Mineralogy, geochemistry, fluid inclusions and genesis of Fe-Cu-Au mineralization associated with Ahmadabad intrusions, Semnan

Veins and stockwork hydrothermal iron ore are formed by hydrothermal fluids at various depth, from shallow to deep environments (Guilbert and Park, 1997). Ahmadabad deposit is located in 30 km northeast of the Semnan province, between the Alborz and Central Iran structural zones. According to previous studies in the Ahmadabad ore deposit (Haji Babaei and Ganji, 2018; Ketabforoush, 2016), there are major uncertainties about the origin of mineralization and hydrothermal process. In previous study based on fluid inclusions data, Ahmadabad hematite barite ore deposit is considered as low-temperature hydrothermal barite ore deposit and also Ahmadabad barite iron oxide ore deposit is also considered as a veins-type hydrothermal-magmatic ore deposit (Haji Babaei and Ganji, 2018). Ketabforoush (2016) based on lithological, mineralogical and alteration assemblage characteristics of Ahmadabad iron ore mineralization, consider it as an Iron Oxide-Cu-Au (IOCG) type hydrothermal mineralization. This study attempts to use mineralogy, geochemistry and microthermometry of fluid inclusions data in quartz and barite for investigating the genesis of Fe-Cu-Au mineralization and possible style of mineralization at the Ahmadabad deposit. Material and methodsDuring the field work, 54 samples were collected from the host rocks and alteration and mineralization zones. For petrography, mineralogy and paragenetic sequence studies, 48 thin-polished sections were prepared and studied by ZEISS Axioplan2 polarized microscope at Kharazmi University Tehran branch. After ore petrography 10 suitable ore samples were selected for chemical analysis. Preparation, crushing and pulverizing of the samples were carried out in Kharazmi University and samples were analyzed in the Zarazma and Iranian Mineral Processing Research Center (IMPRC) labs. for major, minor and rare earth elements by using WD-XRF and ICP-MS methods. Geochemical analyses results are presented in Tables 1 and 2. Microthermometric analyses were carried out on 3 doubly polished thin section from quartz and barite minerals using a Linkam THMS 600 freezing-heating stage, mounted on a ZEISS Axioplan2 research microscope at the IMPRC. Discussion The formation and associated process of iron ore deposition has been much debated and discussed, with the focus on hydrothermal and magmatic origin (Naslund et al., 2000). In Ahmadabad deposit, it seems that monzonite and monzodiorite subvolcanic intrusions has been emplaced through a volcanic sequence. During magma emplacement and crystallization, magmatic fluids due to lower density, rising to the upper part of the intrusions and penetrated into the volcanic host rocks causing vein-type iron mineralization. On the basis of mineralogical and microthermometric studies of fluid inclusions, the mineralizing fluid was possibly of magmatic origin; cooled and diluted by mixing with meteoritic fluids. Temperature and pressure drop following the migration of the magmatic fluid to the shallow depths may changes the nature of mineralizing fluids from reduced to oxidant state and deposition of iron as hematite after sulfide (pyrite and chalcopyrite) and sulfate (barite) precipitation. Salinity and homogenization temperature of fluid inclusions show that high temperature-salinity fluid mixed with low temperature-salinity fluid and by decrease in temperature and salinity, followed by cooling and dilution, provided the favorable condition for iron oxide deposition. Based on current studies, Ahmadabad deposit formed in following stages:- Intrusion’s emplacement in the shallower depth, caused migration and circulation of mineralized fluid in fractures and faults act as fluid channeling conduits.- Circulation of these fluids through fractured systems may also cause some metal leaching from the wall rocks.- Moderate to high temperature and salinity magmatic fluid, while approaching the shallow depth were mixed with meteoric fluids and by cooling and dilution process, and possibly transition from the reduction-oxidation boundary, ore bearing fluid nature changed resulted in hematite deposition after sulfide and sulfate phases. ResultsAhmadabad iron ores mineralization based on field geology, mineralogy, geochemistry and microthermometry data is similar to epigenetic deposits formed by magmatic-meteoric fluids due to fluid mixing. Mineralization in the Ahmadabad deposit be divided into two stages of mineralization: 1) primary hydrothermal mineralization stage (hypogene), and 2) secondary stage (supergene). The main iron ore mineralization in Ahmadabad is hematite (specularite) which is mainly formed later than sulphides including pyrite and chalcopyrite, as open space-filling, vein-veinlet, massive and disseminated style of deposition. Barite, calcite and quartz are the main gangue minerals, though in some place’s barite has economic potential. Based on field data and mineralogical studies, the subvolcanic intrusions of the Early Eocene age, after emplacement within the volcanic units controlled by fault and fracture zones, have caused extensive low-grade alterations and limited mineralization in intrusion and volcanic host rocks. Microthermometric studies of fluid inclusions show that the best possible model for formation of the Ahmadabad ore deposit, is mixing of hot and high-salinity magmatic fluid with cold and low-salinity meteoric water.Although iron ore grade changes greatly with variation in the silica contents, high grade Fe mineralization mainly occurred in the fault and fractured zones away from widespread intensive silicification which is mainly associated with Cu±Au mineralization. These features are the key exploration criteria for future exploration program in the region.