Arab J Sci Eng (2013) 38:1841–1849 DOI 10.1007/s13369-013-0550-0

RESEARCH ARTICLE - EARTH SCIENCES

Geophysical Investigation of the Hematite Zones in Koçarlı-Demirtepe (Aydın/Turkey) Tolga Gönenç · Emre T˙ımur · Mehmet Utku · Co¸skun Sarı · Zülfikar Erhan · Mümtaz Çolak

Received: 5 January 2012 / Accepted: 5 June 2012 / Published online: 9 February 2013 © King Fahd University of Petroleum and Minerals 2013

Abstract The Koçarlı-Demirtepe iron ore deposit area was investigated with geophysical methods (magnetic and resistivity) in order to determine the presence of iron deposits, which are important in cement production, in the Koçarlı/Aydın region. The study area is locally known as “iron hill” and iron ore was produced in ancient times from three galleries. In this study, magnetic, vertical electrical sounding (VES) and electrical resistance tomography (ERT) methods were applied to determine the actual geometry and reserves. Magnetic investigations were carried out in order to obtain a general idea about the approximate boundaries of the possible hematite zone. The resulting total magnetic field anomaly map was used to define the boundaries of the hematite zones. The depth of the structure was calculated to be about 27 m with a 2-D inversion method for a dyke model. Three profiles of ERT and five vertical electric soundings were measured in order to define the iron ore geometry. A 2-D inversion method was applied to the multi-electrode data to create an ERT model of the hematite zone geometry and the vertical structural variations. In order to obtain the vertical cross sections of the survey area, a 2-D inversion method was applied to the ERT and VES data. The data sets of vertical electric sounding were combined and modeled as a profile. Results of the 2-D inversion were supported by the ERT model. The depth values obtained from the ERT are in agreement with those from 2-D inversion of magnetic data.

According to these results, the thickness of the hematite zone was determined to be 10 m. Finally, the iron ore and its geometry were defined. The strike of the ore body was found to be northwest–southeast and its thickness was found to be about 10–12 m. With respect to these dimensions, approximately 10,000 m3 of iron ore was estimated. Keywords Hematite zone · Electrical resistance tomography (ERT) · Vertical electrical sounding (VES) · Magnetic anomaly · 2-D inversion

T. Gönenç (B) · E. T˙ımur · M. Utku · C. Sarı · Z. Erhan Faculty of Engineering, Department of Geophysics, Dokuz Eylul University, 35160 Tınaztepe, Campus Buca, Izmir, Turkey e-mail: [email protected] M. Çolak Faculty of Engineering, Department of Geology, Dokuz Eylul University, 35160 Tınaztepe, Campus Buca, Izmir, Turkey

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1 Introduction Mineral exploration in Demirtepe-Koçarlı was started in 1962 by MTA (Mineral Research & Exploration General Directorate-Turkey). Various explorations were carried out by MTA from past to present, such as uranium, hematite and geothermal research. There are ruins of old mining galleries in the area, mainly within the borders of the township of Koçarlı, Aydın. It is observed that these galleries open in the east and southeast directions in general. The aim of the present study is to determine the commercial potential of the field by finding the locations of the hematite, which is required in cement production. The survey area lies 24 km southwest of the city of Aydın, which is situated in the Aegean region in the west of Turkey (Fig. 1). Gneiss constitutes the basement of the study area which covers about 90,000 m2 . Overlying the gneiss is a unit which is made up of alternating micaschist and marble. The gneiss is represented by banded gneiss that shows thick, irregular foliation in colors of gray-green and yellowish brown. Foliation of this gneiss has a north–east orientation and slopes to the southeast [Candan and Dora, pers. comm.]. The slope varies between 30◦ and 60◦ . An iron formation with the appearance of bedding is observed in the gneiss (Fig. 2a, b), but there is no precise study on the formation of the iron (Candan and Dora, pers. comm.). The UTM coordinates in the diagrams are in zone 35 (central meridian 27◦ E). In their study, Cihnio˘glu et al. [1] stated that the Fe concentration was estimated to be 44.51 % and reached 54.46 % in the reserve where the average silica ratio was 28 %. In the same study, two preliminary reserve estimates were deter-

