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Stable isotope and fluid inclusion evidence for the origin of Sorkhe Dizaj iron oxide-apatite deposit, NW Iran

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Ghasem Nabatian, Majid Ghaderi

Department of Geology, Tarbiat Modares University, Tehran, Iran,

gh.nabatian@hotmail.co.uk

 

 
 

Sorkheh-Dizaj iron oxide-apatite deposit is located in Alborz-Azarbaijan structural zone and is mainly hosted by quartz monzonitic to monzodioritic rocks and a lesser proportion within volcanoclastic rocks. The magnetite-apatite mineralization at the area occurs in three main zones with W-E direction, all of which have similar host rocks, alteration, mineralogy and texture. The mineralization is characterized by lenzoid, stockwork type and vein forms with massive, brecciate, vein-veinlet and banded magnetite-apatite textures. In the Sorkhe-Dizaj deposit, mineralization fills fractures and faults of the same trend (W-E direction) along parallel fault zones. The size of the vein mineralization varies from 1 to 20 m in width, 5-250 m in length, and up to 60 m in depth. The iron orebody in these deposits mainly consists of magnetite, apatite, actinolite, diopside and minor sulfide minerals such as pyrite and chalcopyrite. The host rock at the Sorkhe-Dizaj deposit is altered by actinolite, secondary K-feldspar, silica, sericite, chlorite and epidote. Alteration has mainly occurred in the fault zones and is declined with distance from the faults zones. In the altered rocks, there is a replacement of pyroxene by actinolite, chlorite and epidote and primary plagioclase phenocrysts by secondary K-feldspar, sericite and clay minerals. In this area, K-feldspar alteration is associated with secondary biotite. Primary alteration types such as actinolization and secondary K-feldspar are overprinted by hydrothermal alteration containing silicification, sericite, chlorite and epidote.

Fig. (a) Last ice-melting temperature for inclusions in apatite, (b) Fluid inclusion homogenization temperatures for inclusions in the apatite, (c) Last ice-melting temperature for inclusions in vein type last stage quartz, (d) Fluid inclusion homogenization temperatures for inclusions in the quartz, and (e) Last ice-melting temperature and homogenization diagram of fluid inclusions in apatite and quartz.

Fluid inclusion studies were carried out on apatite and late stage quartz to understand characteristics of the ore-bearing fluids. Most of the quartz crystals are clear with few small numbers of fluid inclusions the majority of which are 5-30 µm but fluid inclusions of the apatite are typically 20-100 and up to 150 µm. Three types of primary fluid inclusions are identified in the apatite (three-phase (L+V+S), two-phase (L+V), and mono-phase (L or V)) during microscopic studies and on the basis of their morphology. Some of these inclusions comprise trapped crystals of dark magnetite and/or red color hematite. The two-phase inclusions are abundant within these apatite crystals which are usually liquid rich. Also, two-phase (L+V) fluid inclusions are more abundant than mono-phase inclusions in the late stage quartz veins. Homogenization temperatures of apatite fluid inclusions are of 209º-520 ºC and indicate salinities of 9.08-21.61 wt% NaCl equiv (Fig.). δ18O values in the magnetite associated with apatite are 9.72-11 per mil at 341.8 ºC and those from disseminated magnetite within the host rock are 9.0-11.32 per mil at 341.8 ºC. Fluid inclusions in the late-stage veins of quartz homogenize at 186.4º-262.6 ºC and δ18O values are 2.5-7.4 per mil at 220 ºC (Fig.). Oxygen isotope in the late stage veins of carbonate has values of 3.28-6.14 per mil at 100 ºC. Based on these data in the late stage veins of quartz and carbonate, fluids may have been derived from the introduction of a cooler, less saline and isotopically depleted fluid such as meteoric water component. Fluid inclusions and δ18O isotope data show that the deposit generated by predominantly magmatic fluid. In the final stage of mineralization, mixing of cooler meteoric water with magmatic fluids has reduced δ18O values in magmatic fluids and may have facilitated precipitation of sulfides, quartz and carbonate veins.



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