MINERALIZATION STUDY ON IRON DEPOSITS FROM DEHBID AREA, FARS PROVINCE, SOUTH IRAN: GEOCHEMICAL AND MINERALOGICAL DATA

M.A. Rajabzadeh, S. Rasti and A. FaghihDepartment of Earth Sciences, Faculty of Sciences, Shiraz University, Shiraz, Islamic Republic of Iran azam.rasti@yahoo.com

The Dehbid Fe deposit is located 176 km to the NE of Shiraz (the Capital of Fars province), in south Iran. In the classification of the structural units of Iran [4], this area is situated in the metamorphic Sanandaj-Sirjan zone along the northeast Zagros orogenic belt. The zone extends for 1,500 km along strike from Sanandaj in the NW to Sirjan in the SE of Iran. The rocks in this zone are the most highly deformed in the Zagros orogen [3, 5, 6 and 7]. The oldest units at Dehbid are metamorphic rocks composed of quartzitie, slate, phylite and recrystalized limestone intercalations (Sillurian) overlain unconformably by Permian rocks composed mainly of shallow water limestones and dolomites (Fig. 1). Extensively altered subvolcanic masses with maximum 10m2 extent and dioritic in nature are observed in contact with silicified dolomites at some places. The ores are hosted by metasedimentary rocks of late Paleozoic (Permian) and extensively silicified dolomite of early Mesozoic (Teriass). The ores are usually associated with faults and brecciated zones in host dolomite.

The center parts of iron veins are remarked by the presence of vuggy structure (Fig. 4). Locally, hematite has replaced magnetite and much of the hematite shows magnetite cores, indicating of precursor magnetite (Fig. 5).

DiscussionAt Dehbid area the presence of dolomite, microcrystalline quartz, amphibole and minor chlorite in brecciated zones associated with Fe mineralization suggests a hydrothermal origin for ores. The most plausible explanation for the origin of the mineralization is penetration of hydrothermal fluids along faults related to extension and orogenic collapse. The proposed genetic model involves early sedimentation of limestone and dolomite, introduction of silisic fluids followed by silicification, deformation and metamorphism, finally formation of magnetite ores by hydrothermal fluids. In Dehbid area, the observed diorites which crosscut the Teriassic rocks could represent the surface expression of larger magmatic systems at depth, that are regarded as sources of much of the heat, the metals, and at least a part of the hydrothermal fluids. The ore component originated at depth, were transported as highly mobile hydrothermal fluids along permeable fault zone or other controlling structures, and then emplaced at favorable rocks. The faults are thought to have tapped the sources of ore-bearing fluids at depth, and to have acted as channel-ways for their passage [2]. The ore fabrics suggest that ore formation took place as open space fillings. The presence hematite shows evidence of a fluid source from above and of oxidation, possibly by meteoric fluids. The high variation in Fe2O3 contents in short distances across the orebody along with the petrograpgic characteristics of ores in Dehbid could be explained by the ore formation by open space filling phenomenon. The high K2O (3-3.7 wt %) and relatively Na2O contents in the samples just near the ore –host contact show an immature potassic alteration halo that is indicated by formation minor K-feldespar and biotite in the altered zones. The preferential presence of Mn in some magnetites can be attributed to the variations in activity of the Mn ion in the hydrothermal fluids at the time of magnetite crystallization and/or dependent on compatible electronic spin states between Mn+2 and Fe+2. Co and Ni have ionic radius near to Fe+2 so as well as substitute it. Co/Ni ratio increases during magma differentiation so high Co/Ni ratios characterizes hydrothermal magnetite [1]. The ratio of the iron ores from Dehbid area varies from 0.17 to 6.3 with an average of 2.6 that is in the range of hydrothermal iron deposits (0.2 to 7) (Fig. 9). The P2O5 content is also typically low that supports the hydrothermal origin of mineralization; it increases from mineralized zones (2.4-91.2 ppm) to host rocks (30.5-233 ppm). The Cu content in ores varies from 9.1-132.3 ppm indicating that the deposit could not interpret to be a member of the IOCG (iron oxide-copper-gold) class of deposit.

Figure 1; Geological map of Dehbid area with tectonic scheme of Zagros in Iran (modified after [9]).

The mineralized zones, as veins with maximum 15m width and 200m length within silicified brown dolomite show a sharp contact with the wall rock. The ore veins trending NW-SE, plunging 65° SW crosscut dolomitic formation, suggestion that at least one deformation phase predated introduction of the Fe-bearing fluids (Fig. 2).

Figure 2; Ore vein within silicified dolomite.

