Sphalerite associated with pyrrhotite-chalcopyrite ore occurring in the kotana fe-skarn deposit (Giresun, NE Turkey): Exsolution or replacement

Abstract

The Kotana prospect is located about 30 km south of Giresun (NE Turkey). The ore mineralization is a Feskarn occurring within the low-grade pre-Lower Jurassic Pi{dotless}narlar metamorphics, consisting of marble-phyllite intruded by the Upper Cretaceous-Eocene Aksu biotite monzogranite. The principal primary ore minerals include pyrrhotite and magnetite along with minor pyrite (I) and chalcopyrite, accompanied by trace sphalerite. Sphalerite is closely associated with chalcopyrite and to a lesser extent with hexagonal pyrrhotite. Secondary ore minerals include pyrite (II), marcasite, martite, hematite, goethite, lepidocrocite, and intermediate Fe-oxides-hydroxides. Gangue minerals are mainly calcite and quartz. Oxidation of the primary sulphides resulted in formation of diverse secondary ore textures containing bird's eye, martitic, spheroidal, colloform, rim, and veinlets. Distinct crystal shapes of sphalerite are of particular interest to this investigation due mainly to their proposed formation mechanisms. Three alternative mechanisms for their formation were considered: (i) quasi-exsolved bodies developed by hexagonal pyrrhotite replacement of chalcopyrite, (ii) interstitial formation between coalescing pyrrhotite crystals during crystal growth, and (iii) as genuinely exsolved bodies, and as such conflict with previous experimental results. Although the general absence of solute mineral outside the solvent mineral suggests solid solution at high temperatures (favouring the third mechanism), textures, modal, microprobe and sulfur-isotope data suggest that these are more likely to be pseudoexsolved bodies formed as a result of replacement of chalcopyrite by hexagonal pyrrhotite. The δ34S values of pyrrhotite and chalcopyrite are between 5.23 and 6.73 per mil (n= 12) and 2.29 and 3.26 per mil (n= 8), respectively, indicating continuous enrichment in heavy sulphur isotopes from prograde stage to retrograde stage within the typical range for skarn-type mineralization. Fluid inclusion analyses of calcite and quartz gangues indicate that the minimum homogenization temperature (Th) averaged 400±20 oC with salinities < 15 wt% NaCl equivalent. Copyright ©TÜBİTAK.