Platinum-group element and gold contents in the Skouries porphyry copper deposit, Chalkidiki Peninsula, northern Greece

Abstract

The Skouries porphyry copper deposit is located in the eastern Chalkidiki peninsula, northern Greece. Along with several other porphyry copper occurrences, this deposit establishes the northeastern part of the Servomacedonian zone as a porphyry copper region. Amphibolite, mica schist, and two-mica gneiss are the predominant rock types exposed in the Skouries area. These rocks have been intruded by the Skouries porphyry stock, which is of subalkaline composition and is Tertiary in age (18 Ma). The Skouries porphyry intrusion and the surrounding country rocks are strongly fractured and intensely altered by hydrothermal fluids. In addition to intense silicification, a prominent alteration type is potassic, whereas phyllic and propylitic types are less common.Mineralization mainly includes chalcopyrite, pyrite, bornite, chalcocite, and magnetite in the form of veins, stockworks, and disseminations. Native gold and electrum commonly are present as small inclusions in chalcopyrite. Although lateral zoning is not defined clearly, a vertical zoning in terms of both metal mineralogy and overall metal content is apparent. A zone of oxidation (azurite-malachite) covers the upper part of the ore and is underlain by a zone of reduced rock rich in covellite and chalcocite, which in turn is underlain by the most important ore type, including a chalcopyrite-pyrite-bornite-magnetite assemblage.Both weakly and intensively altered porphyry samples from the deposit were analyzed for major oxides, trace elements, and all platinum-group elements (PGE). The stock exhibits high levels of Cu, Au, Ag, B, Bi, Co, Se, and Te. Barium and Sr contents are also high while Rb is low, resulting thereby in high K/Rb and low Rb/Sr ratios. Palladium concentrations range from less than 2 to 480 ppb. Ruthenium content ranges from 8 to 41 ppb and Pt from 2 to 10 ppb. Osmium, Ir, and Rh content is below detection limit. The average Au/Ag ratio is 0.87, which is consistent with the range of several worldwide porphyry copper deposits.On the basis of factor analysis of selected geochemical data, a strong positive interelement correlation (r > or = 0.83) between Au, Ag, Pd, nd Te coupled with their association with silica appears to support a correlation of these precious metals with silicification rather than potassic-type alteration. Such a relationship may suggest similar behavior of these metals and a common origin. A positive correlation of Cu with iron (r = 0.86), which is consistent with the association of chalcopyrite with magnetite, and the low correlation of Te and Pd with Cu may suggest that these metals and, to a lesser extent, copper have been redistributed during a late stage of evolution of the Skouries porphyry system. Furthermore, the close association of Te with precious metals, combined with the relatively low temperature for telluride minerals stability, may indicate that at least partly precious metal tellurides have been formed in a relatively shallow and less hot environment.