Silicification is the main hydrothermal alteration that affected pervasively limestone and dolomite. It seems that the phenomenon acted as ground preparation for ore deposition. Potassic alteration is not extensive and is localized around fractures and faults, with alteration up to 1 m on either side of major faults and is distinguished by the formation of feldspar and minor biotite along with the overgrowth of quartz grains in the dolomite. Mineralogical studies show that the iron ores are dominated by magnetite. Mineralogical studies show that the ores are dominated by magnetite which occurred as open space fillings. Hematite, goethite, specularite and martite occurred as secondary minor phases. Gangue minerals include quartz, dolomite and minor feldspar. Quartz and minor dolomite occur throughout the orebody as angular remnants of host rocks cemented by magnetite (Fig. 3).

Figure 3; Silicified dolomite-breccias cemented by magnetite.

The texture of hematite is both granular and platy. Most of hematite resulted from the oxidation magnetite under surface or near-surface conditions and colloform textures are commonly observed in microscopic sections (Fig. 6). Martitization is common, and martite replaces magnetite on grain boundaries and cleavage planes. Tabular crystals of hematite or specularite growing perpendicular to other hematite grains or filling cavities, probably represent a later stage of intrusion of oxidizing fluids [8]. Goethite which is the principal secondary mineral, replaces hematite along grain boundaries and fractures. Rare sulfides (generally pyrite and chalcopyrite) as disseminated grains are also present. Paragenetically, sulfide formation postdates that of the oxides and the phases belong to the latest hydrothermal alteration stage. The very limited dioritic masses are extensively altered that indicated by altered plagioclase phenocrysts in a groundmass composed of amphibole, chlorite and microcrystalline quartz (Fig. 7).

Figure 4; Vuggy structure in vein centers. Figure 5; Magnetite remnants in hematite after replacement.

Figure 6; Colloform texture of hematite. Figure 7; Altered diorite in thin section.

Fe2O3 content in mineralized rocks varies between 38 to 73 wt%. The analyses indicate that Mn (average 2 wt%), Co (average 23.5 ppm) and Ni (average 15.9 ppm) substitute for Fe+2 in magnetite lattice. Fe2O3 with MnO indicates positive correlation (Fig. 8a). Among the other major oxides MgO shows the most concentration (0.7 to 5.3 wt %). Zn (up to 106 ppm) and Pb (up to 43 ppm) have been detected in the mineralized body. The P2O5 content in ore is typically low; it increases from mineralized zones (2.4-91.2 ppm) to host rocks (30.5-233 ppm). The Cu content in ores varies from 9.1 to 132.3 ppm and other precious metals have not been found at Dehbid. Ca in the host rock increase simultaneous with Sr (R=0.63) that show Sr is substituted Ca. The alkali elements content (Na2O+K2O) vary from 1.3 to 7.4 percent. The elements correlated positively (R=0.78) whereas K2O and Fe2O3 correlated negatively (R=-0.74). With increase distance of iron ore veins K2O content increases at host rocks, thus the minimum K2O content is observed in the ore veins center (Fig. 8b).

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Figure 8; a) Positive correlation between Fe2O3 and MnO in the ore samples, b) K2O contents across ore and host rocks (the sampling distance is 5 m) from Dehbid.

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Figure 9; Chemical data of Co and Ni in iron ores indicating hydrothermal origin of the Dehbid deposit.

ConclusionFractured and brecciated rocks within the detachment zone associated with high deformation during metamorphism and magma intrusion are characterized by a high permeability, and these deformed rocks were possible feeder channels for aqueous fluids driven by differential stress along the fault. Field observation and microscopic examination of the Dehbid ores samples indicate that magnetite (major primary oxide) was precipitated mainly in fractures and fissures between the rock fragments in breccias of the host rocks. This deposit is principally constitute of the magnetite that is superficially oxidized to hematite. Hematite also probably occurred during the later stage of hydrothermal event while oxidizing condition changes. The contact between ore and country rock is sharp and shows little evidence of replacement. The ore contains only minor quantities of elements other than the iron oxides. The ore component originated at depth, were transported as highly mobile hydrothermal fluids along permeable fault zones or other controlling structures, and emplaced at favorable rocks as open space fillings. A large magmatic system at depth could be regarded as the sources of much of the heat, the metals, and at least a part of the hydrothermal fluids. Field relationships and ore textures along with geochemical data on iron ore deposits at Dehbid area are consistent with a hydrothermal origin. Temperature decrease, redox changes, and fluid mixing represent effective mechanisms of ore precipitation.

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