mined through the cross-section method, giving 19,000 tons of high grade and 36,000 tons of low grade iron ore and ore with high silica. The old galleries that open at the surface in the east and south directions in the survey area indicate that there was periodic demand for the ore. One of the galleries is along the road side and was utilized in the interpretations (Fig. 3). In the current study, various geophysical methods were used based on the different physical properties of rocks in order to evaluate the economic feasibility of the area. For this purpose, total magnetic field readings were obtained on seven profiles. In addition electrical resistivity tomography (ERT) measurements along three profiles and vertical electrical sounding (VES) measurements at five points were carried out in the survey area. The locations of dense hematite content in the study area were determined in general with the magnetic measurements. After determining the boundaries of the area from the results of the magnetic profiles, the ERT and the VES measurements were carried out. Through the ERT, the distribution and the direction of the iron-ore-rich zones in the study area in general were identified, and using the VES results the thicknesses of these zones were obtained. Total magnetic field readings were mapped and the probable structural depth was obtained by taking cross sections on the observed high amplitude anomaly. The ERT data were evaluated by a 2-D inversion method [3] and vertical cross sections were obtained. The VES data taken along one profile were modeled by a 2-D inversion technique [4] and a probable structural cross section was obtained.

2 Geophysical Procedures and Field Plan

Fig. 1 The location of the Kocarlı-Aydın, Turkey, survey area (UTM Zone 35 ED50)

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In the study area, three geophysical prospecting methods were employed as stated above. In accordance with the field research license, the number of profiles and their directions were defined for maximum efficiency in the entire area. Total magnetic field measurements were carried out along seven profiles. Resistivity studies were utilized along three profiles of ERT and five points of VES were carried out (Fig. 4). The ore zone should show higher magnetic anomalies compared to the surrounding rocks. On the other hand, it could have variable electrical resistivity compared with the rocks in the vicinity. Hence the magnetic measurements were carried out in order to be able to define the area of ore. Afterwards, the electrical resistivity studies were undertaken to define the structural geometry of the probable ore zone, its thickness and geometrical parameters (Fig. 4). The VES points were chosen in order to determine the vertical variation of the hematite zone which is observed also on the surface [2]. Depending on the directions of the galleries, the field plan was carried out in four steps.

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Fig. 2 a Geological map of the survey area (Cihnio˘glu et al. [1]; Candan and Dora, pers. comm.), b petrographic cross section of the study area (revised from Öztunalı [2])

1. Hematite zone—iron ore distribution outlined with the magnetic method. 2. Determination of the geometry and direction of the structure, with the multi-electrode resistivity method. 3. Definition of the structural thickness with vertical electrical sounding (Schlumberger array). 4. Joint interpretation of combined result.

3 Magnetic Survey A Scintrex ENVI MAG/VLF instrument was used for the magnetic data acquisition. All profiles were placed parallel to the road trending NE–SW with a profile interval of 10 m, measurement interval of 5 m and length of 30 m. According to the total magnetic field anomaly map (Fig. 5a), a clear structural anomaly was determined on the

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Fig. 3 Gallery entrances and directions in the survey area (modified from Google Earth)

Fig. 4 Geophysical investigations (vertical electric sounding points, electrical tomography profiles, magnetic profiles modified from Google Earth)

western part of the map. A reduction to the pole or RTP [5,6] operation was applied to the total magnetic intensity data and the RTP anomaly map was obtained (Fig. 5b). Along the AB profile, on which total magnetic field measurements were carried out with a sampling interval of 1 m, a 2-D inversion method for a dyke model [7] was applied to the data. According to the petrographic cross-section of Öztunalı [2] (Fig. 2b), this model was selected as dyke. Point A of the profile is the entrance of gallery 1 as shown in Fig. 3. The low magnetic anomaly indicated with blue color in Fig. 5b is related to the schist formation in the field (Fig. 2). The high (red colored) anomaly was interpreted as the hematite ore zone in gneiss.

