ALSAR DISTRICT, MACEDONIA: PLIOCENE SRHDG MINERALIZATION ASSOCIATED WITH SHOSHONITIC MAGMATISM

Historic mining in the 8x4 km Alsar district exploited As and Sb veins. Recent exploration shows that it contains disseminated Au mineralization (1-10 g/t Au). It is located in the N-trending Vardar Zone, a tectonic suture located on an E-dipping subduction zone produced by Alpine convergence. The Neogene magmatism and N-S extension in the region is associated with NE subduction of the African plate below the Aegean/Eurasian plate. Mineralization is hosted in Triassic carbonate rocks unconformably overlain by Miocene lacustrine dolomite and volcaniclastic rocks, and capped by Pliocene tuffs and volcaniclastic rocks. Pliocene subvolcanic shoshonitic dacites intrude pre-Tertiary and Tertiary rocks and are locally altered and mineralized. Reconstructions indicate ore formation was at shallow depths of ~250 m or more. Mineralization is localized along N-, NW- and NE- trending high-angle faults and extends outward in the clastic debris-rich base of the lacustrine dolomite. Silicification is most intense in carbonate, clastic and volcanic rocks and is enveloped by variably developed argillized, dolomitized, and decarbonatized rocks. Gold mineralization is spatially associated with replacement silicification and the higest grades are in pyrite/marcasite- and/or stibnite-bearing jasperoids. Arsenian pyrite/marcasite is auriferous and the outermost rim has the highest As and Sb contents. Ore fluids introduced Si, K, Fe, S, Au, As, Sb, Tl, and Ba along with lesser amounts of Te, Se, F, Mo, and Zn and resulted in the formation of pyrite, marcasite, stibnite, realgar-orpiment, cinnabar, arsenopyrite and Tl-As-Sb-Hg sulfides and sulfosalts. Gangue minerals include drusy quartz, minor fluorite, and later barite and calcite. The district is zoned with increasing paleodepth from N to S in the sequence (As-Tl>>Hg-Sb-Si+/-Au) to (Sb-As-Si-Au+/-Tl-Hg-Ba) to (Si-Sb-Au-As+/-Hg-Ba-Tl). Fluid inclusions in realgar have Th of 144-170°C and salinities of 7.9-12.9 wt %. d 34S of realgar, orpiment and stibnite correspond to H2S values of -2.6 to 3.4‰. d D of fluid inclusion water in orpiment and realgar range from -75 to -99‰. The S and H isotope data suggest that magmatic fluids mixed with meteoric ground water. Together, the evidence indicate that Alsar is genetically related to a shoshonitic magmatic center.