are of strategic importance due to their growing use in advanced technologies and automobile catalyst converters. They traditionally are associated with mafic-ultramafic complexes, and have been described in a wide range of geotectonic settings. The majority of the world supply of PGE is produced from magmatic ores derived from basaltic magmas and is associated with primary magmatic. Major Pt and Pd economic mineralization is hosted in well-defined stratiform reefs of large layered intrusions, as is exemplified by the Bushveld Complex (South Africa), the Great Dyke (Zimbabwe) and the Stillwater Complex (USA).

The types of mafic-ultramafic complexes dominant in Greece, which belong to the Balkan-Carpathian system, are ophiolites associated with orogenetic zones. Although chromite is major collector of the PGs, their content in chromite deposits, such as the large deposits of Othrys and Vourinos, and chromite occurrences hosted in ophiolites of Pindos, Rhodope and Serbo-Macedonian massifs, Edessa-Veria-Vermio, Euboea and Skyros islands, is generally low (few hundreds of ppb). However, PGE-enrichment: (a) in all PGE, (b) only in Os, Ir and Ru or (c) in Pt and/or Pd, are a common feature of disseminated chromite and/or relatively small chromite occurrences, of both high-Cr and high-Al type, in the uppermost parts of the mantle and/or in the lowest crust sequence. Examples of PGE-enrichment include the ophiolites of Pindos, Skyros Island and Veria (Greece).

The platinum group minerals (PGM) may be precipitated directly from silicate melt (S-poor), immiscible sulfide liquids, and the magmatic volatile phases. PGM can be classified into two subgroups: the more Os-, Ir- and Ru-rich or IPGE (Ir-goup) and Pt, Pd-rich or PPGE (Pt-group) assemblages. The more Pt- and Pd-rich assemblages (Pd–Pt, Pd–Pt-alloys, Pt-arsenides, most likely sperrylite) occur interstitial to chromite grains. On the basis of field and experimental data small grains of PGM (average 25 μm) of the IPGE-goup, commonly laurite, as inclusion in unaltered chromite have been interpreted as an early magmatic phase formed by direct crystallization of a basaltic magma. The presence of members of the irarsite-hollingworthite solid-solution series and other Os-, Ir, Ru- and Rh-bearing PGM in PGE-enriched altered chromitites from some ophiolite complexes may indicate either in situ alteration or/and remobilization and re-deposition of PGE. A salient feature of the latter case is the presence of extremely large (over 1.3 mm) PGM grains and extremely abundant PGM small grains/fragment (over 100) dispersed along a highly fragment chromitite zone, in a distance over 3 mm. They occur within small chromite occurrences located along a shear zone of strongly brecciated chromite ore of Veria having high PGE (up to 25 ppm) content. Such fluid-driven multistage platinum-mineralization and subsolidus reactions are considered to be widespread, but the system is considered to be a closed one with respect to PGE. The relatively high IPGE-enrichment in chromitites seems to be related to post magmatic processes covering a long period of deformation episodes, starting from the asthenosheric mantle flow (plastic deformation). Thus, most targeting locations general in ophiolite complexes may be are (a) for the chromitite-IPGM associations exclusively small chromite occurrences along shearing zones of ophiolite complexes, postdating their initial/magmatic PGE deposition, and (b) for the PPGE the uppermost parts of the mantle and the lowest crust sequence.

Platinum and Pd contents in sea-floor massive sulfides are very limited. However, elevated contents, reaching values up to 1 wt % Pt in marcasite and chalcopyrite from massive sulfides on the East Pacific Rise, 1000 ppb Pt in disseminated pyrite and chalcopyrite from brecciated pipeform diabase, underlying the massive ore of the Pindos ophiolite complex (Greece) may indicate that Pt and Pd are quite soluble under a range of hydrothermal conditions. Recently, elevated Pd and Pt contents, reaching values over 5 ppm were determed in certain porphyry Cu-Au deposits (average ≥ 0.4 ppm Au). British Columbia, Colorado and Late Cretaceous to Miocene porphyry Cu deposits, extending from Romania, through Serbia and Bulgaria to Greece are the most important porphyry intrusions of that type, all formed in a supra-subduction zone geotectonic environment.

The Pd-telluride, merenskyite occurs mostly as inclusions or/and at margins of chalcopyrite and bornite or forms intergrowths with Pd-Pt-Bi- and Ag-tellurides. Although the potential for PGE mineralization associated with such large Cu and Au-Cu porphyry deposits is still unknown, the average (Pd+Pt) values (over 5 ppm) are considered to be encouraging for Pd and Pt as by-products, with Au being by- or co-product, and porphyry deposits a good target for Pd & Pt exploration. Porphyry Cu-Au-Pd±Pt deposits show a similarity in terms of their associations with alkaline rocks, in particular those characterized by (a) SiO2 <65 wt%, (b) a major contribution by crust material, as is exemplified by the 87Sr/86Sr and 207Pb/204Pb values, (c) their association with alkaline or K-rich calc-alkaline systems, characterized by relatively high of REE, Th and halogen (F, Cl) contents (d) the close association of the Cu-minerals with the main Pd-bearing mineral, merenskyite, and Au–Ag tellurides, (e) the association of the elevated Pd, Pt and Au contents with magnetite-bornite-chalcopyrite assemblages, and the pervasive potassic alteration type at the central parts of the deposits, and (f) the transportation of Cu and precious metals, as chloride complexes, by relatively hot (400 to 700 ºC) and saline to hyper-saline (>70 wt% NaClequiv) hydrothermal fluids. Thus, critical factors controlling base/precious metal potential of porphyry Cu+Au+Pd±Pt deposits are considered to be the composition of parent magmas (contribution of mantle, oceanic and continental crust) and the physico-chemical conditions during the formation of porphyry Cu deposits. In addition, experimental work on Cu-concentrates derived from large ore samples, after roasting showed that the leaching process (HCl and H2O2 in appropriate proportion) resulted the recovery of >97% for Pd, Pt and Au. These results are considered to be encouraging for te evaluation of Pd-Pt as an economic factor (as a by-product) for that type of deposits.