Discovery of the Siah Diq porphyry (Cu–Au) prospect in the foothill of Dam Koh volcano is a recent exploration success story of mineralization buried under a 46 m alluvium cover in an exploratory mature Chagai belt. Acquisition of geophysical data followed by drilling and logging was key in the discovery. Integrated magnetics and induced polarization (IP) surveys in an area of 7.5 km2, pointed out magnetic-low, IP-high, and resistivity-low anomalies corresponding to porphyry Cu–Au type sulfide mineralization. Three bore holes were drilled to test the geophysical anomalies. After careful observation and geoscientific logging of core, porphyry style Cu–Au mineralization was revealed. The porphyry prospect was further characterized based on host rock lithology, petrography, alteration mineralogy, ore vein characterization, and Cu/Au geochemical assays based on core samples. Rocks hosting the mineralization include andesite, granodiorite, coarse, as well as fine grained diorite and pink granite, all highly altered, mineralized and porphyritic. Propylitic alteration was dominant in all the three bore holes and developed earlier followed by phyllic, potassic, and argillic alterations. Sulfide mineralization is present as cross-cutting stockwork veins and disseminations. Average copper and gold assays of drill core are 0.17% Cu and 0.78 ppm Au, respectively. Economically insignificant values of molybdenum and silver have been noted in some samples.
With the advent of remote-sensing and stream sediment-based geochemical exploration for mineral deposits, there is a bloom in the discovery rates of porphyry copper deposits (PCDs) during the last three decades. Specifically, the remote-sensing techniques are significantly contributing to most of these discoveries since PCDs have a distinctive, widespread, and remote-sensing-based detectable alteration system [1,2,3]. This discovery bloom is leading to a gradual drop in the success rates of discoveries and is progressively leading to more sophisticated exploration techniques for hidden mineralization at depth or those covered by alluvium . Usually such hidden mineral prospects have a very little or no direct signs of mineralization at outcrops. In such situations, indirect exploration techniques, including geophysical surveys and drilling, allow the exploration beneath cover. Thorough understanding and detailed assessment of the geophysical anomalies of such blind porphyry mineralized systems, its surrounding geological environments, and subsequent proposal of drilling sites are critical factors of success in such situations . If the geophysical anomalies are significant, the concealed target is confirmed by drilling. Integrated geophysical surveys are a cost-effective, efficient, and swift methods that provide quantifiable important subsurface information about lithology, mineralogy, and structure in alluvium-covered terrains . Induced polarization (IP), ground magnetic, and gravity surveys are the widely used geophysical methods to discover and further characterize subsurface porphyry copper alteration systems and associated sulfide mineralization in three dimensions [7,8,9,10]. This contribution describes one such opportunity of discovery of the hidden Siah Diq porphyry Cu–Au prospect, with integrated geophysical surveys and subsequent drilling program playing a major part in its discovery in a thick alluvium cover. This new prospect escaped almost 40 years of exploration of Chagai belt through various geological, remote-sensing, geochemical, and geophysical surveys. Here, the top of hypogene sulfide mineralization is located under a 50 m alluvium cover and the only sign of mineralization at the surface is a patch of weak propylitic alteration located 2 km from the core of mineralization that could easily be overlooked, or even if observed, this weak alteration would be easily classified of little or no importance. During this work, we covered the area by ground magnetic and IP surveys. An initial low magnetic anomaly was detected in an otherwise high magnetic area. We covered this low magnetic area by several IP profiles. A pronounces anomaly of high chargeability and low resistivity was noticed, with dimensions and characteristics corresponding to that of a porphyry style sulfide mineralization. Drilling of three boreholes was the next step to check the geophysical response that confirmed the mineralization present below a 50 m alluvium cover and continues beyond the drilling limit of 443 m depth. The project area is the Block-2 lease of Saindak Metals Limited (SML), Pakistan, where the Geological Survey of Pakistan did geophysical survey, drilling, and other relevant geological work in an area of 7.5 km2. The center of the main geophysical anomaly was outside this lease area, so drilling could not be done on the more promising area, rather only the marginal part of the main geophysically anomalous zone was tested by drilling.
