Lulak Abad Iron Occurrence, Northwest of Zanjan: Metamorphosed and Deformed Volcano-Sedimentary Type of Mineralization in Central Iran

The Lulak Abad iron occurrence is located in the northwestern part of the Central Iran, 55 km west of Zanjan. Mineralization at the Lulak Abad area was originally identified by Zamin Gostar Company (2007), during a geophysical exploration. The present paper provides an overview of the geological framework, the mineralization characteristics, and the results of a geochemical study of the Lulak Abad iron occurrence with an application to the ore genesis. Identification of these characteristics can be used as a model for exploration of this type of iron mineralization in the Central Iran and elsewhere.
Detailed field work was carried out at different scales (give scales in parentheses) in the Lulak Abad area. About 16 polished thin and thin sections from host rocks and mineralized and altered zones were studied by conventional petrographic and mineralogical methods at the Department of Geology, University of Zanjan. In addition, a total of 7 samples from ore zones at the Lulak Abad occurrence were analyzed by ICP-OES for minor and trace elements and REE compositions at Geological Survey of Iran, Tehran, Iran.
Rock units exposed in the Lulak Abad area consist of schists and metavolcanic units the Kahar Formation; Lotfi, 2001) that were intruded by granite and microdiorite bodies. The schist units consist of chlorite-biotite-muscovite schist and muscovite schist that show granolepidoblastic texture with foliation-parallel disseminated magnetite. The metavolcanic units consist of metadacite, rhyolitic metatuff and meta-andesite with porphyritic textures. They are marked by dominant mylonitic foliation surrounding feldspar and quartz porphyroclasts. Alkali feldspar and quartz are the principal minerals of the granite. The intrusion is characterized by intense deformation features and is highly mylonitized. Based on field and microscopic studies, the microdiorite postdated metamorphic and deformation events and shows neither schistosity nor mylonitic foliation. It is composed principally of plagioclase with minor disseminated magnetite and a microgranular texture. Two deformation events are recognized at the Lulak Abad area, one principally ductile, the other brittle.
Iron mineralization at Lulak Abad occurs as veins, veinlets and lens-shaped bodies in schist units, mylonitic metavolvanic rocks and mylonitic granite. The main ore vein extends up to 100 m in length and averages 3 m in width, reaching a maximum of 6 m. It trends NE, dipping steeply SE. The ore lenses are parallel to the mylonitic foliation and variably boudinaged, about 10 m in length and vary in thickness up to 5 cm. Two stages of mineralization can be distinguished at Lulak Abad. Stage 1 mineralization is recognized as stratiform and stratabound lenses, laminated and disseminated crystals of magnetite in volcano-sedimentary host rocks. Stage 2 is characterized as hematite-pyrite-calcite veins and veinlets cutting the mylonitic foliation of the host rocks. Hydrothermal alteration is restricted to silicified, calcitic and chloritic altered parts of the ore zones.
The ore minerals at Lulak Abad formed as vein and hydrothermal breccia cements, and show vein-veinlet, brecciated, disseminated and open space filling vein and veinlet textures. Hematite is the main ore mineral, accompanied by minor magnetite and pyrite. Goethite occurs as a supergene mineral. Quartz, calcite and chlorite are present in the gangue minerals that represent vein-veinlet and vug filling textures.
The Lulak Abad mineralized veins and breccias show lower concentrations of LREE and HREE (i.e., Pr, Er, Ho, Dy and Yb) relative to barren granitic host rocks but higher Tm, Gd, Eu and Lu concentrations. Chondrite-normalized REE patterns (Sun and McDonough, 1989) of host barren granite and the mineralized samples at Lulak Abad indicate that mineralized samples are depleted in LREE (except Ce) but enriched in most HREE (beside depletion in Dy and Ho). These signatures indicate high wall rock interaction (e.g., Lottermoser, 1992; Liegeois et al., 2003).
Comparison of the geological, mineralogical, geochemical, textural and structural characteristics of the Lulak Abad occurrence with different types of iron deposits reveals that iron mineralization at Lulak Abad was originally formed as volcano-sedimentary, and then reconcentrated as vein mineralization (Karami et al., 2012; Karami et al., 2013).