Skarns and skarn deposits of Turkey

Abstract

The skarns and skarn deposits, an important class of mineral deposits in Turkey, are formed during major periods of subduction, continental collision, and post-collisional evenst in response to the convergence between Eurasia and Afro-Arabia between Late Cretaceous and Oligo-Miocene period. The skarns in Turkey are generally classified as calcic exoskarns and occasionally as magnesian exoskarns. The economic mineralization are largely confined to calcic exoskarns with exception of some Fe-skarns hosted by endoskarns. They are widely distributed over the calcitic meta-carbonates and recrystalllized limestones within metamorphic sequences or intercalations within sedimentary and/or volcanic sequences. The skarns are classified as Fe-, Fe-Cu, Cu-, W-, Fe-W, Mo and Pb-Zn-skarns according to dominant metal they contain. No gold or gold-only skarn is present while gold occurs as sub-economic to anomalous enrichment associated with Fe-Cu skarns. The Fe and Pb-Zn skarns have been the prime target for companies for centuries. The Fe and Fe-Cu skarns are commonly associated with oxidized I-type intermediate to mafic intrusives or sub-volcanic domes Late Cretaceous to Eocene in age that also formed economic porphyry Cu deposits. However, Fe and W skarns are co-genetic to porphyry Cu depoists associated with oxidized to mildly oxidized intrusives of Middle Eocene to Oligocene in age. The skarns that coexist as proximal calc-silicate assemlages within major high-level porphyry Cu systems are largely controlled by regional, large-scale structures such as strike-slip faults or core-complex systems. Where both Fe, Fe-Cu and Pb-Zn-skarns are observed in the same region, the Fe- and Fe-Cu skarns are classified as proximal, and Pb-Zn skarns as distal skarns. The vast majority of Fe-, and Fe-W skarns are associated with endoskarn assemblages whereas some Fe-, and Pb-Zn-skarns are associated both with calcic and magnesian exoskarns. The wall rocks span age range rom Cambrian to Permian, and to Upper Cretaceous. The most common era for skarn generations are likely to be Late Cretaceous, Middle Eocene and Oligocene. No or a few skarn associated with intrusions younger than Miocene period is present. The main skarn-producing events in Turkey are related to the Late Cretaceous obduction of oceanic crust at the collision sutures resulted in the formation of a series of magmatic arcs, that also resulted in porphyry systems at the Pontides, but not at the Bitlis-Zagros suture. The second major skarn-producing events took place during Late Cretaceous post-dating the collision between the Anatolide-Tauride Block collided and the Eurasian plate. This era is characterized by the generation of post-collisional (late-orogenic) I- to H-type calc-alkaline granitoids. These granitoids are the products of mixing/mingling of two coexisting magmas derived from the mantle and continental crust. This is the major period during which the most economic skarn deposits in central Anatolia have been resulted in. The third skarn-producing events took place at the Mesozoic-Tertiary boundary (Uppermost Cretaceous to Lower-Mid Paleocene during which a major crustal extension is predominant throughout Turkey.