Formation of skarns at Gümüşhane (Northeastern Turkey)

Abstract

Skarn deposits are located in the NE Black Sea region of Turkey belonging to the Alpin metallogenic province. The Fe skarn deposit at Arnastal is hosted by Late Cretaceous carbonate rocks comprised of limestone and the skarn occurrence at Camiboğazı dolomitic limestone, adjacent to the Eocene Zigana Granitoid. The prograde textures include rhythmic banding, spots, and mossiness, and the retrograde textures are shock-induced and brecciated. These are identifi ed in the Fe skarn deposit at Arnastal, where there are mineral parageneses of andradite, actinolite, tremolite, epidote, quartz, magnetite, and hematite, and less pyrrhotine and pyrite. Additionally, well-crystallized vesuvianite, well-crystallized phlogopite, and diopside in the skarn occurrence at Camiboğazı are identifi ed. Some of the skarn minerals are calcic and have high Mg/(Mg + Fe) ratios. Clinopyroxene consists of diopside (Di96.66–99.04 Hd0.67–2.80Jo0.29–0.55). Vesuvianite is mostly Mg-rich, and has a very low MnO content (< 0.1 %). Other skarn minerals are Fe-rich. Garnet is characterized by oscillatory zoning and has an andradite composition (Ad72Gr28 to Ad98Gr2). Epidote has an Fe content of 8.96 to 12.42 wt%. Magnetite is generally massive and sporadically banded with some martitization. The formation temperatures of skarn minerals range from 330 to 600 °C. In addition, mass change calculations from the granitoid and andesite toward the skarn zone indicate that there is considerable mass transfer of Si, Al, Fe, and Mg to the carbonate rocks during skarn formation. The average mass changes are estimated at a depletion of 41 g/100 g in granitoid and a gain of 8 g/100 g in andesite. Fe skarn deposit and skarn occurrence are characterized by early, high-temperature mineral assemblages dominated by the anhydrous minerals garnet and diopside and late low-temperature assemblages with hydrous minerals, such as vesuvianite, phlogopite, and epidote. The type of garnet and the abundance of magnetite with minor pyrite suggest that the Fe skarn deposit at Arnastal forms under relatively oxidized conditions. The type of clinopyroxene in the skarn occurrence at the Camiboğazı shows oxidized skarns.