GEOSCIENCES
Year:2001 | Volume:9 | Issue:37-38|Papers Count:6

The Ab-e-Torsh volcanic district contains the Paleogene outcrops of trachyandesitic and dacitic lava flows and fall out pyroclastic deposits (equivalent to Karaj Formation) which have been hydrothermally altered, The, abundant fractures and faults produced a cauldron subsidence. Subsequently, fissure eruptions occurred and later hydrothermal solutions were intruded through the country rocks.One the basis of the formation of the sulfate mineral (alunite), it seems that the acidic solutions (e.g. H2 8 and H2SO4) have been formed by the interaction of hypogene volcanic vapours with hot solutions. Because of the interaction of the acidic solution with the country rocks and their chemical leaching, silicic, allunitic, argillic and advanced argillic alteration zones were developed. The high permeability and reactivity of the pyroclastic deposits with respect to the hydrothrmal fluids facilitated the chemical reactions and hence the alteration products have been well developed in the latter unit. However, due to the presence of fractures the other units (e.g. trachyandesitic and dacitic lava flows) were also intruded by the solutions and altered.

Dasht-e Moghan in north-west Iran, is a part of Koura- Uoni region and contains one of the most complete paratethys sequences in Iran. In this area flyschoid facies deposited from Paleocene to Late Miocene and is underlined by Akchagyl Formation. The angular unconformity between Akchagyl Formation with respect to older rocks is related to the Attic orogeny event. Eustatic sea-level fluctuations occurred in the Upper Pliocene- Lower Quaternary which is indicated by fluvial lacustrine deposits equivalent to the Apsheron Formation.The passadenian orogeny caused another angular unconformity between young horizontal alluvium and older formations. The most complete outcrop of the Akchagyl Formation has been found in the Moghan section. The faunal assemblage of the Akchagyl Formation reveals an age corresponding to the Lower to Middle Akchagylian stage.

Dashkasan antimony-arsenic-gold deposit is located in west Iran, 42 km NE of Qorveh (Kordestan provice). The area is a p'art of Sanandaj- Sirjan magmatic-metamorphic zone. Based on geological observations, the oldest rock units in the area belong to the Jurassic serie and are slates, phyllites and quartzites. The youngest units are Neogene volcano-clastic conglomerates, basaltic flows, block lavas and Quaternary agglomerates. The plutonic rocks in the area consist of a Neogene (Pliocene?) calc-alkaline microgranite-microgranodiorite intrusive with microgranular porphyritictextures. The sub volcanic equivalent of these plutonic rocks, is represented as Agh Dagh and Sari Dagh dacite-rhyodacite domes Dashkasan mine is considered as a vein-type deposit in which its related mineralization is controlled by tectonic structures. The deposit is hosted by dacite, rhyodacite and microgranodiorite sub volcanic rocks which are mainly associated with silicic, argillic, and pyritic alterations. The ore parageneses in the veins includes quartz, stibnite, pyrite, realgar, orpiment, pyrrhotit, chalcopyrite, bornite, galena, boulangerite, aurostibite (?), gold, stibiconite, kermesite and iron-hydroxide. The Gold content of the veins ranges between 55 and 266ppm, whereas in sulfide-silicic zone, it ranges from 10 to 18 ppm. Gold grains are between 10 and 150 microns in diameter commonly occurring with quartz and in smaller sizes within the oxidized pyrites. Scanning Electron Microprobe (SEM) studies display quantities of Sb, Ag and Hg within the gold grains. The analytical results for samples from six litho geochemical profiles vertically cutting the main stibnite veins in Agh Dagh and Sari Dagh areas indicate that high-grade gold is limited to stibnite veins and also to silicic-pyritic alteration zones. Fluid inclusion studies reveal that homogenization temperatures range between 183 C and 255 C and salinity from 8.9 to 18.8 wt % NaCl equivalent. Field observations and laboratory studies show a close relationship between the antimony-arsenic-gold mineralization, and silicic solutions drived from sub volcanic body in the area. It is therefore suggested that, the mineralization belongs to epithermal group and acid-sulfate type.

