The southern Abbas Abad iron skarn deposit is located in the western part of the Central Iran zone, SW edge of the Urumieh-Dokhtar magmatic arc. The skarn alteration occurred mainly at the contact of the Abbas Abad gabbro and Triassic Nayband Formation. The Triassic Nayband Formation in the southern Abbas Abad area consists of limestone, shale, siltstone, and marl with interlayers of tuff. The Abbas Abad gabbro is calc-alkaline with adakitic signatures and continental arc setting affinity. Four iron ore bodies occur along the NW-SE trending normal fault along the southern margin of Abbas Abad gabbro within sedimentary host rocks of the Nayband Formation. Three main paragenetic stages of skarnification and ore deposition have been recognized: i) prograde stage, ii) retrograde stage, and iii) supergene stage. Spinel, garnet, and pyroxene formed during the prograde stage. Epidote, tremolite-actinolite, calcite and ore minerals, such as magnetite, hematite-I, pyrite, and minor chalcopyrite formed the retrograde stage assemblage. Magnetite is the main economic ore mineral, which is diverse in terms of fabric and texture, including massive, disseminated, needle-like, blade-like, prismatic, crustification, and cubes. Stable isotope data from Abbas Abad iron skarn deposit indicated that the interaction of non-magmatic brine (connate) fluids with Triassic carbonate host rocks at high temperature and its mixing with meteoric water has led to changes in the isotopic composition of the mineralizing fluid. Subsequently, garnet and magnetite with relatively high estimated fluid delta 18O values (average 12.3 and 14.6 parts per thousand, respectively) formed from this modified fluid. The estimated delta 18O values of fluid in equilibrium with calcite decreased as a result of the greater influx of the meteoric water into the hydrothermal system in the late stages of ore formation. Fluid in-clusion data from this iron skarn deposit are consistent with minor boiling during the prograde stage, although cooling and dilution seem to be principally responsible for Fe precipitation. The petrogenetic model proposed for the Southern Abbas Abad iron skarn deposit suggests that during the Eocene a gabbroic stock intruded the sedimentary rocks of the Nayband Formation and acted as a heat source for the non-magmatic brine (connate and meteoric) fluids convection cells, then reacted with cooler Triassic carbonate rocks. In this model, leaching of the gabbroic rock provided the iron, whereas carbon came from degassing and dissolution of carbonate rocks during contact metamorphism-metasomatism.
Jamali, H., Dilek, Y., Daliran, F., Yaghubpur, A., Mehrabi, B.
International Geology Review (00206814)52(4-6)pp. 608-630
The Cenozoic Ahar-Arasbaran volcanic belt (AHAVB) in northern Iran is part of the Alborz-Azerbaijan magmatic zone, which developed along the southern margin of Eurasia. Upper Jurassic-Cretaceous flysch deposits and platform carbonates were deposited adjacent to this margin, are overlain by Eocene volcanic rocks, and are intruded by Oligo-Miocene, shallow-crustal to hypabyssal plutons that collectively make up the AHAVB. The volcanic and plutonic rocks have medium- to high-K, calc-alkaline to alkaline compositions and show similar geochemical features, indicating a common subduction-metasomatized continental lithospheric mantle source. Granodioritic, tonalitic, and quartz monzonitic plutons provided the heat flux and magmatic fluids that played a major role in the precipitation of porphyry, skarn, and epithermal copper, molybdenum, and gold deposits. The distribution of mineralization shows three major zones parallel to the general NW-SE trend of the AHAVB. Most of the major CuMo porphyry and skarn deposits occur in Zone A in the north, whereas porphyry and epithermal Cu, Au, and As deposits are present in Zone C in the south. Epithermal Au mineralization occupies a narrow, intermediate Zone B. NW-trending transtensional fault systems locally control the distribution of both late-stage volcanism and mineralization. Cenozoic magmatism and attendant mineralization in the AHAVB were a result of post-collisional, slab breakoff-induced heat and fluid flow from upwelling asthenosphere beneath the young orogenic belt. Magmatically induced transient increase in the geothermal gradient and associated extensional deformation weakened the orogenic crust and produced enhanced permeability, facilitating the emplacement of granodioritic to monzonitic plutons at shallow crustal depths as well as the associated mineralization.
