Magnetometric geophysical studies in Khosrow Abad iron deposit, northeast of Sonqor (Kermanshah Province)

Abstract

Khosro Abad iron ore deposit is located 130 km of Kermanshah, 30 km northeast of Sonqor city close to Khosro Abad village, in the Sanandaj-Sirjan zone. Based on the field observation, exposed units in the area include; basaltic-andesite volcanic rocks and limestone of the Sonqor Volcanic-Sedimentary Complex. The quartz-monzonite body intruded the sequence and caused a contact metamorphism of up to greenschist facies, and development of hydrothermal alteration halos. Discharged iron bearing hydrothermal fluid destabilized due to physicochemical changes and deposited the iron ore in the contact of limestone, basaltic-andesite and quartz-monzonite intrusion. Control of iron ore mineralization by the contact of quartz-monzonite and volcano-sedimentary sequence and presence of skarn mineralogy is evidence of skarn mineralization in Khosrow Abad. Geophysical survey was carried out using magnetometry technique (1305 stations), covering an area of 3.6 Km2 for defining the high potential areas of iron mineralization and depth estimation. In this research, total magnetic, reduction to pole, analytical signal, and upward continuation maps were prepared, and based on the obtained results four anomaly blocks of; An(A)-An(A´)-An(B)-An(C) were identified. An(C) anomaly is shallower than the others and extend approximately down to 50 meters, while other anomalies are much deeper. The maximum magnetic anomaly in total field intensity and RTP maps is 46426.1 and 49474.6 nanotesla, respectively, along a northeast-southwest trend, aligned with the contact of meta-andesite-basalt and crystallized limestone units. Introduction The magnetometric exploration technique is one of the low-cost, widely used, and indirect methods for mineral exploration and tracking of oxidizing intrusive bodies containing Fe-Ti oxides, such as magnetite, titanomagnetite, and titanohematite, as well as some iron sulfides such as pyrrhotite (Donohue et al., 2012). The Khosrow Abad iron ore deposit is located in west Iran’s iron metallogenic zone of the Sonqor series. It is located northwest of the Sanandaj-Sirjan zone; northeast of the Sonqor town (Kermanshah province) (Ghorbani, 2007) in Sonqor-Baneh volcanic belt (Azizi and Moinevaziri, 2009). In the northeast of Sonqor, hypabyssal acidic intrusive bodies penetrated the Upper Jurassic rock units, including basaltic meta-andesite volcanic rocks and limy slates (calc-shale), causing skarn development and iron ore mineralization. Mineralization is mainly hosted in the meta basaltic andesite unit's skarn zone, differentiating it from other common carbonate-hosted skarn deposits (e.g., Ebrahimi Fard et al., 2022a; Ebrahimi Fard et al., 2022b). The main purpose of current research is to examine the magnetometric geophysical explorations' accuracy in detecting possible ore zones as well as the depth of oxidized (magnetic) intrusive bodies and current emplacement depth and possible associated mineralization. The main challenges were locating the magnetite ore zone covered by recent alluvium and ore zone has limited exposure, and skarn ore facies without classic carbonates as a host rock. Calc-silicate minerals in association with oxide and sulfide minerals formed the skarn zones in the Khosrow Abad deposit. Magnetite, pyrite, chalcopyrite, garnet, clinopyroxene, sodic plagioclase (albite), actinolite, epidote, chlorite, calcite, quartz, and iron hydroxides such as hematite, goethite, and limonite are the main constitute of the skarn zones. The main fracture system in the Khosrow Abad is a large strike-slip fault with an approximately north-south (NNW-SSE) trend and more than 80 degrees dip towards the east. Iron ore mineralization is mainly located in the eastern edge of this fault which formed a shear zone in the metamorphosed volcanic unit. Material and methods Based on the geological map, field observations, geological structures, mineralization limited exposure, and geophysical exploration were carried out perpendicular to apparent structures (probable veins and faults trend) along the northwest-southeast direction. The exploration pattern of the survey was, 25 10 m (profile distance of 25 m and 10 m reading intervals) in the ore zone and a 20 25 m pattern in surrounding rocks (1350 stations). Measurements were carried out by a Proton magnetometer model G856 manufactured by Geometrics. Magnetic surveys were mainly focused on meta basaltic andesite and quartz monzonite units. Qualitative and quantitative data processing and interpretations of data were carried out by using Geosoft Oasis Montaj and Magmap software. The highest field intensity recorded in the area is 52049 nT and the lowest recorded is 43619 nT, which according to the IGRF of 47515nT, is possibly due to the changes caused by the presence of medium-grade magnetite mineralization. Discussion In the total magnetic intensity map, magnetic anomalies were detected with the northwest-southeast trend which is related to discontinuous lenses along the main fault. In the IGRF-REM map, the maximum intensity value of anomalies is equal to 4534 nanotesla. All 4 detected anomalies; (A), (A´), (B), and (C) are high-intensity anomalies in this range. Anomaly (C) is marked on the map with moderate to relatively weak intensity. The upward continuation map indicates that the ore zone continues at least to a depth of more than 70 meters. The Euler's deconvolution method shows an estimate of the anomaly depth, An(C) anomaly is shallower than the others and has a depth of about 50 meters. Based on Spector and Grant's method, the estimated depth of Khosrow Abad iron ore mineralization is about 0.03 km, which is a reasonable estimate. The minimum depth of anomalies in most areas is about 30 meters. The model prepared by Encom Quickmag software shows that linear anomalies are located in line with the regional faults and parallel to each other with medium depth. For validation of the modeling results, based on the data interpretation (analytical signal map, pole-to-pole map, and transversal continuation), several drilling points were proposed for borehole drilling. Conclusion Based on the geophysical data processing and interpretation (analytical signal map, reduction to pole map, and upward continuation) as well as modeling results, several locations were proposed for borehole drilling. The BH-1 borehole is located in the An (A) block anomaly central point, and according to the core logging data, this anomaly is introduced as an anomaly with good intensity, and according to the continuation map with a depth of more than 30 meters is almost match the geophysical interpretation and model. After the drilling campaign, it was discovered that geophysical prediction shows a good agreement with the drilling data, which confirms the effectiveness of the proper design and implemented geophysical survey for iron ore exploration in the Khosrow Abad deposit, which could be used for other skarn iron ore deposits of the region.

Keywords

• magnetometry
• depth estimation
• modeling of magnetic anomalies
• khosrow abad