Mineralogy, Geochemistry, and Origin of Nickel-Bearing Laterites in the Northwestern of Noorabad (Lorestan Province)

The nickel-bearing laterites of Noorabad area are seen in the high Zagros zone along with Kermanshah ophiolitic complex. The ophiolitic complex in the studied area includes serpentinized peridotites, isotropic gabbros, plagiogranite, sheeted dykes, basaltic lavas, andesite, and radiolarite. The peridotites of this ophiolitic complex include dunite, harzburgite, and lherzolite. The tectonic activities have caused tectonization in these rocks in such a way that it has facilitated their alteration process and created laterite zones. The alterations of peridotite rocks include serpentine, dolomite, hematite, and silicification. The laterite zone is layered and lens-shaped red horizon on peridotite rocks and is covered by Miocene limestones. Based on XRD analysis, mineralogy of the lateritic zone includes dolomite, quartz, smectite, and serpentine. The ratio of La/Y changes indicates that the studied laterites were formed in alkaline pH. According to the discrimination diagrams of the source rock, the studied laterites are karst bauxites with ultrabasic origin. Introduction Laterites are formed in areas with long tectonic stability and low erosion rates (Brand et al., 1998). Usually, the source rock of these laterites is serpentinites. Nickel is washed from the profile upper parts and concentrates in the lower parts, causing the enrichment and formation of nickel-bearing laterites. Most nickel-bearing laterite profiles have two ore types, an oxide component and a hydrous silicate or clay silicate component (Brand et al., 1998). The richest deposits of nickel-bearing laterite are formed where oxide-rich regoliths rise and nickel is washed down to the new silicates in saprolite (absolute accumulation) to be concentrated (Butt & Cluzel, 2013). This study tried to investigate the field geology, mineralogy, lateritization process, and geochemistry of Noorabad laterites (northwest of Lorestan province, Iran). Materials and methods The field operations were carried out in several rounds and led to the collection of 30 samples from serpentines and laterites of the studied area. Then, by preparing 25 thin and polished sections, the geological characteristics of ores, especially their texture and associated minerals, were discussed. Then three soil laterite samples, for mineralogical study by XRD method were sent to the central lab of Lorestan University and 10 rock and soil samples from the laterites of the studied area were sent to the Canadian SGS laboratory for determining the amount of major and minor elements by ICP-MS method. Results The peridotite rocks of Noorabad ophiolite complex in the northwest of Lorestan province include dunite, harzburgite, and lherzolite, which have been strongly affected by alteration. The tectonic activities have caused crushing in these rocks in a way that has facilitated their alteration process and created laterite areas. As a result of different alteration processes such as serpentinization, carbonation, hematitization, and silicification, primary minerals such as olivine, pyroxene, and spinel have been altered and secondary minerals such as serpentine, dolomite, quartz, hematite, and smectite have been formed. The laterite zone is a layered red horizon and in some areas is lenticular, which is located on peridotite rocks. In some areas it is covered by Miocene limestones. In some areas, the secondary fractures of laterites formed by tectonic and diagenesis activities are filled by dolomite and silica. According to XRD analysis, there are dolomite, quartz, hematite, smectite and serpentine group minerals in the laterite section. The chemical analysis of the studied samples shows the nickel content of 381-2660 ppm for Noorabad laterites. According to the discrimination diagrams of source rock, the studied laterites are derived from ultramafic rocks. Moreover, the investigations carried out on the La/Y ratio showed that the formation environment for the studied laterite samples is alkaline. Discussion The peridotite rocks of Kermanshah ophiolite are related to suprasubduction ophiolites (Kiani, 2011). Due to the multi-stage replacement mechanism, these types of peridotites undergo the most serpentine alteration, so that their severe crushing during replacement and their migration on the margins of the continents facilitate the intensity of the altered. The peridotites of northwestern of Lorestan province were altered into serpentine during these alterations in the initial stages, and then underwent carbonate, hematite, and silicification alterations. During these changes, the primary minerals of these peridotites, such as olivine, pyroxene, and spinel, have become secondary minerals. The serpentine minerals are the first group of secondary minerals that were formed. Then, the secondary minerals of the second stage replaced the serpentines in such a way that the minerals of the smectite group can directly replace the serpentine (Dixon, 1989; Nahon et al., 1989; Gaudin et al., 2005). Other minerals are formed by substitution and saturation of other elements and form minerals with simple chemical formula such as quartz, dolomite, and hematite. During the first stages of alteration of peridotite rocks, the main elements Si, Mg, Ca, Fe, and Al were washed from the rock and caused the formation of secondary minerals in the lateritic zone. A significant amount of silica is included in the structure of silicate minerals such as serpentine and smectite, and the rest is filled in fractures in the form of chert and chalcedony. During the weathering process, magnesium-bearing minerals are destroyed and Mg2+ is removed from them, so that some of the magnesium in the serpentine mineral are replaced by Ni2+ (Sagapoa et al., 2011) and nickel-bearing serpentine minerals. The Ca element is removed from the pyroxene group minerals during alteration and enters the structure of smectite minerals or forms dolomite mineral. The Ni element has a positive correlation with the Co, Mg, Cr, and Fe elements, and this correlation can be due to the substitution of nickel instead of these elements in the crystal network of secondary minerals or due to the surface absorption of the mentioned elements by the hematite mineral.

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