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 (ReS2) 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 A in pure molybdenite to 2.355 A), 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 2 H polytype and not the 3 R 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 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.