Mineral chemistry and garnet U-Pb dating in the Bizmişen iron skarn deposit, Erzincan, East-Central Türkiye

Abstract

In Bizmişen area, Middle Eocene (46.3–42.0 Ma) plutonic rocks (quartz diorite) were intruded into Triassic-Cretaceous limestones and Upper Cretaceous ophiolites and caused to skarn zones containing prograde (garnet, clinopyroxene) and retrograde (amphibole, epidote, calcite, magnetite, phlogopite, serpentinite and chlorite) minerals. Due to contact relationships of quartz diorites with both serpentinized ultramafic rocks and limestone blocks, the mineralogical associations represent calcic (grossular/andradite, amphibole, chlorite and epidote) as well as magnesian (diopside, serpentine, phlogopite, talc) skarns. During the skarnization process, garnet, diopside, epidote, scapolite and amphibole minerals developed in the endoskarn zone within quartz-diorite, and diopside, tremolite, serpentine, phlogopite, talc, Mg-chlorite and calcite minerals developed in the exoskarn zones in ophiolites (serpantinite and listwaenite) and limestones. Skarn and magmatic clinopyroxenes in serpentinized ultramafic hosted exoskarn zone have a compositional range of Wo_45-53En_35-53Fs_0–15 and Wo_24-51En_38-76Fs_0–11, and classified as diopside and augite, respectively. The end members of garnets are Adr_46–54Grs_43–50Prp_1–2Sps_1.1–5 in amphibole-epidote-calcite-garnet skarn in endoskarn zone, whereas Adr_40–93Grs_4–57Prp_0.2–2.3Sps_0.9–2 in epidote-calcite-garnet skarn in exoskarn zone, and classified as andradite-rich grandite garnets. FeO, MnO and partially MgO contents gradually increase from the edges to the center in zoned garnet grains. Amphibole compositions represent to calcic (Ca >1 atoms per formula unit) group (pargasite) and indicate the high oxygen fugacity conditions which caused the increase in the rate of magnetite crystallization. Al₂O₃ and FeO contents of epidotes range 23.68–27.63 and 9.10–14.45 wt%, respectively, correspond to composition of epidote and clinozoisite. The iron oxide contents (FeO, wt%) of magnetites change from 91.72 to 98.36 correspond to structural Fe³⁺_1.88–1.97 and Fe²⁺_0.91–1.0 values. The contents of Ti, divalent and trivalent metal cations correspond to ulvospinel-magnetite compositional line at the magnetite end-member. The mineral chemistry of skarn minerals shows similarities to those of most iron skarn deposits. Garnet U-Pb ages (46.95–45.63 Ma) are consistent with the cooling ages of the Bizmişen pluton. The prograde stage of skarn should be occurred syn-intrusion, whereas retrograde stage should be developed later than youngest garnet age (45.6 Ma) and before the oldest late stage argillic alteration (37.5 Ma).