PETROLOGY
Year:2017 | Volume:8 | Issue:30 |Papers Count:8

The Vejeh Skarn is located at 70 km Northeast of Isfahan. The Vejeh intrusive body with granodiorite to tonalite composition have been intruded the lower Cretaceous marl carbonate which caused the formation of skarn rocks. On the basis of field and mineralogical studies skarn type is calcic with low Mg amount. The mineral assemblage including wollastonite, pyroxene, garnet, epidote, quartz and calcite, have been taken place at progressive and retrogressive stages. Wollastonite and pyroxene (diopside – hedenbergite) were formed at primary progressive stage whereas garnet (grossular-andradite) was formed at secondary progressive stage through metasomatism of pyroxene and wollastonite. In the retrogressive stage with increasing ¦O2, the garnet was altered and replaced by epidote and secondary calcite and quartz. Regarding mineral assemblage it could be deduced the temperature of skarn formation have been reached to 500 -700oC in the progressive stage and then decreased to 470oC in the late stage.

The Siahbaz granite-gabbro-appinite outcrops from the Northwest Khoy is related to one of the complex of the Late Ordovician-Early Permian. The complex is located on the north Sanandaj-Sirjan Zone. Synchronous activities of the mafic and felsic melts indicate that hydrous mafic melts during or after injection to the crust have been able to mingle with felsic magmas produced from the base of the crust. Fractional crystallization in the magma chambers of the hydrous mafic melts caused to form appinite mafic rocks c.25 to 60 modal percent of hornblende and gabbros. The felsic melts could form A-type granitic rocks. The mineralogical observations along with geochemical and tectonical studies show that subduction of the Paleo-Tethys under an island arc (Siahbaz) in the Northwest Iran were produced all rock types at the Late Ordovician-Early Permian. The hydrous mafic melts were produced in the mantle wedge because of the presence of subduction fluids and then transferred to the base of the island arc, formed the A-type felsic liquids as a result of the partial melting of the base. Convection in the mantle wedge in the supra subduction zone, led to the formation of an extensional setting in the region (the extensional region in the active margin); hence, the conditions for the formation of A-type felsic melts were provided.

The Ardestan study area lies in the northeast of Isfahan and at the outer margin of Urumieh -Dokhtar Volcanic Arc (UDMA). In this area, copper mineralization is associated with dikes. Mineralization occurred as sulfides (chalcocite, chalcopyrite and bornite) and oxides (malachite and azurite). According to field studies as well as petrographic and geochemical investigations, two different types of dikes are present. The first type, trending NW-SE and comprising fine crystalline gabbro, whereas the second type with relatively E-W trend are gabbro and pyroxene diorite. Geochemically, these rock are characterized by SiO2=45.8 to 52.8 wt.%, MgO with 6.9 wt.% (average), Na2O+K2O=5.6 wt.%, and Al2O3/TiO2=16.8%. All dikes are alkaline, related to back-arc tectonic setting in a wider concept associated with changing in source of magmatism. The second type shows enrichment in Ba, Sr, Rb, K, Zr, Nb, Ti, Cr and Ni in comparison to the first type. The first type is generated as a result of a subducted modified mantle while the latter shows an enriched astenospheric mantle source. It appears that there is a weak correlation between ore-forming and volatile elements in the mafic dikes. Overall, the same tectonic stresses are an essential controlling factor for the formation of second type E-W dikes associated with mineralization.

Monogenic basaltic cinder cones and lava flows from west of Khash are part of volcanic arc of northern Makran, formed as a result of subduction of Oman oceanic lithosphere beneath the Eurasian plate. The basalts belong to medium-K calc-alkaline series as they contain high Al2O3 (16.5- 19.04 wt. %) and CaO (8.4- 12.0 wt. %) and moderate amounts of K2O (0.5- 1.1 wt. %). They share arc geochemical features such as high LILE/HFSE ([Rb/Zr]N-MORB up to 19) LILE/LREE ([Ba/La] N-MORB up to 4.86) and LREE/HREE ([La/Yb]N-MORB up to 10), and depletion of Ta, Nb, Zr, and Ti relative to N-MORB. Partial melting models indicate that near-primary basalts were derived from an enriched source type mantle wedge peridotite after low to medium degrees (2-10%) of partial melting. This source peridotite was enriched in LREE and LILE, by subduction derived fluids in the supra-subduction zone. Negative correlation of Th/La vs. Sm/La, and relationships between Pb/Ce and Th/Nb values of the studied basalts which are between two end compositions of global subducting sediment (GLOSS) and N-MORB are indicative of significant contribution of subducting sediments to the genesis of the basaltic rocks. Estimates made using binary mixing model are indicative of about 16% of sediment participation in the magma genesis. Low Pb/Ce ratio (1.6 -11.1), compared to OIB (>20) may be a signature of participation of fluids resulted from dehydration of the subducting slab.

