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GEOMETRIC AND KINEMATIC EVOLUTION OF PROGRESSIVE DEFORMATION IN SHEAR ZONES, SAGHEZ GOLD MINE, NW IRAN
Abstract
The Saghez shear zone is part of metamorphic belt of the Sanandaj-Sirjan zone in northwest Iran that uplifted during continental collision between the Afro-Arabian continent and the Iranian microcontinent. The mine is being exploited by open-pits mining, with present reserves estimated at 3.2gr/t gold. The currently favored genetic model is an example of porphyry system, with mineralization being closely to shear bands and emplacement of granit and pegmatitic veins. Rocks from Sagez shear zone consist of metavolcanic, phillite and phillonite are foliated, compositionally laminated and folded at different scale from kilometer to sub-millimeter. Shear bands show compression and shear features which form both a planar foliation and a weak compositional banding within the rock. In samples from study area, these features include pressure solution surfaces, sigmoidal and planar shear surfaces, cleavage planes, kink-bands, zones of crystal brecciation and cataclasis. Three episodes of deformation were recognized in the Sagez shear zone. First stage (D1) is not mapable in mesoscopic scale but observed by bedding parallel continues shistosity (S1) under the optical microscope. The second stage (D2) is well characterized by isoclinal recumbent folds with axial plane schistosity (S2) folded by the later generations. Numerous quartz veins and veins with pegmatite textures were folded with the S2. The third one (D3) developed parallel to the axial plane of close to tight upright to overturned folds. At least, two main metamorphic events accompanied with deformation episodes observed by mineral assemblages. The variation in mineral chemistry allows some estimation of P-T conditions through (D1) and (D2) stages of deformation, and indicates that the rocks experienced minimum of 550 В° C and 450 В° C during (D1) and (D2), respectively, and a minimum pressure of 4kbar during both (D1) and (D2).
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