Mineral chemistry and chromite mineralization in the Qaranaz-Alamkandi area, west Zanjan

Malek Ghasemi and Karimzadeh Somarin (2005) reported that Chromite deposits in Iran occur in Paleozoic and Mesozoic ophiolite complexes in association with serpentinite and serpentinized peridotites and dunites (Ghazi et al., 2004; Shafaii Moghadam and Stern, 2014). There are more than 74 chromite deposits that have been explored in these complexes and they are mainly of alpine-type (Ghorbani, 2013). These ophiolite complexes are part of the Tethyan belts which link to other Asian ophiolite belts such as Pakistan and Tibet in the east as well as ophiolites in the Mediterranean region such as Turkey, Troodos, Greek, and east Europe in the west (Yaghoubpur and Hassannejhad, 2006; Hassanipak and Ghazi, 2000).New data in the current research study are used to infer the geology, mineralization, mineralogy, mineral chemistry and origin of the Qaranaz-Alamkandi chromite.

After preparing 72 samples from the study area, microscopic studies were carried out on 18 thin sections and 23 polished-thin sections for recognition of the microscopic features of the host rock as well as the mineralogy and texture of the ore body. Then, two chromite samples were analyzed at the Iran Mineral Processing Research Center, Karaj, Iran using electron microprobe and scanning electron microscope (SEM) methods.

The Qaranaz-Alamkandi chromite occurrence is located in the west of the Zanjan province and in the northern part of the Sanandaj-Sirjan zone. This area is composed of ultramafic sequences associated with Precambrian metamorphosed rocks such as amphibolite, marble, granitic gneiss and schist.According to petrographic studies rock units in the Qranaz-Alamkandi area consist of serpentinized harzburgite, serpentinized lherzolite, serpentinized dunite, serpentinite, amphibolite, amphibole schist, gneissic granite and mica shcist. This study show that the peridotitic rocks in this region include dunite, harzburgite and lherzolite. Olivine, orthopyroxene and lesser amounts of clinopyroxene associated with secondary minerals (such as serpentine, chlorite and calcite) and opaque minerals (chromite and magnetite) are the main minerals in peridotites.Mineral chemistry of olivines in the peridotites shows magnesium rich olivine with forsterite composition, slightly tending to chrysolite. The composition of olivines falls in the olivine spinel mantle array. Moreover, the olivines of dunites are comparable with those from the oceanic supra-subduction zone peridotites.Clinopyroxenes and orthopyroxenes are Fe-Mg-Ca rich. Furthermore, clinopyroxenes show augite composition and are mainly of the calcium- magnesium type. Orthopyroxenes show mainly bronzite and minor samples showing hypersthene composition. The composition of clinopyroxenes is similar to those of boninites and arc related magmas. This result and the given fact of the low contents of TiO2 and high contents of SiO2 in the structural formula of the pyroxenes suggest that the pyroxenes of the study area are comparable with those from the subduction tectonic settings.Chromite mineralization in the Qaranaz-Alamkandi area has occurred within the ultramafic rocks with serpentinized harzburgite and serpentinite composition. Due to the limited expansion of the peridotitic host rocks, chromite mineralization is also limited and it has occurred as lenses with maximum length up to two meters and one meter width. Chromite mineralization has occurred as massive, disseminated, lenzoid and vein- veinlets form in this area.Mineral chemistry of Chrome spinels from the Qaranaz-Alamkandi area indicate that the chromite samples plot within the ophiolite complexes and high- magnesium chromite field (Leblanc and Nicolas, 1992), which classifies them as podiform chromite deposits in terms of mineralization type (Arai et al., 2004).Chromite mineralization in the Qaranaz-Alamkandi area indicates an Alpine type deposit which is enriched in Cr and Mg and depleted in Ti. The Qaranaz-Alamkandi chromite mineralization has been formed from boninitic magmas which were derived from the subduction process in a supra-subduction zone and fore-arc tectonic settings (Ahrabian, 2018).