Petrology and geochemistry of the shoshonite-hosted Skouries porphyry Cu–Au deposit, Chalkidiki, Greece
The Skouries porphyry Cu–Au deposit, containing an indicated reserve of 206 Mt at 0.54% Cu and 0.80 g/t Au, is hosted by at least four hypabyssal monzonite–porphyry phases. In decreasing age, they are: (1) pink monzonite, (2) main monzonite, (3) intra-mineral monzonite, and (4) late-stage porphyry. High-grade ore is directly associated with the main and intra-mineral monzonite phases. All intrusive phases are cut by late-stage monzonite dykes that are barren. The monzonites have porphyritic textures with phenocrysts of plagioclase, alkali feldspar and amphibole as well as apatite and titanite microphenocrysts in a fine-grained feldspar-dominated groundmass. Mineralized samples are affected to varying degrees by potassic alteration, ranging from weak biotite–magnetite disseminations, through cross-cutting veinlets of hydrothermal orthoclase, to zones with pervasive orthoclase flooding. The high halogen contents of the Skouries intrusions are reflected in the high Cl and F concentrations of mica phases (up to 0.19 and 2.48 wt% respectively). The presence of magmatic magnetite in all intrusive phases implies high oxygen fugacities of the parental melts. All four monzonite phases have relatively evolved compositions, as reflected by their high SiO2, low MgO and low mg#, and variable but low contents of mantle-compatible elements such as V, Ni and Co. However, their mg# suggests increasing degrees of fractionation of the parental melts with decreasing age. Their high K2O (up to 5.8 wt%) and K2O/Na2O ratios (>1), as well as their high Ce/Yb and Th/Yb ratios (>34 and >21 respectively), which are believed to have been unaffected by alteration processes, are typical of alkaline rocks of the shoshonite association. Importantly, the Skouries intrusions are characterized by very high U and Th contents (up to 18.9 ppm and 62 ppm, respectively) that are consistent with accessory thorite and rare allanite in several samples. The high initial 87Sr/86Sr ratios (0.7082) for the Skouries intrusions suggest crustal contamination during emplacement. The use of geochemical discrimination diagrams assigns the rocks to a continental arc setting in accord with the interpretation of previous workers.
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