| Summary: | In this thesis the results of a study on the geology, geochemistry and magnetite-apatite mineralization of the Avnik area, southeast Turkey, are presented. Conclusions are drawn with respect to the origin and the way of emplacement of the mineralization. The study area is part of the Bitlis Massif which is characterized by various stages of deposition, folding, faulting, erosion, and intrusion of a coarse granite. An area of about 120 square km has been mapped at the scale of 1:25,000. Major mineralized zones have been mapped separately at the scale of 1:2,000. The present basement rocks of the study area consist of a metamorphosed, metasomatosed and hydrothermally leached volcanic and sedimentary series, deposited during Cambrian-Precambrian times. Although the coarse granite covers large areas outside the mapped area and clearly cuts across the Basement Series only small outcrops are exposed in the study area. The radiometrically determined age of the granite is Devonian-Carboniferous. The Basement Series are unconformably overlain by pelitic and calcareous sediments (Epimetamorphic Series) which were deposited during a Late Paleozoic-Early Mesozoic time. Both series were regionally metamorphosed at biotite (Cretaceous) and chlorite (Eocene-Oligocene) zone greenschist facies conditions. The early greenschist metamorphism (M1) was accompanied by a region- Wide overturning. In Late Miocene-Pliocene times the above series were thrusted southward over the Baykan Series along the Frontal Thrust. The Baykan Series consists of unmetamorphosed Upper Cretaceous-Eocene ultrabasic rocks, basic volcanics, and sedimentary rocks. Further south, outside the area, the latter series overrides the sedimentary rocks (Cambrian-Miocene) of the Arab Platform. The highest grade of metamorphism (Mo; epidote-amphibolite facies) is tentatively attributed to a period of thermal metamorphism related to intrusion of the coarse granite. Much of the mineralogical and geochemical evidence for this period of metamorphism was erased by metasomatic/hydrothermal alteration. Geochemical investigations of the metaVOlcanic rocks using major and trace element abundances, and oxygen and strontium isotope distributions support the complex nature of these rocks. Metasomatic alteration of the Basement Series is manifested by a region-Wide zoning of the alkalis whereby aNa-rich inner- and a K-rich outer zone was formed. The invasion of coarse granite supplied the required temperature gradients for such zoning. In the early stages the metasomatic fluid consisted of an alkali-rich vapor phase, which probably originated from the invading granite. The alkali-rich metasomatic fluid evolved into an aqueous phase as a result of cooling upon interaction with the wall rock and mixing with connate and underground waters. Degassing of the intruding granite and a change in volume of rock as a result of alkali metasomatism opened widespread fractures in the brittle, felsic, volcanic country rock. Decrease in the pH of the aqueous solutions created a relatively corrosive fluid which attacked and leached the host rock and primary ore layers. The metasomatic/hydrothermal alteration appears to disturb the REE distribution of the Basement Series to a great extent. Using the variations in the REE abundances as a criterion, the least modified (relatively "fresh") felsic metavolcanic rocks are assigned to a (high-potassium) calcalkaline magma. The main magnetite-apatite mineralization (Main Ore) occurs essentially in felsic metavolcanics from the Basement Series and consists of several massive ore layers running roughly concordant with the stratification of the host rock. Cross-cutting contacts of coarse granite through layered ore, and evidence for hydrothermal alteration of the latter indicate that the concordant ores were emplaced before intrusion of the coarse granite. Extensive drilling in the area indicated the presence of over 30 million tons of minable ore with a grade of 40 wt.% Fe and 0.8 wt.% P. Mineralogically the major ore layers consist of varying amounts and combinations of magnetite, apatite and actinolite and subordinate to accessory amounts of sphene, epidote and salite. The occurrence of up to 15 cm long apatite crystals is one of the unique features of the Avnik ores. Presence of ores which cut across the Main,Ore and of mineral assemblages different from the Main Ore suggest the occurrence of subsequent stages in the history of mineralization. These ores are called Post Main Ores and include redistributed and recrystalized ore types. The major redistributed ores consist of disseminated and laminated magnetite and apatite ores associated with considerable amounts of amphibole, sphene, biotite, chlorite, epidote, quartz, feldspar, muscovite and small amounts of calcite, tourmaline and pyrite. Cross-cutting veins related to the redistribution stage are generally barren and contain usually quartz, feldspar, epidote, amphibole and hematite. Recrystallization is associated with the regional metamorphism and affected both the massive and redistribut~d ores. The observed mineralogic assemblages, the transition element distribution in magnetite and the REE distribution in apatite, all show a strong similarity to the Kiruna type ores of Sweden. The occurrence of similar deposits is also reported from the Bafq region in Central Iran. It is concluded that the Avnik apatite-bearing magnetite ores were evolved in at least 3 separate stages: (1)" Volcanogenic Ore Stage "; (2) "Redistributed O~e Stage "; and (3)" Recrystallization Stage" The occurrence of the Main Ore is related to aerial/sub-surface volcanism during Cambrian-Precambrian times. An eXhalative-sedimentary origin for the Main Ores is rejected on mineralogical and geochemical grounds. Surface eruption and sub-surface injection of ore-magma are proposed as the mechanisms of ore emplacement. Based on rare-earth element distribution in apatites liquid immiscibility is proposed as a process of primary ore generation. However, textural evidence to support the hypothesis of liquid immiscibility is mostly destroyed by younger metasomatic/hydrothermal modifications and recrystallization. The redistributed ores are generated by redeposition of leached material. No eVidence is found to suggest that the coarse granite itself supplied any mineralizing solutions. The Early Alpine regional metamorphism caused: (1) recrystallization and weak to moderate penetrative textures in the mineralized zones; and (2) a strong re-equilibration of oxygen isotopes
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