Geophysical exploration and geological appraisal of the Siah Diq porphyry Cu–Au prospect: A recent discovery in the Chagai volcano magmatic arc, SW Pakistan

5.1 Ground magnetic survey

Magnetic anomalies in igneous rocks may be a product of primary processes of crystallization and deposition, or they may be linked to secondary hydrothermal alteration. The secondary hydrothermal alteration processes may remove or introduce magnetic minerals into the system resulting in the decrease or increase, respectively, of magnetic intensity of the resulting altered rocks. In such cases, magnetic surveys may outline areas of fossil hydrothermal alteration activity that are directly related to the rocks that host ore deposits [40]. In porphyry style mineralization, circular to semi-circular low magnetic anomalies may be directly associated with the hydrothermally altered zones where primary magnetic minerals are replaced with low magnetic minerals like chlorite, biotite, serecite, and epidote. Intense propylitic, argillic, and phyllic alteration related with porphyry mineralization causing intrusions tend to destroy magnetite within the intrusion itself as well as the host rock [9,41]. Therefore, most of the porphyry (Cu–Au) deposits are noticeably associated with and usually centered on low magnetic anomalies [42].

Result of the magnetic survey conducted in Siah Diq is shown in the form of total field intensity magnetic map at contour interval of 50 gammas (Figure 4). This map shows variation in magnetic intensity from 45,300 to 48,000 nT. Small intrusions with 46,700–47,500 nT magnetic intensity show shallow intrusions with magnetite type of occurrence, on the northern and southern margins of the map. The cool blue colors show the relatively low magnetic anomalies at three principal locations, east of Siah Diq, southwest of Siah Diq and in middle of Siah Diq. These zones are main areas of interest, since they most probably represent demagnetized areas due to porphyry mineralization-related propylitic, argillic, or phyllic alterations in an otherwise magnetically higher anomaly area. The higher magnetic values occur at south and north ends of Siah Dik Range where sporadic mining for iron and manganese is being carried out.

Figure 4

Total field intensity magnetic map at the contour interval of 50 gammas of the project area. The cool blue colors show magnetic-low anomalies where the IP profiles were conducted.

5.2 Chargeability/resistivity: IP

In mineral exploration, the geo-electrical properties, associated electrical anomalies of ore deposits, and their host rocks play an important role in the discovery, location, shape, nature, and size of the respective ore deposits [43]. Induced potential, self-potential, and resistivity surveys are the commonly employed electrical methods in the exploration of both metallic as well as nonmetallic mineral deposits. IP survey measures the resistivity as well as chargeability of the host rock and associated ore body hidden in the ground. Sulfide mineralization dispersed as stock work and disseminations reduce resistivity while enhance the chargeability of the host rocks, therefore, induce polarization method is widely used in the exploration of porphyry deposits [44,45,46]. Nine IP profiles were done in the three low magnetic intensity zones of Siah Diq area to gauge the resistivity and chargeability parameters of the underlying rocks (Figures 2 and 4). From the results of IP survey resistivity and chargeability, pseudo sections for each profile line were made that are discussed separately below.

Profile C-1 is located near the eastern edge of the anomaly area. Pseudo section shows that the high chargeability ranging from 20 to 40 mV/V is at the northern side of the area (Figure 5). Low resistivity of 2–20 ohm-meter is supporting the chargeability anomaly. The anomaly starts from the depth of 150 m and continuous down to more than 300 m depth.

Figure 5

Resistivity and chargeability pseudo sections constructed for the IP profiles (C1–C6). For detailed description, see text under section chargeability and resistivity.

Resistivity and chargeability pseudo sections along profile C-2 (located 200 m west of profile C-1) show almost similar pattern as profile C-1 (Figure 5). Higher chargeability values of 37 mV/V occur at the depth of 300 m indicating continuity of the anomaly toward north. A low-resistivity pattern correlates with the high-chargeability zone.

Profile C-3 that was conducted in the middle of magnetic lows has the highest chargeability values, i.e., 150 mV/V, as shown in the chargeability pseudo section (Figure 5). The anomaly starts at the shallow depth of 150 m and correlates very well with low-resistivity anomaly in the northern part of the profile.

Profile C-4 is 200 m west of profile C-3 and shows some high chargeability of 70 mV/V with corresponding low resistivity of 7.5 ohm-meters at shallow depth of 150 m. Both the resistivity low and chargeability high anomalies seem continuous to the north as observed in the pseudo sections (Figure 5).

The IP profile C-5 was conducted at the eastern edge of Siah Diq Range and the western edge of the low magnetic anomaly zone. The resultant pseudo sections of resistivity and chargeability (Figure 5) indicate a low resistivity with a corresponding moderate to high chargeability zone to the north. The high-chargeability values start from 150 m depth continuing down to 300 m.

