Ore-fluid evolution at the Sasa Pb-Zn skarn deposit,Republic of Macedonia

The Sasa Pb-Zn skarn deposit is located in the Serbo-Macedonian massif approximately 100 km east from Skopje, Republic of Macedonia. Ore reserves are estimated at 10 million metric tons with 7.5% lead and zinc. The deposit is hosted by the Lower Palaeozoic metamorphic complex composed of gneisses, marbles and quartz-graphite schists. The mineralisation is spatially and temporary related to Tertiary calc-alkaline magmatism. The volcanic rocks in the area are mostly quartz latite and andesite with K/Ar ages between 27 and 24 Ma. The mineralisation is represented by skarn and hydrothermal minerals as two end-member associations. The skarns occur in the form of replacement of marble, whereas the hydrothermal mineralisation appears as replacements and as open-space fillings. The skarn association has characteristic zonal structure and contains calc-silicate minerals (Fe-Mn-pyroxenes, Fe-Mn-pyroxenoids, garnets, ilvaite, epidote), magnetite, pyrite and pyrrhotite. The hydrothermal association, which is superimposed onto the skarn assemblages, contains argentiferous galena, sphalerite, pyrite and minor chalcopyrite. Carbonates and quartz are the most abundant gangue minerals. Fluid inclusion studies recognised three major stages of mineralisation: (1) Metasomatic formation of skarn minerals during the prograde stage of mineralisation from saline and high-temperature fluid, probably of distal magmatic origin. The absence of mineralised contacts between the magmatic and the host rocks as well as the manganese rich mineralogy suggests metasomatism by infiltration of the mineralising fluids into the host rocks, rather than mineralisation by diffusion-driven contact metasomatism. (2) Hydrothermal alteration of skarn minerals (ilvaite, chlorite, magnetite, pyrrhotite, carbonates, quartz) followed by precipitation of ore and gangue minerals resulted from the mixing of magmatic and meteoric fluids during the retrograde stage. (3) Deposition of post-ore gangue minerals, mostly carbonates, from cooler and slightly diluted fluids.