AN APPROACH TO THE ORIGIN OF KESİKKÖPRÜ (BALA-ANKARA) IRON DEPOSIT

Abstract

This study discusses the ore geology and origin of the Kesikköprü (Bala- Ankara) iron deposit. The basement composed of gneiss, schist, and quartzites of Paleozoic- Mesozoic Kırşehir massive is overlain by sedimentary and volcanicvolcaniclastic rocks that consist of spilitic basalt, basaltic tuff, diabase dikes, cherty limestone, radiolarite, and mudstonelimestone lenses which are transitional with an ophiolitic melange made of crystallized limestone blocks and ultramafic-mafic rocks in a basin that was formed during the upper Cretaceous time. Rocks of the basin are cut by granitoids such as granite, granodiorite, and porphyries of Maastrichtian-Paleocene age. These units are unconformably covered by Eocene Çayraz formation consisting of sandy, clayey, fosiliferous limestone, Miocene-Pliocene (?) İncik formation composing of siltstone, claystone, anhydrite-gypsum alternations, and sandstones with limestone and fosiliferous limestone blocks, volcanic rocks made of rhyolite and tuffs, Pliocene-Quaternary Kızılırmak formation consisting of gravel, sand and mud deposits, and finally posttectonic basin sediments of Quaternary alluvium. Possible lithologies for source and country rocks of mineralization are examined under two main groups as ultramafic rocks consisting of crystallized limestone blocks, peridotite, pyroxenite, and serpentinites and mafic rocks consisting of gabbro and diabases while in more detail, they are investigated as serpentinites and mafic rocks subjected to hydrothermal alteration. The association among olivine, pyroxene, and plagioclase minerals is distinctive in field, thin section, and XRD studies. Petrographic works conducted on mafic and ultramafic rocks yield occurrences unique to ultramafic cumulates. Ore samples, in the order of abundance, consist mainly of magnetite and lesser amounts of pyrite, chalcopyrite, chromite, siderite, ankerite, and trace amounts of pentlandite, pyrrhotite, gersdorffite, ilmenite, and sphene. In addition, olivine, pyroxene, tremolite, and actinolite also accompany the ore and calcite and dolomite are also observed in crack and fracture fillings. Geochemical studies performed on the ore samples indicate that granitic fluids have no direct effect in the ore formation. The processes of granitic intrusions associated with hydrothermal convection cell affect the mafic rocks and shape the hydrothermal alteration modes formed by "skarn type minerals" and fels-like textures". Expelling of iron element as a result of serpentinization of ferromagnesian minerals within the ultramafic rocks, such as olivine and pyroxene, is the primary source of iron. Their secondary enrichment by the granitic intrusions indicates another important stage in the formation of Kesikköprü iron deposit. Present study favors the idea that Kesikköprü iron deposit is similar to the Divriği type deposit, in other words, as also stated by previous works, iron was derived directly from the granitic rocks.