The Tudeshk area in southeastern part of Nain, Central Iran is located in Urmiyeh- Dokhtar magmatic belt and is a part of Kajan 1:100000 map of GSI.The stratigraphic sequnce is composed of Mesozoic and Cenozoic strata, consisting of Cretaceous limestone, Tertiary volcanic rocks (Iavas and pyroclastics) and also Quaternary deposits.The Neogene section is indicated by Upper Red Formation (URF), and a dacitic (Ng d), unit. The rock type variations in Neogene volconics along with the geochemical and petrographic evidences indicate a kind of sodic character. Mixing and fractional crystalization in magmas, are important, phenomena which has been resulted in this study.Along with the above processes, the associated basic rocks have been strongly altered, caused by fluid pressures, high temperatures and the nature of late fumarolic fluids. This area has been subjected to a fault and fracture system with a NW-SW trend, indicates that the area has passed a brittle type tectonic regime, which follows the main trend of Urmiyeh-Dkhtar magmatic belt. This kind of extensional tectonic regime controlled the emplacement of igneous rocks The geochemical characteristics of basalts in Tudashak area (drown as spider diagrams in special cases) has been compared with the basaltic rock-typs in orther places of the world as: Parana (South America), Rio Grande (North America), Deccan (India) and Ethiopia. All of analyis results plot in sub alkaline field of diagram, indicating contamination with amphibolites gneisses of sialic crust during the initial stages of ascending. Similarity with the Tudashak Neogene basalts shows a considerable similarity with the Rio Grande Basalts and forms this point of view. It could be deduced that the partial melting of a depleted mantle is responsible for the formation of this basalts, and has a similarity to the steans Basalts of North America Basin and Range.

Strike-slip faults are affected by many variants, which do not remain constant along the strike. The most common factors are differences in the rate of displacement and the width of shear zone. A study of these variants on the Nehbandan dextral strike-slip fault in east Iran has been done and the experimental models shows that the maximun displacement and width of shear zone occurs in the central part of strike - slip faults. But the displacement decreases exponentially toward the termination of faults. The relationship between these two factors can be formulated as Y = C.e gd The gd coefficient in the terminal splays is smaller than in the main fault. These can be seen on experimental models, which have been done by flexy-glass materials. Comparison between displacement - distance and width of shear zone-distance curves from the field traverse (Nehbandan data) and experimental materials (flexy-glass) shows the experimental curves are symmetry but the "natural" curves are asymmetrical. This difference is due to homogeneous nature of manufactured materials while the earth's crust is heterogeneous.

The Tudeshk area in southeastern part of Nain, Central Iran is located in Urmiyeh- Dokhtar magmatic belt and is a part of Kajan 1:100000 map of GSI.The stratigraphic sequnce is composed of Mesozoic and Cenozoic strata, consisting of Cretaceous limestone, Tertiary volcanic rocks (Iavas and pyroclastics) and also Quaternary deposits.The Neogene section is indicated by Upper Red Formation (URF), and a dacitic (Ng d), unit. The rock type variations in Neogene volconics along with the geochemical and petrographic evidences indicate a kind of sodic character. Mixing and fractional crystalization in magmas, are important, phenomena which has been resulted in this study.Along with the above processes, the associated basic rocks have been strongly altered, caused by fluid pressures, high temperatures and the nature of late fumarolic fluids. This area has been subjected to a fault and fracture system with a NW-SW trend, indicates that the area has passed a brittle type tectonic regime, which follows the main trend of Urmiyeh-Dkhtar magmatic belt. This kind of extensional tectonic regime controlled the emplacement of igneous rocks The geochemical characteristics of basalts in Tudashak area (drown as spider diagrams in special cases) has been compared with the basaltic rock-typs in orther places of the world as: Parana (South America), Rio Grande (North America), Deccan (India) and Ethiopia. All of analyis results plot in sub alkaline field of diagram, indicating contamination with amphibolites gneisses of sialic crust during the initial stages of ascending. Similarity with the Tudashak Neogene basalts shows a considerable similarity with the Rio Grande Basalts and forms this point of view. It could be deduced that the partial melting of a depleted mantle is responsible for the formation of this basalts, and has a similarity to the steans Basalts of North America Basin and Range.