The Almas area in NW of the East Azarbaijan Province, is a part of the Soltaniye-Misho zone. The lamprophyric dikes crosscut the Kahar Formation. The principal minerals of these rocks are amphibole (amphibole phenocrysts are longer than 3 cm), biotite, pyroxene, olivine, plagioclase, apatite, calcite and chlorite with porphyritic texture. According to mineralogical and geochemical evidences, the studied lamprophyres are camptonite with alkaline nature. The plotted spider diagrams indicate that the studied lamprophyres are enriched in light rare earth elements (LREE) and incompatible elements in comparison to heavy rare earth elements (HREE). The parent magma is probably generated from spinel lherzolite mantle with low rate partial melting.

The Nabijan ore index located 20 km southwest of Kaleibar city, East Azerbaijan province in the Alborz-Azerbaijan structural zone. Geological rock units outcropping in the area consist of volcano-sedimentary rocks (trachyandesite, andesite, shale and limestone) of the Cretaceous age which intruded by the Oligo-Miocene monzogranite, monzogranodiorite and diorite. These intrusions and related hydrothermal activities caused the Au mineralization in the Cretaceous host rocks, as well as in the monzogranitic intrusion. The mineralization in the monzogranite occurred as silicic stockwork and sheeted veins with pyrite and minor chalcopyrite, sphalerite and galena. The other type of mineralization is skarn, which present as sparse irregular veinlets with less amount of Au in the Cretaceous carbonates. Preliminary estimation of ore reserve shows 320000t reservoir with average of Au grade about 1.37 gr/ton. According to surface and drilling data, two important anomalies determined. One in the contact zone of intrusive rocks with the Cretaceous volcanics and carbonates (skarn zone). In this anomaly, the correlation coefficient index between Au and Cu, Pb, Zn, Ag, and As is low whereas, in the second one which coincides with stockwork mineralization within monzogranite, it is high.

The Agh-Otagh mineralization area in the north of Takab, was formed within the andesistic tuffaceous rocks of the Oligo- Miocene age. Mineralization include polymetallic (Cu-Pb-Zn-Au-Ag) quartz veins and silicified zones, which occurred as breccia and vein- veinlets with comb, cockade and disseminated textures. Chalcopyrite, pyrite, galena and sphalerite are common ore minerals. Alteration zones consist of silicification, sericitization, argillitic, propelitic and carbonatization. Cu-Au mineralization is associated with silicification and sericitization. Analytical results of the samples from the ore- bearing quartz veins and the silicified zones indicate that the highest grade for Au is 664 ppb (ave.181 ppb). The highest and the average grades for Ag, Cu, Pb, and Zn are 120 ppm (300 ppm), 1.3 % (0.38 %), 5.5 % (0.06 %) and 4.5 % (0.28 %), respectively. The investigations indicate that the Agh-Otagh mineralization was formed in four stages. In the first stage or the pre-mineralization stage, the host rock, as a result of hydrothermal process, underwent brecciation and some quartz veins and siliceous cap were formed. In the second stage or the mineralization stage the sulfide minerals formed within the quartz veins and silicification zones developed at the third stage, some unmineralized quartz, barite and carbonate vein- veinlets crosscut the previous stages. The last stage of mineralization related to supergene processes. Based on geological, mineralogical, alteration, structural and textural evidences, the Agh-Otagh base- precious metal mineralization is similar to the medium sulfidation epithermal deposits.

The Shirinak volcanic rocks, known as Dahaj-Sarduieh belt in Kerman province, are exposed southeast of Urumieh-Dokhtar volcanic belt. Petrographically, the volcanic rocks are basalts and andesite, which consist mainly of plagioclase, clinopyroxene, olivine as well as calcite, quartz and chlorite as the secondary minerals. All of these minerals set in fine grain matrix with porphyric and glomeroporphyric textures. Based on mineral chemistry data, plagioclases range from labradorite to bytownite and have been undergone compositional and thermal mixing. They mostly show sieve texture. CSD (crystal size distribution) study shows that the shape of plagioclase microlites is tablet with aspect ratio of 1: 7: 10 for short: intermediate: long axes, respectively. Moreover, three-dimensional shape of plagioclase crystals, nucleation and growth time were estimated 40.27 years, which is completely consistent with the nature of basalt. Based on dip of CSD diagram, magma mixing process has been clearly involved in the magma genesis. The pyroxenes studied are augite in composition that were physically crystalized in moderate to high pressure and temperature of 550-1110oC. They crystallized from a magma likely with about 10% fluid and in variable ¦O2 condition. On the base of pyroxene chemistry, the basic rocks from Shirinak belong to tholeiitic to calcalkaline series in volcanic arc setting (Neo-Tethys subduction).