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Maryam Abdi, Department of Geology (Islamic Republic of Iran)
Majid Ghaderi, Department of Geology (Islamic Republic of Iran)
Nematollah Rashidnejad-Omran, Department of Geology (Islamic Republic of Iran)
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Various types of tungsten and base metal mineralization are reported in southwest of Shazand, west Central Iran. There are several stratiform-stratabound tungsten mineralizations in the carbonated sediments (lightly metamorphosed), with alternative meta-tuff and meta-volcanic rocks (e.g., Bamsar, Bowaki and Aghbolagh); and so in the volcano-sedimentary rocks (metarhyolite, semipelitic schist) which metamorphosed in amphibolite facies (e.g., Azna) near the southern and central parts of this district. The hydrothermal W-Cu (Sn, Pb, Zn, Au) quartz-tourmaline veins at Nezam-abad and Deh-hossein areas have occurred in north of the studied area, by middle Jurassic biotitic-granite and Jurassic meta-sandstone host rocks, respectively; both of which show open space filling and veinlet textures.
Based on field and textural criteria, three groups of tourmaline-rich rocks are distinguished: (1) Pre-granitoid intrusive bodies (clastic metasedimentary rocks of the upper Triassic?early Jurassic volcano-sedimentary units, orthogneisses and myllonitic granite rocks) affected by both regional metamorphism and deformation during the Tethys subductions; W stratiform-stratabound mineral deposits occur in these units. (2) Tourmalinites related to the granitic complex (granitic pegmatite and biotitic granite) of Nezam-abad - Malmir. (3) Tourmaline-quartz ore-veins. Petrographic observations, X-ray analysis, and other analytical data demonstrate different chemistry for the tourmalines. Overall, variations in the Mg/(Mg+Fe) ratios decrease from metasediments and orthogneisses through myllonitic granite and ore veins (first group) to granitic/pegmatitic rocks (second group). Type of tourmaline in the ore veins and metasediments (first group) is dravite with chemical formula as Na(Mg,V)3Al6(BO3)3Si6O18(OH)4; whereas the those from the granites and pegmatites is schorl with chemical formula as NaFe3Al6(BO3)3Si6O18(OH)4.
Textural and geochemical data are consistent with a pelitic/metamorphic source for the protolith of the tourmalines from the ore veins. Presence of compatible element of V in the tourmalines of ore veins and its low content in granites and pegmatites (about 20 ppm) as well as its high content in the ore-bearing metasediments (160-190 ppm), show that the tourmaline in the ore veins (vanadium dravite) have a sedimentary/metamorphic nature. Barren granites and pegmatites, and their different type of tourmaline and REE patterns compared with those of the ore veins, show that the ore veins aren't related to the granitic complex. Therefore, tungsten and other metals could not have a granitic origin.
The observations support an important crustal cycling of boron, in southwest of Shazand where tourmalines from the ore veins were remobilized by a combination of mechanical and chemical processes during deformation, metamorphism, anatexis and hydrothermal fluids cycle, leading to the formation of sedimentary-metamorphic tourmaline-bearing veins.
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