RHENIUM-RICH MOLYBDENITE AND RHENIITE IN THE PAGONI RACHI Mo-Cu-Te-Ag-Au PROSPECT, NORTHERN GREECE: IMPLICATIONS FOR THE Re GEOCHEMISTRY OF PORPHYRY-STYLE Cu-Mo AND Mo MINERALIZATION

Abstract

The porphyry-type Pagoni Rachi Mo-Cu-Te-Ag-Au prospect, in northern Greece, is a porphyry-epithermal system hosted by an Oligocene dacite porphyry and quartz-feldspar porphyry dikes. The rare mineral rheniite (ReS 2 ) and molybdenite, with very high contents of Re (up to 4.7 wt% Re), occur in quartz veins along with Fe-Cu sulfides, Pb-, Sn-, and Cl-bearing oxides, hematite, ilmenite and tellurides of Bi; these veins are spatially related to sericitic and transitional sericitic-sodic-potassic alteration. Earlier-formed gold-bearing quartz and magnetite veins with sodic-potassic-calcic alteration and late precious-metal telluride-rich carbonate-quartz veins with argillic alteration contain only minor amounts of molybdenite. The composition of rheniite ranges from almost pure, stoichiometric rheniite with low Mo content to rheniite with up to 5.99 wt% Mo. Petrographic, scanning electron microscope, and structural studies suggest that the high Re content of molybdenite from Pagoni Rachi is the result of the isovalent Re-for-Mo substitution in molybdenite. This fact is corroborated by the progressive shortening of the Mo-S mean bond-distance (from 2.414 Å in pure molybdenite to 2.355 Å), as well as by the isotropic decrease of the unit-cell values with the increase of the Re:Mo ratio. The structural analysis of four molybdenite crystals, with the highest Re content ever reported in nature, demonstrates that they crystallize as the 2H polytype and not the 3R polytype, as previously hypothesized, thus suggesting that Re concentration does not correlate with a specific polytype. The fluid inclusions in quartz in the Re-bearing molybdenite-rheniite veins at Pagoni Rachi show that they homogenize to either the liquid or vapor phases (354 to 428°C) or by halite dissolution at 317 to 585°C, which equates to salinities of 40 to 59 wt% NaCl equiv. Rheniite and molybdenite likely precipitated as temperatures fell below 400°C during phase separation under relatively oxidizing conditions, at elevated chlorine activity, and from relatively acid hydrothermal solutions. However, the presence of Pb oxides, Sn-bearing minerals and tellurides § E-mail address: voudouris@geol.uoa.gr 1014 thE CanaDian MinERalogist is compelling evidence that rheniite and Re-rich molybdenite may have formed directly from the vapor as sublimates, in a manner similar to the way they are deposited at the Kudriavy volcano, Kurile Islands.Keywords: porphyry-epithermal deposit, rheniite, Re-rich molybdenite, polytype, phase separation, tellurides, sublimates, Pagoni Rachi, Greece.soMMaiRE L'indice minéralisé en Mo-Cu-Te-Ag-Au de Pagoni Rachi, de type porphyre, situé dans le nord de la Grèce, est un système épithermal encaissé par un porphyre dacitique oligocène et des filons de porphyre quartzofeldspathique. Nous y avons trouvé la rhéniïte (ReS 2 ), espèce rare, et la molybdénite très enrichie en Re (jusqu'à 4.7% Re, poids), dans des veines de quartz avec des sulfures de Fe-Cu, des oxydes contenant Pb, Sn, et Cl, l'hématite, l'ilménite et des tellurure...

Geological, Mineralogical and Geochemical Aspects for Critical and Rare Metals in Greece

“…The porphyry Cu-Mo and Mo ± -Cu prospects of Pagoni Rachi, Maronia and Melitena, in western Thrace contain molybdenite with high to ultrahigh Re content (reaching 4.7 wt %), which are the highest concentrations of Re-in-molybdenites ever measured [31][32][33]. The microprobe analyses demonstrate that there is a positive correlation between Mo and Re supporting the concept that Re substitutes for Mo in the structure of molybdenite.…”

“…The extremely high contents of rhenium in molybdenite at the Greek porphyry prospects could be explained by a rhenium release from the asthenospheric mantle wedge by subduction-related fluids and melts [33]. The Re-enrichment in the western Thrace molybdenites, suggest that this region should be a potential target for future Re-and Mo exploration and exploitation.…”

“…The microprobe analyses demonstrate that there is a positive correlation between Mo and Re supporting the concept that Re substitutes for Mo in the structure of molybdenite. Moreover, the very rare mineral rheniite (ReS 2 ) is found in the porphyry systems of the Pagoni Rachi and Konos/Sapes prospects [31,33].…”

Molybdenite as a Rhenium Carrier: First Results of a Spectroscopic Approach Using Synchrotron Radiation

“…The possible correlation between the rhenium content of a natural molybdenite and its polytype has been addressed long ago [33] but so far not fully interpreted [9,34].…”

“…This very scarce element has been found in granite pegmatites and quartz veins (e.g. in Japan [2]) as well as in volcanic gases, namely from the Kudryavy Volcano, Kurile Islands [3], but it occurs mainly in ores of porphyry copper-molybdenum deposits [4][5][6][7][8][9] associated to molybdenite, MoS 2 , generally recognized as the main Re-carrier in nature [10,11]. Indeed, specific rhenium minerals are so far confined to very few sulphides among which rheniite, ReS 2 [12]expected to be structurally identical to synthetic rhenium disulphide [13] despite its crystal structure has not yet been fully determined [14], and tarkianite (Cu, Fe) (Re, Mo) 4 S 8 [15], a lacunar kenotetrahedral [16] thiospinel.…”