Abstract
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
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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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