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Depths to the top and bottom boundaries of the hematite zone were calculated. Average depth to the top of the structure was determined to be about 27 m (Fig. 6).

4 Electrical Resistivity Survey From the magnetic survey the possible iron rich zones were defined. According to this, electrical resistivity imaging surveys were performed along three profiles in the survey area. All profiles were placed parallel to the road. Data acquisition was performed with a system consisting of two multi-core

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(a)

(b)

Fig. 5 a Total magnetic field map, b reduced to pole map and A-B cross-profile

Fig. 6 Cross-profile A-B and 2-D inversion result

cables, an automatic switching unit and a resistivity meter. The profiles trending NE–SW are about 300 m in length and the interval between the profiles is 25 m. A Wenner–Sch-

lumberger configuration was used with an electrode spacing of 5 m (Fig. 7). This electrode configuration was chosen to be able to investigate lateral and vertical resistivity variation of the sub-surface structure. Measurements were carried out with RVA resistivity equipment produced by the Atikol Company using steel electrodes. In order to reduce the contact resistance, salty water was added between ground and steel electrodes. Processing of the data was carried out using a 2-D inversion method [3] with a maximum RMS error of 5 %. Profile 1 is the nearest to gallery 1 for the resistivity measurement in the survey area and is perpendicular to the gallery 1 entrance, as are Profiles 2 and 3. The inverted ERT cross section is shown in Fig. 8. There are high resistivity (>1,600 m) zones in the lateral intervals 70–90, 120–150 and 200–240 m (red colored) in Fig. 8. The third high resistivity zone of profile 1 (in the lateral interval 200–240 m) is next to gallery 3. These two anomalies were interpreted as the effect of the hematite zone according to the studies carried out by Öztunalı [2]. Another anomaly centred at 180 m along the profile was thought to be the effect of the space in gallery 1. Figure 9 illustrates the results along Profile 2. This profile was also taken perpendicular to the direction of the galleries 1 and 3 shown in Fig. 3. This profile is also over gallery 3. There are high resistivity zones (>1,600 m) in the lateral ranges 10–100 and 195–260 m which show similar features

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Fig. 7 Electrode configuration of the Wenner–Schlumberger arrays, where C and P are the current and potential electrodes (after Loke [8])

Fig. 8 Profile 1, ERT cross section

Fig. 9 Profile 2, ERT cross section

to Profile 1. In both Profile 1 (in the lateral range 120–160 in Fig. 8) and Profile 2 (in the lateral ranges 10–100 and 195– 260 m in Fig. 9) the same relatively high resistive zones are seen. The ERT of Profile 3 shows that the thicknesses of high resistivity zones in the lateral range 20–100 m (Fig. 10) are greater than those which are defined along Profile 1 (Fig. 8) and Profile 2 (Fig. 9). In contrast, the resistive shallow layer from 100 to 200 m laterally in Fig. 10 is thinner or even

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absent compared to the other sections in both Profile 8 and Profile 9. This anomaly was also interpreted as the effect of the hematite zone. The VES studies were carried out at five points along a NW–SE profile perpendicular to the ERT profiles. The area around gallery 3 is not observed along this profile. The 2-D inversion algorithms were applied to the VES data for the interpretation using the methods of Uchida and Murakami

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Fig. 10 Profile 3, ERT cross section

5 Conclusions and Discussion

Fig. 11 Inverted resistivity cross section of the combined VES points

[9] and Uchida [4]. Figure 11 illustrates the cross section of this profile from the combined VES points. Area I and Area II are represented by high resistivity sections along the profile (Fig. 11). The deeper structure named as Area II in Fig. 11 is interpreted as the same zone as the anomaly from 70–210 m laterally in Fig. 10. The thickness of the shallow resistive structure (Area I in Fig. 11), which is the nearest to the first gallery and the road, was determined to be 10 m. This anomaly was considered to be due to the space of the gallery 1. These results are in agreement with the geological observations and the drilling which is given in Öztunalı [2].

