Evolution of the Tepeoba porphyry-skarn Cu-Mo-Au deposit, NW Turkey: New mineralogical and geochemical findings

This study recently investigates the evolution of the Tepeoba porphyry Cu-Mo-Au deposit (Cu ∼ 3 wt%, Mo ∼ 0.16 wt%, Au ∼ 10 ppm) with associated skarn prospects in the Biga Peninsula (NW Turkey) using the geochemical and mineralogical data. Ore-related alteration types which include biotite (±muscovite)-K-feldspar at the center of the mineralization zone, actinolite-albite alteration, and outer chlorite-epidote-calcite mineral facies, are observed in the Tepeoba mine area along the southern contact between the Karakaya Complex (mainly hornfels at the mine area) and porphyry Eybek intrusion. Porphyry style alteration pattern has been detected recognized in the porphyritic microgranite host rock. On the other hand, the calcareous and carbonate as well as the mineralized hornfels refer to the skarn style alteration pattern associated with a porphyritic felsic stock. Sulfide-rich stockwork and mineralized brittle brecciated, and fracture zones are associated with these hydrothermally altered wallrocks. Ore mineralogy includes chalcopyrite, molybdenite, pyrrhotite, pyrite, subordinate cubanite and native gold. These minerals are replaced by marcasite, hematite, and goethite in the oxidized cap rocks. EPMA data of the alteration and sulfide minerals revealed that Cu-, Fe-, Mo-sulfides partially replace biotite ± muscovite-K-feldspar/actinolite-albite and chlorite-epidote-calcite alteration facies. The estimated ore formation conditions returned wide range of temperatures of 546–471 °C (biotite thermometry) to 340–264 °C (chlorite thermometry), at which sulfides had most likely precipitated while the oxygen fugacity of the magmatic-hydrothermal ore fluids was gradually lowered and developed during the stage of retrograde metasomatic reactions. Chalcopyrite and cubanite was likely to have formed via un-mixing of intermediate solid solution (iss) under oxidizing hydrothermal conditions. Under these conditions, thiosulfide complexes could have been responsible for the transport and deposition of gold. This also refers to native gold, which could have been liberated from sulfide ores (i.e., pyrite) in the oxidation zone due to the processes of supergene enrichment.