Mineralogy of Iron-Rich Sediments of Benavi Area in Kurdistan Region—Northern Iraq

Abstract

Benavi iron-rich sediments (BIRS) is located about 20 km N–NW of Amadia district in Duhok Governorate – Northern Iraq. It occurs as an elongated E–W body within the Jurassic–Cretaceous sequences and extends about 2 km. The true thickness of BIRS outcrop ranges from 2.5 m to 12.8 m enclosed within highly fractured carbonate beds. Forty-eight samples collected from 10 sections represent the whole BIRS body. The sampling was systematic according to variations in color, hardness, texture, and iron content. Mineralogical study using instrumentations such as XRD, SEM, EDS, and TGA besides petrography showed that BIRS is composed of mineral assemblages: carbonates (calcite, siderite, ankerite), iron oxides/hydroxides (hematite, magnetite, goethite), sulphides (pyrite and arsenopyrite), silicates (kaolinite, chamosite, glauconite, quartz), and phosphates (collophane and apatite). Calcite is the predominant mineral in the whole BIRS, whereas siderite and ankerite are minor, and dolomite is trace. Shifts of the main real peaks values of calcite about the theoretical value in its d-space in all BIRS samples gave the indication that there is no pure calcite in most studied samples, but in a few samples it appears to be pure. Impure calcite presence is most likely due to Mg²⁺ and Fe²⁺ substitutions in the space lattice of the calcite crystals; this is supported by the EDS analysis which showed the presence of these elements in the calcite spectra. XRD results also showed that there is a negative relationship between calcite and iron oxides/hydroxides in BIRS. This is attributed to the loss of carbonate that took place during the oxidation of Fe²⁺ to Fe³⁺ and release of H¹⁺ where the initial calcite had been dissolved and replacement of iron oxides/hydroxides occurred, frequently, yielding interpretation for partial and complete replacements of calcareous constituents by iron minerals. The main iron minerals are hematite and goethite. The former is higher at the upper part of BIRS; contrarily, the latter is higher at the middle part. This study proved that different goethites coexist. They differ in crystallization and Al³⁺ content; these are Al-poor and Al-rich goethites, supported via the wide range of decomposition temperature by TGA (263ºC–350ºC); besides the peaks shifts to higher 2Ø angles indicating Al³⁺ substitution for Fe³⁺. Most of the primary Al-poor goethite altered to Al-poor hematite (by dehydration), hence the prevailing of the latter at the upper part, whereas Al-rich goethite which dominated at the middle of BIRS is a secondary and it is produced by goethitization of pyrite, siderite, and chamosite under surface weathering circumstances. High specific surface area of iron oxides/hydroxides probably acted as important sorbents to dissolve species: particularly heavy metals, phosphates, and arsenate, then under reduction conditions most of the adsorbed species released from iron oxides/hydroxides surfaces. This may give a clue about the presence of phosphate minerals, as well as the presence of arsenopyrite in the lower part of BIRS. Clay minerals of BIRS were represented by kaolinite and chamosite as well as a trace of glauconite noted in some samples. Paragenetically, kaolinite may have recrystallized or transformed to chamosite under reducing conditions in the availability of Mg²⁺ and iron oxides. Arsenopyrite, which has not been mentioned before, was confirmed by EDS and it is found restricted at the base of BIRS. The presence of arsenopyrite mineral in BIRS may give vital evidence to hydrothermal activations in this area. Frequently, finding other ores in the study area is promising.

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