Geochemistry, Petrology, and Provenance of Magnetite-Rich Ortaklar Cu Deposit Hosting Basalts from Koçali Complex, Gaziantep, Turkey

Abstract:

Magnetite-rich Cyprus type VMS deposit has been recently discovered from the Ortaklar-Gaziantep region within Koçali complex, SE Turkey. Magnetite rich sulfide ore bodies are in close contact with underlying footwall spilitic basalts. These basalts are part of Koçali mélange, which represents an accreted oceanic complex during closing of southern Neotethys. These extrusives are low-K, low alkali tholeiites with Ca rich, partially sericitized plagioclase subophitically enclosed by augite with disseminated Fe-Ti oxides and pyrite. Mineral crystallization sequence of plagioclase followed by augite and opaque is typical of MORB.

Major and trace element analyses for least altered basalts based on LOI (1.5~3.6 wt%), Ce/Ce^* (0.9~1.1) exhibit limited range of element abundances. Low Mg# (59~60) suggests that basalts were derived from moderately evolved magma with fractional crystallization. HFSE (Th, Nb, Hf, Zr) were used for tectonic discrimination and basalts were plotted within MORB end spectrum, near MORB-IAT boundary. N-MORB normalized La to Lu ranges from 0.4 to 0.9 times N-MORB with LREE depletion [(La/Sm)_N = 0.58~0.67] and flat HREE [(Tb/Lu)_N = 0.95~1.05]. Chondrite normalized REE patterns resemble those of N-MORB but characterized by severe LREE depletion [(La/Sm)_CN = 0.35~0.45]. LREE depletion coupled with high Sm/Nd (0.36~0.43), high CaO/Na₂O (5.0~6.2) and low Nb/Yb (0.23~0.39) suggest depleted N-MORB composition derived from the refractory mantle source.

Analyzed basalts are similar to those found from other rift (Costa Rica Rift Hole 504b) and intra-transform fault (Siqueiros transform). Magnetite emplacement occurring close to the ore-host boundary and lack of pyrrhotite from hosting basalts imply an involvement of oxidized hydrothermal fluids. Basalts probably have formed by late stage, partial melting of the refractory mantle at low pressure, shallow depth, and H₂O rich environment. Possible source of mantle heterogeneity can be identified by isotope analyses.