Application of the magnetic geophysical method to exploration of the Potassic Zone in some Porphyry Copper Deposits (Iran)

Shohreh Hassanpour1*, Mehmet Salih Bayraktutan2 and Snežana Komatina3 investigate the utility of magnetic data to understand the distribution of concentrating on porphyry systems in NW Iran.

Abstract Haftcheshmeh, Masjeddaghi and Nowchun are three giant porphyry systems located in Iran. Because of the presence of potassic alteration outcrops in the Haftcheshmeh ore deposit (NW Iran), we decided to detect borders of this alteration zone by a ground magnetic method. Goal of exploration is to understand the distribution of concentration in porphyry systems, as the level of magnetization commonly differs across alteration boundaries. Further, 3D interpretation of magnetic data is important to improve our understanding of geology and alteration in the system.

In this paper, qualitative and quantitative results of geophysical surveys are presented in the form of total field intensity, second derivative, upward continuation and pole reduction maps. Comparison of magnetic results with borehole logging provides a good pattern to a field geologist to assign and guide an exploration project. According to this pattern, all drill holes in Haftcheshmeh, Masjeddaghi and nowchun porphyry system were designed.

According to the results for the mentioned copper systems, anomalous magnetic features are related to the sub-volcanic intrusions and high anomaly zones over the potassic alteration probably show the existence of magnetite. The potassic zone pattern can be used as a key in all further exploration of porphyry systems.

Introduction At all map scales, the foremost role of the geophysical mineral exploration is to provide geologic information in three dimensions, particularly in terrains concealed by younger rock systems. At large scales (deposit and district-size studies), high-resolution magnetic and gravity data are often used to define structure directions related to mineralization (e.g., strike-slip faults: Isles et al, 1989; Henley and Adams, 1992; Mckinlay et al., 1997; Whiting, 1986). On the other hand, at smaller scales, geophysical data can help to elucidate the regional geologic framework, mainly by defining major compositional boundaries or structural zones that may be favourable environs for mineralization (e.g., Gunn et al., 1997a, b; Jaques et al., 1997; Leclair et al., 1997; Moore et al., 1998).

Here we investigate the utility of magnetic data to understand the distribution of concentrating on porphyry systems in Iran (Figure 1). Because of the fact that magnetization commonly differ across alteration boundaries, interpretation of magnetic data in three dimensions is important to further our understanding of geology and alteration information in the system.

For copper exploration, the magnetic survey is of particular importance, because it may map areas of structural complexity, hematitization outcrops in the area, that we have proposed to explore as potassic alteration zones by the magnetic geophysics exploration method. All acquired data helped us to design precise drilling points as well (Figure 2). It should be mentioned that for the drilling plan, an exploration geologist must pay special attention to geochemistry and geophysical data all together. We could get the best result especially in potassic zones of some porphyry systems in the northwest and southeastern part of Iran in Arasbaran and Kerman porphyry systems (Hassanpour et al., 2009).

Haftcheshmeh The Haftcheshmeh Cu-Mo porphyry deposit is located in the Arasbaran porphyry belt (in NW Iran). This ore deposit is a typical porphyry Cu-Mo porphyry system in terms of its all mineralization, alteration and association with granodioritic intrusion and predominance of quartz vein hosted copper mineralization. The Haftcheshmeh, has been explored by drill holes in 2009 and, is a world-class giant porphyry system, at about 0.5 per cent Cu and 180 ppm Mo at 180 Mt ore. It occurs in gabbrodiorite and a felsic granodiorite which influenced Cretaceous limestones and hornfelses. Skarn-type metasomatic alteration and mineralization occurred along the contact between upper Cretaceous impure carbonates in the south and west of the deposit. The Haftcheshmeh intrusion is intersected by different dykes ranging in composition from granodiorite to monzonite, gabbrodiorite and diorite (Figure 1). There are very interesting potassic outcrops and mineralization into gabbrodiorite and granodiorite bodies (NICICO, 2008).

1 Payame Noor University | 2 Ataturk University | 3 Technical Faculty “Mihajlo Pupin”, University of Novi Sad * Corresponding author, E-mail: hassanpour@pnu.ac.ir DOI: 10.3997/1365-2397.fb2022067

Figure 1 Studied porphyry systems locations on Iran geology map.

