Geochemistry and Petrology of Ortaklar-Gaziantep mafics within Kocali complex, Gaziantep, Turkey: implications for seawater-hydrothermal alteration and provenance studies

Abstract

Magnetite-rich Cyprus-type VMS deposit has been discovered from the Ortaklar-Gaziantep region within Kocali complex, Southeast Turkey. Magnetite-rich sulfide ore bodies are in close contact with underlying footwall basalts. These basalts are part of Kocali mélange, which represents an accreted oceanic complex during the closing of Southern Neotethys. These extrusives are nonvesicular, olivine-free, plagioclase-clinopyroxene phyric basalt with minor Fe-oxides and pyrite. Mineral crystallization sequence of plagioclase followed by augite and Fe-oxides is typical of MORB. Electron microprobed clinopyroxenes are primitive with high Mg# and XFe and thus used as petrogenetic indicators. Analyzed pristine clinopyroxene phenocrysts indicate that they are part of sub-alkaline, non-orogenic tholeiitic basalts. Studied mafics have abundant secondary hydroxides and sulfates which hinder original strontium isotope values. Multi-step 6N HCl leaching experiment was performed to remove non-primary signatures. Unleached basalt powders have 87Sr/86Sr values greater than 0.7045, which are much more radiogenic compared to MORB and arc-related mafics. On the contrary, 87Sr/86Sr values of leached basalt powders are within the MORB range, which coincides with the result from clinopyroxene electron microprobe analyses. Low Mg# (59~60), MgO (< 8%) and Ni (~120 ppm) suggest that basalts are not primitive and rather derived from moderately evolved magma with olivine fractionation. Mafics exhibit similar REE pattern compared to N-MORB with severe LREE depletion [(La/Sm)CN = 0.35~0.45]. These mafics also exhibit depletion in hygromagmaphile elements such as Nb, Th, and Zr, which imply that mafics are derived from depleted magma. Analyzed basalts are similar to those found from Costa Rica Rift Hole 504b and Northern Chile Ridge. Mafics most likely formed by the process called dynamic melting in Triassic following the rifting of A-T block from the Arabian platform. Depletion in hygromagmaphile elements is most likely due to prior melt loss as suggested by Pearce and Peate (1995). Projecting the mafics onto the melting models calculated by Shervais et al. (2005) indicate that mafics probably have formed by approximately 5% melting of a mantle that previously lost 1% melt.