علوم زمین
سال:1392 | دوره:- | شماره:88 |تعداد مقالات:6

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.

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.

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.