Geology, mineralogy and geochemistry of ultramafic-mafic hosted Ni-Co mineralization; Gunes Ophiolite (Eastern Central Anatolia, Türkiye)

The Güneş Ophiolite is located in Eastern Central Anatolia, contains number of mineralization in different types in different settings. Nickel-Cobalt-Copper mineralization are generally in association with large mantle-derived igneous complexes or basic igneous rocks of the ophiolitic sequences. Relatively high Ni, Co and Cu contents in addition of sulphur from different sources prompted to form sulphide minerals as a result of gravity segragation coupled with fractional crystallization. Since the Late Cretaceous Güneş Ophiolite’s Ni-Co-Cu potential had been confirmed, through the drilling of the first exploration hole, further drilling programs were conducted by different companies for several years. Ophiolitic sequence is composed of tectonites (harzburgite and orthopyroxenite), cumulates (clinopyroxenite, olivine gabbro), and their serpentinized equivalents, pegmatitic gabbro, massive gabbro, sheeted dike complex and basaltic lava from bottom to top. Whole sequence is cut by sub-volcanic diabasic intrusion and younger distal dykes which differs in terms of lithology but with similar strucural condition. Dykes in association with mineralization strikes generally ENE-WSW and dips 50 to 60° NW.  Three settings of mineralization were identified; a) mineralization occurs in the contact zone between the dykes and ultramafic host rocks as massive, semi-massive and net textured b) in fractures of the dyke c) veins and fissure-fillings with disseminations in ultramafic host rock. Alteration minerals are represented by phlogopite-biotite in the proximal (potassic) part of the mineralization, amphibole group actinolite, tremolite and chlorite are observed towards the middle parts as a result of uralitization of pyroxenes. The distal parts are represented by albite, epidote, chlorite and serpentine associated with unmineralized ultramafics. Ni values in the rock samples vary between 500-4700 ppm and the Co values range between 50-1050 ppm, in the soil samples these ranges are 250-4500 ppm and 50-700 ppm, respectively. Pyrrhotite (Fe_1-xS_x), pyrite (FeS₂), pentlandite ((Ni,Fe)₉S₈) and chalcopyrite (CuFeS₂) were identified in core samples as representative of  primary ore mineralization stage. Early stage oxide minerals are represented by magnetite (Fe₃O₄), rutile (TiO₂) and ilmenite (FeTiO₃). According to field and drill core observations, it is though that nickel-saturated ultramafic rocks of Güneş Ophiolite were cut by diabasic dykes of a sub-volcanic intrusion and younger acidic dykes from different origin. In this situation nickel in ultramafics were leached by diabasic and more acidic apophyses and remobilized along ENE-WSW trending structures and dykes. Based on geological setting and structural evolutution and spatial distribution, we suggest that mineralization is mostly related to acidic dykes at the upper parts, whereas at the lower parts, it is mostly associated with highly altered diabasic dykes. It seems like that the enrichment of the main ore minerals shows direct relationship with the thickness of the enclosed individual dyke and it also reflects proximity to the main body of subvolcanic intrusion, in addition to the nickel saturation rate of the ultramafic rock.

Keywords: Ni-Co deposits, Ophiolites, Ultramafics, Ore petrography, Eastern Central Anatolia, Turkey.