The study was carried out to determine the ore-rich zone by utilizing three geophysical prospecting methods. It can be said that the results of the geophysical studies were in agreement with the information obtained from three galleries which were present in the study area. Firstly, total magnetic field investigations were performed to achieve a general idea about the borders of the hematite zone. Then resistivity observations were carried out to determine the location and geometry of the ore zone. According to the study by Candan and Dora (pers. comm.), the gneiss shows thick, irregular foliation in colors of greygreen and yellowish brown, while banded gneiss is also represented and foliations of the gneiss show north eastern trends and south eastern slopes that change between 30◦ and 60◦ . According to the result of the magnetic method applied in the first stage of study, it was determined that the highest magnetization is along the A-B cross-section that has a northeast–southwest orientation (Fig. 5). The location of point B corresponds to the place where gallery 1 is located (Fig. 3). Probable depths of the hematite rich zones were defined as 27 m from 2-D inversion of magnetic data. This approximate value is consistent with the high resistivity anomalies, centered on 150 m laterally in Figs. 9 and 10. In the second stage, the first ERT profile was taken in the vicinity of gallery 1. In the cross section for profile 1 (Fig. 8), an anomaly identified between 120–150 m corresponded to the iron rich zone that was observed in the vicinity of gallery 1. Anomalies near gallery 3 were identified in the cross section between 200–240 m along the ERT profile (Fig. 8). It was found that signs of gallery 1 disappeared in profile 2 but the high-resistivity signs of gallery 3 continued (defined in red in from 130 to 170 m in Fig. 9). It was observed in the ERT cross section that the effect of the zone observed in the vicinity of gallery 3 continued in the cross section in Fig. 10 from 80 to 210 m.

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Fig. 12 a Boundary and direction of the hematite zone defined by magnetic and electrical prospecting methods and b magnetic methods

High resistivity anomalies seen toward the northeast end of each of the three profiles are considered to be areas that are rich in hematite content, which is also observed along the surface. When the cross sections are examined, the presence of an iron rich layer can be expected in the deeper horizons. The VES section (Fig. 11) which is in the NW–SE direction perpendicular to the ERT profiles revealed that the zone observed at the entrance of the gallery continued in the southeast direction. It has been thought that Area I, which is rich in ore, is confined within the entrance of the gallery. From the combined vertical electrical probing section, it can be said that the thickness of the ore-containing zone is about 9–10 m. Although the topography shows an increase towards the southeast, at VES point three indications of a second zone (zone II in Fig. 11) were detected that show similarities to zone I. This anomalous zone has a more pronounced continuation and is deeper and thicker than the first zone.

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As a result of the study, the following conclusions have been reached: 1. The ore mineralization trends in a northwest–southeast direction and the boundary of the zone was defined in Fig. 12. 2. According to magnetic data, the hematite enrichment of the area is in the northwestern part of the survey area. Using the polarization caused by the bedding in the geological settlement of the area which is related to the gneiss and schist contact, magnetic data has been modeled by the cross-section A-B along the NE direction. 3. According to VES data that traverses the red-colored area in the RTP magnetic map (Fig. 5b), the ore mineralization thickens in the southeast direction where the altitude of topography increases.

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4. According to the calculated dimensions, approximately 10,000 m3 of iron ore was estimated in the studied area. Density and grade of the hematite are expected to have variations in the area. Reliable density and grade calculations will be determined on samples from the borehole drillings. 5. For the purpose of investigating the second zone seen in the VES cross section, it is thought that further studies (such as drilling) are required for the area.

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3. 4. 5. 6. 7.

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