Masjeddaghi The Masjeddaghi deposit is located on the northwestern part of Iran, 35 km from east of the Djolfa town. This prospect has been studied since 2000 by GSI and by NICICO recently. Mineralization occurred in two epithermal and porphyry forms in andesite to trachyandesite and dioritic subvolcanic rocks with Paleocene (Hassanpour and Alirezaei, 2017) age rocks. These rocks set in an area of 500 km2 contains argillic, phyllic, propylitic, and potassic alterations zones.

The magnetic method has been applied for investigation of copper mineralization and geological features such as contact, fault, and probable alteration especially potassic zones in the region for the first time in this research. The results have been used as an exploration pattern in all NICICO prospects from 2006 (NICICO, 2008).

Nowchun The Nowchun copper ore deposit is situated on the southeastern slope of Kuh-e-Manzar about 4 km southwest of the Sarchashmeh copper open pit mine and 10 km northeast of the town of Pariz. The Eocene andesite and tuffs are the oldest rocks in this region, and formed alternations of stratified flows and tuff horizons in variable thickness. They have been subjected to intensive folding and faulting, and have been altered by contact metamorphic and hydrothermal processes (NICICO, 2008, unpublished report).

Method Field procedure In this method, to obtain the database in the field, it is necessary to choose two base points for total field variation measurements, one inside and another outside the surveyed area. In this case, two sets of the magnetometer will be used. One of them is located in the base (outside) to take measurements every five minutes.

A second magnetometer will be used in the area to produce data quality; each station should have three readings, which are divided by three. Finally, after dataset correction as drift correction, Ref correction and diurnal corrections, a total field intensity map will be produced.

A number of advanced techniques for data enhancement or filtering, as employed in ground survey data, will be discussed here as: vertical derivative, upward and downward continuation, reduction to the pole, most of which are applied to two-dimensional data.

Total field intensity instrument Proton magnetometer G-856 has been used in the whole of the porphyry copper prospect area. The precession frequency, typically 2000 Hz, is measured by modern digital counters as the absolute value of the total magnetic field intensity with accuracy of 1 gamma, and in a special case 0.1 gamma, in the earth’s field of approximately 50,000 gamma.

Total field intensity map The geomagnetic survey is recorded through a 100 m by 20 m grid network and after averaging between three separate records for each point. Whenever variation between two successive points was greater than 50nT, there was an additional record between the two points. After the common correction processes, such as drift correction, ref. correction and diurnal correction, the total magnetic field map is plotted by Oasis Montaj software, with 25nT contour interval.

Magnetic field intensity in each point is proportional to the magnetic field and remnant magnetization of rocks on that point. Remnant magnetization of rocks is a property related to the amount of iron-bearing minerals in rock content, and is different in rocks. So, the rocks containing these kinds of minerals reveal

a greater total magnetic field anomaly. As a matter of fact, there are usually more magnetized minerals in intrusive rocks than sedimentary rocks. Therefore, measurement of total geomagnetic field variation over ground surface, in addition to showing magnetic mineral anomalies, will also reveal layer contacts and probable faulting systems (NICICO, 2008, unpublished report).

Discussion and results Masjeddaghi copper ore deposit By working with a well-known applicable program; IGRF, the background value of the Masjeddaghi area is proposed 48,373nT. The maximum and minimum total fields measured in the area were 49,974nT and 47,047nT respectively.

Magnetic anomalies are recorded in the south and the southeastern part of the total magnetic field intensity map (Figure 3). Recorded anomaly in the southern part of the prospect (at the location of zone), which is surrounding the Arpachay River, can be related to potassic zone which shows high intensity. The southeastern part of the prospect is also so important due to a high-intensity anomaly which is recorded and detected alterations. High intensities are generally recorded on an andesite unit which contains magnetite minerals. As a result of all borehole information, Porphyry mineralization, which has limited outcrop around the Arpachay river, is an elliptical mass with 500×400×600 (L×D×h). Based on estimations its reserve is 300 Mt with about 0.35 copper assay, and 150 ppm gold (based on report of geological survey, 2005 and NICICO 2009).

Investigation of the total magnetic field intensity map demonstrates that magnetic anomalies are connected with an elliptical body with 500m×400 m (L×D) and this issue confirms in geological reports, too.

As it has been shown on magnetic field intensity maps, phyllic and argillic alterations have lower magnetic field intensity and are demonstrated with yellow to green colours (Figure 3). Finally, the studied area is composed of two absolutely different zones in the case of magnetic field intensity. High intensities are related to alteration and occurred at the location of copper mineralization, and medium intensities connected with phyllic and argillic alterations are recorded around the mineralization.

