CHARACTERISTICS AND GENESIS OF SEDIMENT-HOSTED IRON DEPOSITS IN SANANDAJ-SIRJAN BELT, WEST-CENTRAL IRAN

Numerous sediment-hosted Iron deposits occur in Sanandaj-Sirjan metamorphic-plutonic Belt (SSB), Iran. Host rocks vary from sandstone to siltstone, shale, limestone, dolostone and less commonly chert and volcanic materials metamorphosed under low-medium P-T conditions. Four main periods of sediment-hosted iron formation can be distinguished: Late Precambrian, Ordovician-Silurian, Permian, Upper Triassic-Lower Jurassic. The ore bodies occur as conformable layers and lenses and in massive and banded textures, the latter distinguished by alternating magnetite (hematite) ± silica ± barite ± carbonate (calcite, dolomite, siderite) bands. Small oolitic iron deposits occur, particularly in the Upper Triassic-Lower Jurassic sandstones. Here we report data from Mooteh, Abpooneh and Chahchaghok deposits in central and southern SSB.

Deformation and metamorphism have modified the original mineralogy, textures and structures. The iron ore in Mooteh is partly associated with calc-silicate assemblages, and the footwall schists in Chahchaghok contain Fe-rich amphiboles. The ore bodies are locally sheared, boudined and dismembered. Representative samples, analyzed at ACME Labs, Vancouver, are characterized by low TiO₂ (<0.1%), P₂O₅ (<0.2%), Ni, Co, V (up to 10, 15, 120 ppm) and Hf, Nb (<1 ppm) but high and variable MnO (up to 5%). REE display large variations (7-62 ppm) and high La_N/Yb_Nratios (5-20). Ce/Ce* varies between 0.8-1.20; Eu/Eu* is highly variable (0.3-7). A distinct correlation exists between the abundances of Ba and LREE and the Eu/Eu* values.

The iron ores are enriched in Zn, Pb, Cu, Mo (up to 8600, 2300, 760, 38 ppm) and Au, Ag (0.07, 2 ppm). The δ³⁴S values for barite from banded ore in Mooteh and Abpooneh analyzed at GG-Hatch Stable Isotope Labs, University of Ottawa, fall in the range19.5-28‰ and 3.4-6.6‰, respectively, consistent with the Late Proterozoic and modified Triassic marine sulfates and the stratigraphic ages of the associated rocks, respectively. The ore mineralogy, textures and geochemistry suggest a sedimentary-diagenetic origin dominated by chemical precipitation. Ore deposition occurred in a marginal basin with episodic volcanic inputs. The iron deposits may represent the distal or shallow-water and oxidized equivalents of black shale/tuff -hosted exhalative base-metal ores.