The geological, mineralogical and geochemical properties of the magnetite-rich Cyprus type Cu-Fe-Au VMS system in Ortaklar, Gaziantep

Abstract

Located in the Şahinbey district of Gaziantep in the southeastern part of Turkey within the Koçali complex, the Ortaklar Fe-Cu-Au Deposit is a magnetite-rich VMS deposit lying within volcano-sedimentary units including N-MORB type basalts of Upper Triassic - Lower Cretaceous ophiolite characterized by nappes and thrusts. With an estimated reserve of 2.5 million tonnes of ore comprising 45% Fe, 3% Cu, and 0.5 ppm Au, the deposit is composed of tectonically separated discrete Ore lenses of various lengths. This deposit consists of pyrite, magnetite, chalcopyrite and sphalerite as ore minerals in decreasing abundance with chlorites, carbonates and quartz as gangue phases. Hydrothermal alteration has affected a wide portion of the study area and is characterized by an inner propylitic zone at the central section characterised by Mg-rich chlorites (chlinochlore) and the calculated chlorite geothermometry produced temperatures in the range of 225 to 2550C. It is underlain by minor silicification grading into argillic alteration zone. Late-stage carbonate alteration crosscuts all the volcano-sedimentary strata. δ34S values obtained from pyrites and chalcopyrites range from +2.5‰ to 4.6‰ indicating a magmatic source for the sulphur with a calculated temperature range of 202 to 3640C. The δ18O values for the quartz and magnetite spans from 8.9 to 28.9‰ and from -5.0 to 8.3‰, respectively with a calculated temperature range of 250–4270C. Calculated δ18O values for the fluids in equilibrium with these minerals have a broad range between 3.39 and 23.32‰. δ2H isotopic composition of the waters extracted from the fluid inclusions in quartz associated with the mineralization vary from -52 to -73‰. Such δ18O and δ2H compositions are highly likely related to a magmatic fluid source modified by interaction with underlying rocks. The geothermometric data together with mineral paragenetic studies and field observations indicate that ore deposition occurred in two stages. A high temperature magnetite-rich first stage and a sulphide-rich second stage followed by a waning stage.