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Magmatic-hydrothermal evolution and post-ore modifications of the Halilağa porphyry cu-au deposit, NW Turkey Brunetti, Paula


The Biga Peninsula of northwest Turkey is an emerging copper-gold province characterized by numerous epithermal and porphyry-type deposits and prospects associated with Eocene to Oligocene calc-alkaline post-collisional magmatism. Neotectonics in the Biga Peninsula are dominated by a NE-SW dextral strike slip regime that is associated with the southern branch of the North Anatolian Fault System (NAF) and a N-S extensional regime. The Halilağa Cu-Au deposit, in central Biga Peninsula, is a porphyry system that also includes other mineralization types such as high-sulfidation and skarn prospects. The deposit is bounded by two transtensional faults, which have a relevant post-mineralization role. The Halilağa porphyry Cu-Au system comprises two dominant intrusive phases: a mineralized quartz monzonite porphyry with crowded phenocrysts of plagioclase and rounded quartz, and a poorly-mineralized, phenocryst-poor, quartz monzonite to granodiorite porphyry. Andesitic dykes cut and post-date the porphyry intrusions. Porphyry Cu-Au mineralization is spatially associated with biotite + magnetite ± K-feldspar alteration and intense quartz veining. This alteration assemblage has been variably overprinted by pervasive sericite±quartz alteration, in particular at shallow levels. At depth, selective pervasive chlorite+sericite alteration has overprinted the early biotite + magnetite ± K-feldspar assemblage. Epidote + chlorite + calcite alteration occurs at the edges of the system. Sulphide mineralogy is dominated by chalcopyrite, pyrite, and minor pyrrhotite as inclusions in pyrite. Faults that bound the porphyry stock display evidence of syn- and post-mineral activity, suggesting that these faults have protracted displacement histories. U-Pb zircon geochronology from the Halilağa district, in this study, constrained the emplacement of the porphyry intrusions to Middle Eocene (40 to 37 Ma) and also indicated another magmatic event of ca. 28 Ma is spatially related to a high-sulfidation epithermal system. Middle Eocene and Oligocene magmatism is coincident with the ages of epithermal and porphyry mineralization in the Biga Peninsula (e.g. Kuscayir, Kartaldağ, Ağı Dağı, Tepeoba), suggesting a favorable setting for porphyry systems generation during that time. In Halilağa, the porphyry mineralization is associated with the Eocene magmatic event, confirmed by a Re-Os analysis of molybdenite yielding a mineralization age of 39.56 ± 0.21 Ma.

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