Abstract

The Pagoni Rachi/Kirki Cu-Mo ± Re ± Au deposit is a telescoped porphyry-epithermal system that contains molybdenite with up to 4.7 wt.% Re and the rare mineral rheniite (ReS2). The deposit is spatially related to an Oligocene, calc-alkaline hornblende-biotite- bearing granodiorite-tonalite porphyry stock; however, a genetic link to a buried stock of quartz-feldspar porphyry magma beneath Pagoni Rachi cannot be excluded. Alteration types and vein relationships suggest that mineralization can be divided into four paragenetic stages, which are, from early to late: (1) sodic/potassic-calcic alteration with quartz- and magnetite-bearing veins (A- and M-type) and distal propylitic alteration, (2) sodic/potassic alteration with quartz-pyrite-chalcopyrite-molybdenite veins (B-type), (3) sericitic alteration with “transitional” porphyry to epithermal pyrite-chalcopyrite-molybdenite veins (D-type), and (4) argillic alteration with quartz-calcite base metal and precious metal rich veins (E-type) with epithermal affinity. Bulk ore analyses and mineralogical data are consistent with deposition of precious metals in all mineralization stages: electrum in A- and M-type veins is spatially associated with bornite and chalcopyrite, whereas tellurides (cuprian hessite, stützite, petzite) and Ag-Au alloy occur in the epithermal veins. The D-type veins contain the highest gold grades (up to 5 g/t) and include Ag-, Bi-, Te-, and Se-bearing minerals indicating a possible genetic relationship with E-type epithermal veins. Fluid inclusion data indicate that A- and B-type veins (porphyry style mineralization) were deposited at 360–510 °C and at pressures up to 690 bars (<2 km depth) in A-veins and up to 360 bars (<1.5km depth) in B-veins, from boiling hydrothermal fluids. This process produced coexisting low to moderately saline (1.7–10.7 wt.% NaCl equiv.) and highly saline (36–>74 wt.% NaCl equiv.) fluids. Subsequent dilution of the moderately saline fluid formed a lower temperature (210–340 °C), less saline (1.4–2.9 wt.% NaCl equiv.) fluid in late epithermal E-veins. Based on the Au and Re grades, Cu/Mo ratios, presence of rheniite and the extremely high Re content of molybdenite, Pagoni Rachi may be considered as a distinct class of porphyry deposits, a Cu-Mo±Re±Au system that is derived from rocks transitional between calc-alkalic and alkalic porphyries. Its similarity with other Re-Au enriched porphyry-style deposits in northeastern Greece indicates that this area may represent a new Au-Re-Te metallogenetic province.

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