Profile C-6 was done at the foothill of Siah Diq Range. The chargeability anomaly of >100 mV/V starting from the depth of 150 m continues down to the depth of 300 m with a corresponding resistivity low anomaly as shown in the pseudo sections (Figure 5). As the anomaly is continuously increasing toward west and north, another IP profile was planned on Siah Diq Range, but due to the steep slopes as well as too much variation in topography, the data were noisy.

In the other two low magnetic anomaly zones, three IP profiles C-7, C-8, and C-9 were conducted. Profiles C-8 and C-9 were done in the north–south directions, while profile C-7 was done in the east–west direction; however, no prominent chargeability or resistivity anomalies could be detected in the underlying strata of these low magnetic anomalies.

By combining and merging data from all the IP profiles, a 3D model of resistivity and chargeability was prepared (Figure 6). The resultant model shows a highly chargeable zone with N–S trend with a corresponding low-resistivity zone of the same trend and dimensions.

Figure 6

Resistivity and chargeability 3D model constructed from the IP profiles data. A low-resistivity and high-chargeability anomaly are evident in this model.

5.3 Logging and petrography

Visual and petrographic studies of the drilled core samples, obtained from the three exploratory boreholes, indicate that major rock units hosting sulfide mineralization are andesite porphyry, coarse-grained diorite porphyry, granodiorite porphyry, pink granite, and fine-grained diorite porphyry. All these rock units are highly altered and pervaded by sulfide mineralization in the form of stock work veins as well as disseminations. These rock units, their alteration characteristics, sulfide mineral percentage, and copper grades were carefully logged that are presented in the form of graphical log (Figure 7). In borehole 1 from surface down to a depth of 155 feet, alluvium was intersected with an erosional contact with the underlying mineralized altered andesite porphyry. This andesite porphyry continues down to 625 ft where it has a gradational contact with the mineralized altered fine grained diorite porphyry continuing down to a depth of 1,080 ft (Figure 7). This is followed by a 155 ft thick rock unit of mineralized altered granodiorite porphyry that has an intrusive contact with the underlying mineralized altered pink granite unit at a depth of 1,235 ft (Figure 7). The pink granite unit with associated mineralization might continue beyond 1,320 ft depth but drilling could not be extended beyond this depth as the maximum capacity of the drilling rig was reached. In borehole 2, the alluvium-covered unit extended from the surface down to 135 ft depth after which mineralized altered andesite porphyry rock unit was noted that continued down to 665 ft where it is abruptly truncated by 60 feet thick mineralized altered breccia/fault zone (Figure 7). At 725 ft depth, the brecciated rock unit grades into the mineralized altered fine grained diorite porphyry that continues down to maximum drilling capacity depth of 1,350 ft after which the bore hole was terminated (Figure 7). The alluvium cover in borehole 3 extended from the surface down to a depth of 160 ft after which the mineralized altered andesite porphyry was intersected and at 300 ft this rock unit changed to mineralized altered coarse-grained diorite porphyry down to the maximum drilling depth of 1,330 ft (Figure 7).

Figure 7

Drill core logs of three boreholes showing details of rock units, alteration minerals, copper assays, and sulfide mineral content with respect to depth.

Thin section studies were done to determine the major and accessory minerals in the lithological units and to find the details of hydrothermal alteration mineral assemblages. Thin section of mineralized altered andesite reveal altered phenocrysts of plagioclase and amphiboles in a fine-grained non-crystalline matrix of andesitic nature (Figure 8a). Thin section studies of mineralized and altered coarse-grained diorite reveal large grains of plagioclase and hornblende in a medium to coarse-grained matrix of variably altered plagioclase, hornblende, alkali feldspar, where the plagioclase is mostly altered to serecite, epidote, and/or carbonate (Figure 8b and c). Petrographic studies of the altered and mineralized fine grained diorite indicate a fine-grained matrix of plagioclase, hornblende, alkali feldspar, and quartz with embedded phenocrysts of mostly serecitized plagioclase mineral (Figure 8d and e). Thin section studies of altered and mineralized granodiorite reveal large phenocrysts of feldspar and quartz in a medium-grained matrix of plagioclase, hornblende, quartz, alkali feldspar, etc. (Figure 8f).

Figure 8

Photomicrographs of different rock units intersected in the drill core samples during logging. (a) Highly altered and mineralized andesite porphyry with altered phenocrysts of plagioclase and sulfide minerals. (b and c) Coarse-grained diorite with variably serecitized plagioclase phenocrysts. (d and e) Intensely altered and mineralized fine grained diorite with sulfide mineralization and quartz veins. (f) Altered granodiorite with plagioclase phenocrysts and sulfide minerals. Abbreviation; Slp: sulfide, Pg: plagioclase, Ser: sericite, Qt: quartz, Carb: carbonate, Epd: epidote.