2 Regional geology and tectonics
The Cretaceous Sinjrani volcano-sedimentary group is the dominant lithological unit in Chagai belt [11,12,13] overlain by the clastic and calcareous sedimentary units of Humai Formation . The Late Cretaceous Humai Formation is overlain by Juzzak, Saindak, and Amalaf formations ranging in age from Paleocene to Oligocene, broadly consisting of marine clastic and fluvial units interbedded with andesitic lava flows, volcanic debris, tuffs, volcanic breccia, etc. [11,13,15,16]. This sequence of three formations is overlain by subaerial red cross-bedded sandstone, conglomerate, mudstone with gypsiferous evaporates collectively called the Dalbandin Formation of Late Oligocene age [14,17]. At the locality of Reko Diq (Figure 1), the Dalbandin Formation is conformably overlain by the Miocene age Reko Diq Formation, dominantly consisting of >350 m thick subaerial andesitic volcanics, pyroclastic breccia, volcanic tuffs, debris, etc. . This Cretaceous to Miocene volcano-sedimentary sequence described before is intruded by multiple episodes of huge batholiths (mainly in eastern Chagai) isolated stocks, domes, lopoliths, dikes, and sills (western Chagai) of varying composition of granitic, dioritic, dacitic, and gabbroic nature, ranging in age from Late Cretaceous to Quaternary [19,20,21,22,23,24]. The recent most rock units of Pliocene-Pleistocene are form the Koh e Sultan, Dam Koh, Alam Reg, and many associated small volcanic centers collectively known as the Neogene Balochistan volcanic arc (Figures 1 and 2). This arc includes the Bazman-Taftan volcanic fields of southeastern Iran as well as the Koh e Sultan and associated volcanic fields of Chagai belt in Pakistan, formed by the subduction of Arabian plate beneath the Afghan block of Eurasian plate along the Makran subduction zone (Figure 1) [25,26], resulting in a volcano magmatic rock suite of calc-alkaline nature [27,13]. The Siah Diq porphyry copper prospect of the current study is most probably associated with this recent episode of volcano magmatic activity .
Chagai belt is a segment of the Tethyan metallogenic belt, a porphyry copper–gold–molybdenum, and other base metal-related mineralized zone in the Eurasian continent that has subduction-related (Andean type) rocks. This belt was formed by the subduction of Africo-Arab and Indo-Australian tectonic plates beneath the Eurasian super-continent [29,30,31]. The Chagai segment of the of the Tethyan metallogenic belt hosts more than 48 prospects and deposits of porphyry copper and gold including world class super tier-1 resource of Reko Diq (having 5.56 Mt of Cu metal) along with Saindak (>1 Mt of Cu metal), Dasht e Kaine, Koh e Sultan, and numerous less studied but high potential prospects in eastern Chagai [13,32,33,34].
3 Local geological setting
The Siah Diq area is partly located in Mashki Chah Quadrangle (30K/8) and partly in Pachin Koh Quadrangle (30K/12) of Survey of Pakistan maps. The area lies between longitudes 62°28′50.98 to 62°31′26.14E and latitudes 29°01′24.248 to 29°01′31.24N. It lies some 18 km north of Alam Reg, a railway station on the main road and railway links between Quetta and Zahedan in Iran. The nearest town is Nok Kundi, located 35 km southeast of Siah Diq. The project area is arid and consists of rough mountainous terrain flanked to the west and north by extensive spreads of alluvial gravel. Much of the southwestern part of the area is covered by wind-blown sand. It is located 40 km east of the world class Reko Diq PCD cluster [33,35,36], 27 km southwest of the Koh e Sultan porphyry & epithermal Cu–Au mineralization , and 15 km south of the Durban Chah Cu–Au–Mo prospect . The closest mineralizations in the vicinity include contact metasomatic skarn type iron ore and iron manganese in Chigen Diq and Mashki Chah [38,39].
Andesitic volcanic rocks up to 8,000 m thick, belonging to the Sinjrani volcanic group (Ksv), form the dominant outcrop of this area (Figure 2). The base of the sequence is not seen. Massive lavas dominate the volcanic piles, though, tuff, agglomerate, and locally thin-bedded limestone also occurs, particularly in the upper part of sequence. In some places, notably at the contact of certain intrusives, the Sinjrani rocks appear bright green in color due to epidotization. Dam Koh volcanic rocks, which are probably of about the same age as the older alluvium, are andesitic in composition that believed to be broadly synchronous with the major volcanic center of Koh e Sultan to the east. Volcanic porphyritic andesite (Qa) forms the crater neck of the of Dam Koh volcano in the extreme north, surrounded by coarse pyroclastic deposit (Qvp) including cinder debris and volcanic bombs (Figure 2). Deposits of laminated accretionary limestone or travertine (Qtr) are present in the northeastern part of the area, with active geysers/thermal spring that continuously expels water into the air.