Then by application of several filters such as derivative, upward and pole reduction on raw data, their virtual identity may be revealed. For determination of depth, slope and thickness, the two sections of AB and AB are chosen and studied. Location of magnetic field intensity measurement stations, their UTM coordinates, location of AB and A’B’ sections and the main fault location, are presented in the compiled map.

Upward continuation maps Upward continuation of raw magnetic data in the Masjeddaghi copper prospect area at three altitude level of 50, 100 and 150 m, is done by the Oasis Montaj software, and the results are shown on the figures of 4a, 4b, 4c. In this way, unlike first derivative, the effect of surface weakened anomalies and deeper anomalies (with higher wave length) tend to be shown significantly. As we consider in Figure 4a, Upward Continuation in 50 m altitude made the contour line smoother with respect to the total field intensity map. This can indicate that the anomalies have deep origin and also are transversal and longitudinal distributed and they are continuous respect to the surface masses. This effect is more visible on maps (Figures 4b and 4c), at 100 and 150m respectively. We can see smoothness of contour lines with more details on the map in Figure 4b, and especially on the map in Figure 4c. Study of this map help us to continue Figure 3 Total field intensity map of Masjeddaghi porphyry.

Figure 5 Pole reduction map, drill holes are illustrated on the ore zone.

Figure 6 (a) Numerical modelling of magnetic anomaly on AB section. (b) Numerical modelling of magnetic anomaly on A’B’ section.

with the processes controlling deep magnetic anomalies. At all three maps, we can see elongation of magnetic bodies is coincident with the direction of alteration (i.e. northwest-southeast). So, it can be concluded that two different anomalies appear in these maps, indicating that anomaly 1 (southwestern) is shallower than anomaly 2 (southeastern). However, due to the erosion level in Arpachay River, anomaly 1 leads to outcrop of diorite porphyry intrusion, while anomaly 2 is buried in sediments.

Anomalies due to surface masses and noises that are existing in the total intensity map are eliminated in these maps (Figure4a, b, c).

Pole-reduction map As it is explained before, in order to interpret the total magnetic intensity map, it should utilize some filters on the data. Using the software, we have produced a pole reduction map from a total magnetic intensity map (Figure 5).

Anomalous features in the total magnetic intensity map also exist in the pole reduction map, but comparison between pole reduction map and total magnetic intensity map reveal that anomalies are shifted during the pole reduction process and also became more regular, so that it demonstrated anomalous variations more precisely. It also reveals that magnetic dipoles are also in their actual direction. It’s an important factor in correlation between geologic features and resulting magnetic anomalies (Figure 5).

This profile is proposed perpendicular to the strike of the main structures of the area with A coordinates of x=581400 and y= 4303200 and B coordinates of x=581400 and y= 4304400. Numerical modelling by geometrics shows the anomalous bodies is towards north with 400m width (Figure 6a). The thicknesses of alluvial formations are estimated 7 m in geophysical study.

This profile is proposed perpendicular to the strike of the main structures of the area with A coordinates of x=581600 and y= 4303050 and B coordinates of x=581600 and y= 4304250. Numerical modelling by Mag2dc shows the anomalous bodies is towards north with 600 m width. The thicknesses of alluvial formations are estimated at 2 m (Figure 6b) (NICICO, 2009, unpublished report).

Masjeddaghi The Masjeddaghi deposit has been studied since 2000. Mineralization has occurred in two epithermal and porphyry forms in andesite to trachyandesite and dioritic subvolcanic rock with Miocene host rocks. These rocks set in an area of 8 m2 contain argillic, phillic, propylitic, and potassic alterations zones.

A magnetic method has been applied for investigation of copper mineralization and geological features such as contact, fault, and probable alteration zones. Results of primary raw data interpretations and studies on processed geophysics data of the Masjeddaghi site are summarized below: 1. Zone boundaries are the cause of the magnetic anomalies, which probably related to magnetite components of andesite to trachyandesite igneous masses that are studied completely. 2. Background threshold amount of studied area which is obtained by using the IGRF program is equal to 38,373nT.

Maximum and minimum amount of total magnetic field in studied area is 49,974nT and 47,047nT correspondingly. 3. High magnetic intensities are related to potassic alteration zones, and medium amounts are recorded around in phyllic and argillic alteration zones (NICICO, 2008). 4. Oasis Montaj software was used for preparing a total magnetic field intensity map, upward map and reduction to pole-reduction maps. 5. Upward maps at various depths confirm the importance of recorded anomalies in the southern part of the prospect. 6. Modelling of magnetic anomaly was carried out on the AB and A’B’ sections; their location was posted on the total magnetic field intensity map and sections were prepared by means of Geometrics software. Anomalous mass has northern dip direction and its approximate thickness in the western and the eastern profile is 500 m and 700 m respectively.