5.4 Hydrothermal alteration

Well-studied porphyry Cu–Au–(Mo) deposits in the world have a characteristic, predictive, and zoned hydrothermal alteration pattern with intense potassic alteration in the center followed by phyllic (serecite + chlorite), argillic, and a propylitic (epidote + chlorite ± calcite) alteration in usually a circular to semi-circular pattern around the causative intrusion [47,48,49,50]. This alteration pattern is very helpful in the exploration as well as identification of the center of porphyry Cu–Au–Mo mineralization through remote-sensing, geochemical sampling, and even geophysical methods since the alteration alters the bulk density of the host rocks. In Siah Diq prospect, the nearest outcrops of Sinjrani andesite volcanics have undergone chloritization and epidotization, giving it a greenish appearance (Figure 9a). The remaining account of hydrothermal alteration comes from the drill core samples. Drill core samples selected from different depths of three bore holes of the Siah Diq porphyry prospect were characterized in thin sections and in hand specimens. Hydrothermal alteration in this porphyry system is complex with multiple alteration zones overprinting one another. Propylitic alteration is prevalent and dominant alteration in all the three bore holes that is developed earlier followed by phyllic, potassic, and argillic alterations. Based on petrographic studies and logging, potassic and phyllic alterations are largely developed in granodiorite porphyry, diorite porphyries, granite, and andesite porphyry while propylitic alteration is predominantly reported from the andesite porphyry.

Figure 9

Outcrop (a) and drill core (b–h) photographs of different alteration types and their host rocks from the Siah Diq porphyry copper prospect. An outcrop of propylitically altered Sinjrani volcanic showing chlorite and epidote. (b–d) Drill core samples of propylitically altered andesite porphyry. (e) An intensely serecitized andesite porphyry from the phyllic zone showing plagioclase polymorph (circled) formed by serecite. (f) Another thorough serecitzed granodiorite porphyry with chalcopyrite vein. (g) An argillic altered zone in andesite porphyry. (h) Potassic alteration in the form of potash feldspar (pink cirlces) and biotite (small black circles) from the granite porphyry.

Propylitic alteration zone is indicated in the drill cores by the presence of chlorite, epidote, and calcite with some minor amounts of sericite formed at the expense of plagioclase and hornblende (Figure 9b–d). The main indications observed for this zone include the chloritization of hornblende and other ferromagnesian minerals, the epidotization of plagioclase, as well as the development of calcite veins.

The phyllic zone in the studied samples is mainly represented by sericite, quartz with some fine-grained muscovite in the veins, as well as replacement of feldspar and other ferromagnesian minerals (Figure 9e and f). Copper mineralization is mostly associated with the phyllic zone in the Siah Diq prospect.

Argillic alteration is not very common that is mostly seen near intensely faulted and fractured horizons in the drill core (Figure 9g). For example, in intensely faulted and brecciated zone (between 665 and 725 ft) in bore hole 2, there is kaolinite and associated clay mineral zone identified by XRD technique (Figure 10). The zone seems to be overprinted by phyllic zone with pyrite and chalcopyrite mineralization occurring in the fractures and fault zones in the upper parts of the porphyry system.

Figure 10

X-ray diffraction patterns of an argillic altered sample. Pie diagram shows the relative abundance of each mineral. Symbols are explained in the figure.

Potassic alteration in the Siah Diq prospect is either not very well developed or the actual intense potassic zone lies farther toward the north of the drilling site in the intense geophysical anomalies area. Biotite and potash feldspar are the main minerals rarely observed during logging. In the borehole 1 (1,235–1,320), pink grains of secondary potash feldspars and biotite are noted at depth corresponding to high temperature of formation (Figure 9h).

