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

Ghasem Nabatian, Majid Ghaderi

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

gh.nabatian@hotmail.co.uk

S orkheh-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.