Formation of the Egrikar (Gumushane) Fe-Cu skarn type mineralization in NE Turkey: U-Pb zircon age, lithogeochemistry, mineral chemistry, fluid inclusion, and O-H-C-S isotopic compositions

Abstract

The Egrikar Fe-Cu skarn type mineralization is located approximately 100 km NW of Gumuhane in NE Turkey. Geochemical analyses of diorite, which is thought to cause skarn, indicate medium K calc-alkaline (1.3 to similar to 2.3 wt % K2O), metaluminous to peraluminous (A/CNK = 0.76 to 1.13) and resembles a continental arc granite. The age of U-Pb zircon obtained from diorite is 42.3 +/- 1.0 Ma, which reflects the age of skarn metasomatism after intrusion into Late Cretaceous sandy carbonates. Skarn formation zones are recognized as endo- and exoskarn. The endoskarn zone mainly consists of pyroxene with hedenbergite (Di(4.31-28.14)Hd(59.29-80.82)Jo(5.0-35.55)) at prograde stage. Mn content of pyroxene increases a slight from endo to exoskam. Pyroxene is replaced by epidote and hornblende at the retrograde stage. The exoskam zone comprises garnet with andradite (Ad(91-99)Gr(01-07)) and grossular-andradite (Ad(44.95)Gr(05-54)), epidote and quartz as silicate minerals, and magnetite and hematite as ore minerals. Magnetite is the abundant ore mineral and is accompanied by chalcopyrite and minor pyrite at retrograde stage. On the basis of fluid inclusion values of minerals, the temperature range for hedenbergite in endo-exoskarn is 350 degrees C-425 degrees C, for epidote in exoskarn is 365 degrees C-376 degrees C, and for quartz in exoskarn 200 degrees C-380 degrees C. NaCl equivalent salinity for hedenbergite 6.88-10.73 wt% and for quartz is 3.9-15.38 wt. The delta O-18 values of hedenbergite is 4.1 to 4.7%o and for garnet 3.4%o at prograde stage, for epidote 7.9-9.2%o, for magnetite 1 to 4.4 parts per thousand, for hematite -2.9 to -1.6 parts per thousand and for quartz range between 8.5 parts per thousand and 15.2 parts per thousand. The delta O-18 value of epidote suggests that oxygen is a magmatic fluid whereas hedenbergite, garnet, quartz and magnetite show the mixing of magmatic and meteoric fluids. In addition, the C isotope values (-3.2 to 2.8 parts per thousand) of calcite and marble in Egrikar Fe-Cu skarn reflect those of metamorphic, and skams. Based on oxygen isotope data from mineral pairs, the retrograde skarn formation is characterized by epidote-magnetite assemblage formed at 255 degrees C-438 degrees C. Fluid oxygen isotope compositions of magnetite and epidote pairs of the exoskarn skarn zone indicate a mixing of magmatic and meteoric fluids with narrow delta O-18 values between 7.17 parts per thousand and 8.9 parts per thousand. The delta S-34 values of the pyrites exhibit a narrow range of 3 parts per thousand and 4.4 parts per thousand, thereby indicating that sulfur were probably leached from magmatic rocks. Based on the field and laboratory observation, mineral chemistries, fluid inclusion studies and isotope composition analysis, it is suggested that the Egrikar Fe-Cu skarn mineralization occurs in a magmatic origin fluids being initially under reduced conditions, subsequently under oxidized conditions, and at shallow depth.