Depth of alluvium formations is about 7 m and 2 m in the western part of the AB and A’B’ sections, correspondingly. 7. Presence of F1 and F2 faults was recognized, by NE- SW strike of the magnetic counters (Figure 5) (NICICO, 2008, unpublished report).

Haftcheshmeh Haftcheshme copper deposit 8 km northwest of the Sungun copper open pit mine. The area has been covered with exploration studies since 2006. Mineralization has occurred in gabbroic and granodioritic rocks of Miocene age with porphyritic textures. All kind of alterations in porphyry type deposits are seen in this ore deposit with 4 km square extent.

The magnetic survey method for detecting the alteration zone and various geological aspects such as: contacts, faults and etc., applied a magnetics survey in a 1.7 km2 area with 50 x 20 m grid size, and after probable desirable results, further explorations are predicted. Results of primary raw data interpretations and studies on processed geophysics data of Haftcheshmeh copper deposit are summarized below: 1. The anomalous magnetic features are related to the granodioritic and gabroic intrusive Miocene dated bodies, of which their distributions are very well found. 2. The background level of the region is obtained by IGRF software equal by 48,664nT. The maximum total field measured is 49,250nT and the minimum is 47,290nT. 3. From the mineralization point of view, the most preferable areas seem to be located in the central and southeast of the region. 4. Application of Oasis Montaj has led to producing first derivative, upward continuation and reduction to pole maps. 5. An upward continuation map shows subsurface anomalies over the southeastern part of the region have a deep origin and are of transversal and longitudinal distribution. They are also continuous in respect to the surface masses (Figure 7a, b, c). 6. According to the interpreted magnetic features on the maps, the fault F1 has a NW-SE strike and the mineralization has occurred along this faulted zone in a NW-SE direction (Figure 8a) (NICICO, 2008, unpublished report).

Nowchun The Now Chun copper mineral deposit is situated on the southeastern slope of Kuh-e-Manzar about 4 km southwest of Sarchashmeh copper open pit mine and 10 km northeast of Pariz town. The Eocene andesite and tuffs are the oldest rocks, and form alternations of stratified flows and tuff horizons of variable thickness. They have been subjected to intensive folding and faulting, and have been altered by metamorphic and hydrothermal processes. The hornfelses are developed over a wide area,

Figure 7 Upward continuation map in Haftcheshmeh ore deposit.

Figure 8 (a) Pole reduction map; primary drill holes are illustrated on the ore zone. (b) Total field intensity map of the Haftcheshmeh porphyry.

Figure 9 Upward Continuation Map in Nowchun ore deposit.

Figure 10 (a) Pole reduction map. Drill holes are illustrated on the ore zone. (b) Nowchun total field intensity map.

and show gradual transitions with the surrounding, unaltered andesites.

Results of primary raw data interpretations and studies on processed geophysics data of the Nowchun copper deposit can be summarized below: 1. The anomalous magnetic features are related to the micro-granodioritic intrusive Oligocene dated bodies, of which their distributions are very well found. 2. The background level of the region is obtained by IGRF software equal by 45,935nT. The maximum total field measured is 47,700nT and the minimum is 44,700nT. 3. The north-eastern and north-western part shows high magnetic anomalies. A high anomaly zone over the potassic alteration probably shows the existence of magnetite. 4. Application of Oasis Montaj, has led to producing first derivative, upward continuation and pole reduction maps and analytic signal estimations (Figure 10a). 5. An upward continuation map shows subsurface anomalies over the southeastern part of the region have a deep origin and also are transversally and longitudinally distributed and they are continuous in respect to the surface masses (Figure 9a, b, c). 6. According to the interpreted magnetic features on the maps, the fault F1 has NE-SW strike and the mineralization has occurred along this faulted zone in a NE-SW direction (Figure 10a) (NICICO, 2008).

Conclusions The present investigations, planned and carried out by the exploration department of the National Iranian Copper Industries Company (NICICO) for delineation of potassic alteration zones, have led to useful and applicable results (by first author). These results can be employed for exploration objectives, in the first stage for exploration drilling planning and in the next for optimal gridding.

This method may be applied for geophysical explorations of all porphyry systems and for all deposits containing magnetite minerals. References

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