5.5 Ore mineralization and veins

A thorough understanding and characterization of mineralized veins in a porphyry Cu–Au system is of great significance as most of the ore minerals are distributed in these veins. The average metal content of copper, gold, and associated economic metals in a Cu–Au porphyry deposit has a positive and strong correlation with the vein intensity [50,51]. Similarly, determination of thickness, frequency, lateral continuity, and ore mineralogy of a particular vein type during logging helps in the visual grade estimates of copper and gold-bearing ore minerals like chalcopyrite and bornite. Furthermore, the cross-cutting relationship, mineralogy, and associated hydrothermal alteration of these veins help to establish the paragenetic sequence and distance from the causative intrusion [52]. Through geological logging as well as thin section studies different types of vein were characterized based on the classification scheme of Gustafson and Hunt [53], later elaborated and used by other experts e.g. [50,51,54]. Based on the nomenclature provided by these previous experts, the different vein types have been interpreted in the Siah Diq porphyry Cu–Au prospect, although some of them are hard to be classified based on these previous studies and are noted as such. The interpreted “A-type” veins generally consisting of quartz, pyrite, and chalcopyrite are usually formed the earliest and are being cut by several late veins and veinlets. These are the main ore-bearing veins and usually lack or have a very narrow alteration salvages (Figure 11a, b, and j). Contrary to A-type veins, the inferred “D-type” veins have a thick 1–3 cm alteration salvages consisting of serecite plus quartz besides typically contain pyrite, chalcopyrite, bornite, and/or molybdenite as the main ore phases with quartz as the main gangue mineral phase (Figure 11c–e). Based on cross-cutting relationship, they are younger than the A-type veins, having a fresh appearance and eye-catching alteration salvages (Figure 11c and e). The interpreted M-type veins consist of massive to laminated magnetite and pyrite plus quartz that generally lack alteration salvages (Figure 11f and g). Another group of veins classified here as the epithermal veins having a mineral assemblage of quartz + epidote + chlorite ± anhydrite ± gypsum ± sulfide typically cross-cuts all the other vein types (Figure 11g). The sulfide minerals in these epithermal veins are sometimes sphalerite (Figure 11h) and galena (Figure 11i) associated with pyrite and chalcopyrite. Some mono mineralic anhydrite veins (Figure 11j) having close relationship with the epithermal veins are also present in the Siah Diq porphyry Cu–Au prospect. Molybdenite is very rarely seen in the Siah Diq porphyry since it seems to be a Cu–Au type, but at places disseminated grains of molybdenite in pink anhydrite (Figure 11k) and fine streak in D-type veins (Figure 11l) have been observed. The main characteristics of all the veins and veinlets of the Siah Diq porphyry Cu–Au prospect have been summarized in Table 1 and illustrated in Figure 11.

Figure 11

Drill core sample photographs of the mainly developed veins in the Siah Diq porphyry Cu–Au prospect. (a and b) Pyrite and chalcopyrite A-type veins cross-cutting andesite porphyry. (c) Pyrite, quartz, and chalcopyrite D-type vein with eye-catching alteration salvage of quartz and serecite cross-cutting an A-type vein. (d and e) Thick and thin D-type veins in a granodiorite and andesite porphyry, respectively, with thick serecite and quartz alteration salvages. (f) M-type magnetite vein in a granodiorite, not the lack of alteration salvage. (g) M-type vein present close to some epithermal veins of chlorite, epidote and pyrite. (h) An epidote-sphalerite epithermal vein. (i) Vein cut surface of epithermal type showing galena and chalcopyrite. (j) A monomineralic anhydrite vein cross-cutting A-type veins in an andesite porphyry. (k and l) Molybdenite grains in a P-type anhydrite veins and in an intensely serecitized rock.

5.6 Geochemical assays and fluid inclusion

Average copper assays of drill core recovered from borehole 1 are 0.11%, silver concentration varies from 0.4 to 0.6 ppm while that of molybdenum varies from 0.5 to 63.95 ppm. The mean copper concentration from bore hole 2 is 0.27%, molybdenum concentration ranges from 0.5 to 80 ppm (two samples with 191 and 290 ppm), while silver was found to be below detection limit of the instrument, i.e. 0.5 ppm. Average copper assays from borehole 3 are 0.13% Cu, molybdenum concentration ranges from 0.5 to 30 ppm, while silver was below detection limit. Gold assays collected at regular intervals from all the three bore holes average at 0.78 ppm Au. The association, nature, and occurrence of vapor rich and of liquid vapor solid fluid inclusions studied in in quartz from mineralized veins indicate that ore precipitation occurred at lower hydrothermal temperatures from intermediate to hyper-saline hydrothermal fluids.

5.7 Siah Diq Cu–Au prospect in the regional context

The Siah Diq prospect is located 40 km east of the giant Reko Diq (24 Mt Cu and 1,300 t Au) and 90 km east of the Saindak (2 Mt Cu) porphyry copper gold deposits in the central Tethyan metallogenic belt. Previous exploration using remote sensing, geophysical surveys, and drilling in the central Tethyan belt has resulted in the discovery of Bakirçay, Kişladağ, Aği Daği, and Çöpler deposits in Turkey [55,56], Sar Cheshmeh, Sungun, Kahang, and Meiduk in Iran [57,58], and giant Reko Diq deposit and large Saindak, Dasht e Kain and Koh e Sultan prospects in Pakistan [33,35,58]. The Siah Diq porphyry copper–gold prospect shares similar lithology, hydrothermal alteration pattern, ore vein system, and mineralization to these well-known PCDs of the region and worldwide [50,53,59].