Intrusive rocks include stock like bodies of diorite, granodiorite, and granite intruding the Sinjrani rocks. In places, the rock type grade into one another and the boundaries marked on the map are gradational. In general, the more acidic varieties postdate the intermediate types. Diorite is the dominant intrusive forming extensive stocks around and to the north of Mashki Chah (Figure 2). Two types of granite are present, a younger pink, fine- to medium-grained variety, locally mineralized that is exposed in the Mashki Chah area south of Dam Koh, and a white coarse-grained hornblende granite present in the south of Tuzgi.
The economic mineral occurrences of the area are directly or indirectly related to igneous activity. They include showings of iron, manganese, and copper, as well as deposits of travertine marble. Four major showings of hematite/magnetite are present. They are of hydrothermal origin that occur as veins within intrusive and Sinjrani host rocks. The principal deposit is about 2 km east of Mashki Chah, called the Chigen Diq iron ore deposit. Porphyry type copper showings are present 4 km northeast of Mashki Chah in fine-grained pink granite (Figure 2). The surrounding area is heavily epidotized. Similar mineralization has been noted at Amir Chah and Durban Chah copper prospects in the northeast. Manganese ore of hydrothermal origin occurs as replacement deposit occurs northeast of Mashki Chah.
Geophysical surveys of magnetic and IP were conducted in Siah Diq area from December 2014 to January 2015 on a 100 m × 100 m grid spacing. Magnetic data were gathered using a Geometrix G-856 proton precession magnetometer. IP survey was carried out by using IRIS SYSCAL PRO time domain machine that was run in a 2-D dipole–dipole configuration array. The dipole interval was set to 100 m. Separation factors of n = 1–5 were employed. The IP surveys were carried out on generally the same lines as used for the magnetic survey. At the end of every working day, data from the magnetometers were transferred to a PC. Output data from magnetometer were text files labeled by date and the operator’s name or “base.” Data dumps from the mobile unit show line/station, total magnetic intensity (nano Tesla; nT), and time (decimal hours). Subsequent processing steps include application of diurnal correction and latitude correction (0.0043 nT/m). Then, the contents of the files containing the corrected total magnetic intensity are loaded into Surfer 11 software. Finally, color and line contour maps of the corrected total magnetic intensity are generated from the database using Surfer 11 software.
A total of nine induce polarization profiles were done (C-1 to C-9) (Figures 2 and 4). All the IP profiles were laid down in the north–south direction except C-7, which was laid in the east–west direction. Few planned IP profiles could not be executed near Siah Dik Range due to the steep mountains. IP/resistivity data were dumped from the SYSCAAL PRO to a computer on a daily basis. This output was a text file with the date as the file name. Raw data from each survey line were collected in text files with the line number as the file name. The data were carefully checked for quality. The collected text files for each survey line were taken into a common database. Location coordinates for each line/station of the interpolated grid were added. Pseudo sections were plotted using standard depth and position conventions. Based on the low magnetic signature, three sites were selected for further investigations
Exploratory borehole 1 (depth: 1,320 ft) 29°01′25.3″N 62°31′06.4″E;
Exploratory borehole 2 (depth: 1,350 ft) 29°01′35.3″N 62°30′55.4″E; and
Exploratory borehole 3 (depth: 1,330 ft) 29°01′22″N 62°31′11″E.
Detailed and careful geological logging of the three boreholes was done during drilling operation in the hand specimen. Variations in lithology, alteration mineralogy, ore mineralogy, fractures, sulfide vein density, etc., were carefully observed, noted, and quantified against respective depths. These data were later transferred to Microsoft Excel software for making a digital log. Representative samples were selected from different depths of the three boreholes for ore mineralogy, rock petrography, alteration mineralogy, and fluid inclusion that were studied in Geoscience Advance Research Labs, GSP, Islamabad, Pakistan. Mineral identification and petrography were done by studying both thin section and ore polished section under a Nikon optical microscope with both transmitted and reflected lights, while some mineral phases including kaolinite were identified by X’Pert Pro PANalytical X-ray diffractometer (XRD) machine. For the geochemical concentration of gold, copper, molybdenum, silver, etc., representative core samples from regular depth intervals were sliced into two halves with one half sent for crushing/grinding while the other saved with the drilled cores. From the grinded powder, stock solutions for geochemical analysis of gold, copper, molybdenum, silver, etc., were prepared and analyzed using atomic absorption spectrometer (Model: Perkin Elmer AAS-PEA 700) under the standard operating settings. For cross-verification of the geochemical results, certain samples were also sent SGS Labs (Pvt) Limited, Karachi, Pakistan. The fluid inclusion studies were done using an Olympus microscope with fluid inclusion stage (heating/freezing).
