5th Swiss Geoscience Meeting, Geneva 2007
Petrography and geochemistry investigation on
hyrothermal altered igneous rocks in NE of Arak
Ayati Fezzeh*, Asadi Haroni Hooshang**, Noghreyan Mosa* and
Mahdevari Saeed**
*Geology Department, Isfahan University, Hezar Jarib Street, Iran
(F_ayati@geol.ui.ac.ir) (Mosa.Noghreyan@hotmail.com)
**Mining Department, Isfahan University of technology, Iran
(hooshang_asadi@yahoo.com)(mahdevari@yahoo.com)
The study area is emplaced within the central province of Iran about 120 km
SW Tehran. Regionally, this area lies near the contact of the main Iranian
volcanic belt (Urumieh-Dokhtar) and Sanandaj-Sirjan zone (volcanosedimentary), of which both are subdivisions of the Zagros orogenic belt and
product of subduction and closure of the new Tethys Ocean. Urumieh-Dokhtar
magmatic belt is part of the Alpian-Himalyan orogenic and metallogenic belt and
represents an Andean type magmatism belt formed by subduction along the
active continental margines of central Iranian block, during the Alpian orogeny
(Tackin, 1979, Berberian et al., 1982, Sengor, 1990, Alavi, 1994).
Locally, the area is underlain by Paleogene andesitic volcanics and pyroclastics
intruded by Pliocene lensoid bodies of diorite, q- diorite, tonalite, dacite and
rarely granodiorite porphyries with calcalkaline affinity. The calkalkaline igneous
roks are in porphyritic texture and composed mainly of plagioclase, amphibole,
biotite and quartz as major minerals. The principal opaque phases are
magnetite, hematite and to a lesser extent pyrite. Zircon, apatite and sphene
are accessory minerals. Plagioclase occurs as phenocryst and microphenocryst
of variable size, almost displaying oscillatory zoning. The groundmass of
volcanic rocks is composed of microlites of plagioclase associated with a lesser
amount of sanidine. The average composition of plagioclase is An40-45.
Initial surface sampling, which was carried out to produce the first geological
and geochemical reports in the study area and its surroundings recognized a
porphyry type Cu-Au mineralization (Asadi, 2005). On the basis of detailed
petrographic studies on rock samples, it was established an extensive potassic
alteration zone, which is characterized by the secondary biotite and rare or
without secondary K-feldspar grains. This alteration assemblage is represented
by secondary biotite, feldspar, quartz, chalcopyrite, pyrite, and magnetite.
Magmatic biotite grains subjected to potassic alteration are either partially
chloritized or completely recrystallized to secondary biotite; the latter are
observed as small aggregate clusters. The euhedral to subhedral mica
phenocrysts up to 3 mm in size display khaki to dark brown colors. They show
different degreee of alteration to secondary biotite, chlorite and sphene.
Sometimes they replace amphibole. Pleochroic amphiboles, the dominant
ferromagnesian minerals in these igneous rocks are green with euhedral to
subhedral forms which together with plagioclase and minor amount of pyroxene
phenocrysts in groundmass yield a porphyritic texture to volcanoplutonic rocks.
Amphiboles are very sensitive to potassic alteration and replace by secondary
biotite, sericite and chlorite. Quartz, sericite and pyrite constitute another
alteration assemblege called phyllic zone. This alteration is associated with
quartz veins. It is highly destructive, commonly showing complete replacment of
5th Swiss Geoscience Meeting, Geneva 2007
the original mineral assemblage. In the phyllic alteration zone, plagioclase is
mainly altered to sericite and biotite is altered to chlorite. The Propylitic
alteration forms a halo around the mineralized pottasic zone. It is characterized
by epidote, chlorite, calcite, and locally actinolite. These alteration minerals are
formed by the replacement of primary amphibole and plagioclase in
groundmass. Biotite phenocrysts are partially altered to epidote, chlorite and
sericite. The argillic zone was locally developed in this area. These rocks are
moderate to strongly enriched in highly incompatible elements, whereas
depleted in compatible elements. On the primitive mantle – normalized diagram,
rocks are enriched in LILE with respect to LREE and HFSE. All these evidences
along with strong negative anomalies for Nb and Ti and positive anomalies for
Pb are characteristics of an orogenic magmatism and convergent margin
magmas.
REFERENCES
Alavi, M., 1994: Tectonic of the Zagros orogenic belt of Iran, new data and
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Asadi H., 2005: Preliminary Exploration at Dalli Porphyry Cu Prospect, Central
Province of Iran, DORSA Engineering Limited, 22p.
Berberian F., Muir, L. D., Pankhurst, R. J. & Berberian M., 1982: Late
Cretaceous and Early Miocene Andean – type plutonic activity in northern
Makran and central Iran: Journal of Geological Society of London 139, 605-614.
Sengor, A. M. C., 1990: A new modle for the Late Palaeozoic-Mesozoic tectonic
evolution of Iran and implication for Oman. Geological Society of London
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