Mineralogical, fluid inclusion, and stable isotope constraints on the genesis of Keban PbZn skarn deposit, southeast Anatolia

Abstract

The Keban PbZn deposit is located in the Elazığ district, southeastern Turkey and hosted by the Permo-Triassic/Permo-Carboniferous Keban Metamorphics and the Late Cretaceous-Paleocene Keban Magmatics. Mineralization develops as disseminated, veins and massive types of ore within alkali syenite porphyry, sericite-chlorite banded calc-schist and dolomitic limestone.

Three paragenetic stages of skarn formation and ore deposition are recognized in the Keban PbZn deposit: prograde (stage I), retrograde-sulfide (stage II), and supergene (stage III). The endoskarn forming a narrow zone is composed of grossular (Grt 1), Fe-rich grossular (Grt 2) and andradite (Grt 3) with diopside and plagioclase. The exoskarn comprises grossular (Grt 4), pyroxene and vesuvianite. Ore minerals include galena, sphalerite, chalcopyrite, magnetite, hematite, molybdenite, and pyrite accompanied in small quantities by pyrrhotite, arsenopyrite, manganese oxides, native gold, and sulfosalts. Mineral chemistry of garnets suggests that Grt 1 precipitated under a low water/rock (W/R) ratio and relatively reduced conditions. Grt 2 with strong oscillatory zoning and Grt 3 with high Fe³⁺ contents were formed under infiltration metasomatism with high W/R ratios. When the water–rock intereaction was decreased, Grt 4 and vesuvianite were affected by Al-bearing residual metasomatic fluids that are derived from calc-schist under reduced conditions.

Depletion of δ¹³C and δ¹⁸O in skarn calcites is largely controlled by hydrothermal fluid infiltration and meteoric water influx. Microthermometric measurements support that magmatic fluids comprising the stage I (473 to 572 °C; 11.9 wt% NaCl eq.) were sequentially mixed with meteoric waters of stage II (230 to 524 °C; 0.8.-6.6 wt% NaCl eq). Based on FI trapping pressures and depths of the boiling system, the mineralization developed after boiling during the retrograde stage in a shallow environment characterized by low to moderate temperatures and low salinities, within the pressure and depth range of ∼100–500 bar and < 1.5 km, respectively. δ³⁴S values of sulfide minerals are between −8.5 and + 2.1 ‰ indicating that ore-forming fluids and metals originated principally from a magmatic-hydrothermal source. High Fe, Mn and Ga contents of sphalerites might point to deposition at low to moderate temperature conditions and trace element concentrations imply that mineralization took place at distal part of the skarn system.