5 Results and discussions
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 . 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 .
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.
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 . 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.
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.
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).
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).
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.
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.
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 . Through geological logging as well as thin section studies different types of vein were characterized based on the classification scheme of Gustafson and Hunt , 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.
|Vein type||Metallic/ore mineral||Thickness||Gangue minerals||Texture||Banding||Salvage character|
|A- vein||Pyrite ± chalcopyrite||Ranges from 1 mm to several cm||Quartz ± mica||Massive, drusy||Weak internal banding||Narrow or hardly identifiable alteration halos|
|D-vein||Pyrite ± chalcopyrite ± bornite ± molybdenite||From few mm to several cm||Quartz ± anhydrite||Granular||Banded||Thick 1–3 cm salvages, feldspar destructive constituting quartz, serecite alteration|
|M-vein||Magnetite ± pyrite||1 mm to 2 cm||Quartz||Granular||Massive to laminated||No alteration halos|
|Anhydrite (P-veins)||Absent sometimes has molybdenite||Up to 0.5 mm to 2 cm||Absent||Drusy||Laminated||Absent|
|Epithermal veins||Pyrite ± galena ± sphalerite ±||From few mm to 2 cm||Chlorite ± quartz ± epidote||Granular/drusy||Absent||Narrow or absent|
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].
Electromagnetic surveys in a porphyry copper gold fertile terrain, where the prospect is covered by post mineralization alluvium cover, are best techniques for scanning large areas during initial exploratory work. In such cases, anomalous areas of low magnetics, high chargeability, and low resistivity are prospective indications of a sulfide mineralization beneath the alluvium cover.
Follow-up confirmation of the geophysical anomalies in Siah Diq by drilling at a suitable location was the key to hit the sulfide mineralization.
Typical hydrothermal alteration pattern of a generalized model of porphyry style mineralization is present, with propylitic alteration being dominant in all the three bore holes followed by phyllic, potassic, and argillic alterations. Hydrothermal alteration in Siah Diq porphyry prospect is complex and multiple alteration zones overprint one another. The sulfide mineralization is mostly associated with the phyllic and potassic alterations.
The sulfide mineralization continues beyond the drilling limit depth and is expected to extend to the north too. Therefore, a reserve estimates cannot be made at this stage; however, for the entire rock column intersected in the three boreholes, the average assays of copper are 0.17% and of gold 0.78 ppm. The geochemical concentration of molybdenum and silver is economically insignificant.
The presence of epithermal veins indicates the peripheral or the upper part of a porphyry system, which may indicate a richer and big deposit further below. The paucity of molybdenite mineralization in the veins as well as low geochemical values of molybdenum indirectly indicate that the Siah Diq porphyry copper prospect is a Cu–Au type and not a Cu–Mo type.
Based on the facts that the intense geophysical anomalies are located farther north east of the area, the presence of epithermal veins, the dominance of propylitic alteration, and the paucity of potassic alteration, it can be deduced that the actual center of mineralization may lie farther north east of the drilling site. Therefore future prospecting and drilling should focus that area (outlined red in Figure 2).
The research has been supported by President Foundation of Chinese Academy of Geological Sciences (No. DZLXJK202011). The data were generated during the collaborative project between Geological Survey of Pakistan and SML, Pakistan. Team members from both organizations are thanked for their support. The authors would like to acknowledge Prof. Liu Lei from School of Earth Sciences and Resources, Chang'an University for his assistance with journal selection and other valuable suggestions throughout this work.
Author contributions: Yasir Shaheen Khalil did the field work, mapping, and drafting of the article. Jinku Huang did petrographic and geochemical work, including shaping of final draft, maps, and figures. Syed Ali Abbas did the on-ground geophysical surveys. Hong Jun and Li Wenyuan helped in logging and fluid inclusion studies and are working with Yasir on the porphyry copper deposits of the Chagai belt including the Siah Diq prospect. All authors are in agreement with the content of the article. This work was done as a part of Ph. D. work of Mr. Yasir Shaheen Khalil.
Conflict of interest: The authors state no conflict of interest.
Supplemental data: There is no supplemental data with this manuscript. All the available data has been presented in the manuscript and discussed there.
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