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The Reko Diq cluster of porphyry copper-gold deposits is located within the east-west trending Chagai porphyry copper belt in western Pakistan, ~500 km WSW of Quetta, Pakistan and 130 km ESE of Zãhedãn, Iran, and ~40 km south and 60 km NE of the Afghan and Iran borders respectively (#Location: 29° 7' 47"N, 62° 3' 38"E).
The earliest geological observations from Baluchistan recorded lead occurrences in the Chagai hills region (McMahon and McMahon, 1897; Vrendenburg, 1901). An extensive regional reconnaissance conducted by Hunting Survey Corporation in 1960 as part of a Colombo plan project, focused on defining coherent geo-stratigraphic units in the Chagai belt. A regional mineral exploration program undertaken by the Geological Survey of Pakistan and US Geological Survey in 1961 identified porphyry copper style mineralisation near Saindak in the western Chagai region (Schmidt 1968; Ahmed et al., 1972; Khan 1974). Between 1971 and 1974, a more comprehensive, including geological mapping, rock geochemistry, geophysical surveys and drilling was conducted at Saindak(Taghizadeh 1974; Khan 1974; Sillitoe 1974; Farah and Nazirullah, 1974; Wolfe 1974; Menzies and Trenholme, 1974). At the same time, reconnaissance mineral exploration programs continued over the entire Chagai Hills region (Sillitoe, 1975). Subsequent work sponsored by the United Nations Development Program identified a cluster of porphyry copper-gold systems at Saindak (Sillitoe, 1975; Sillitoe and Khan, 1977). Regional 1:50 000 scale mapping by the Geological Survey of Pakistan during 1978-79 detected porphyry style alteration and copper oxides at unspecified centres at Reko Diq (formerly known as Koh-e-Dalil; Khan and Ahmed, 1981).
BHP Minerals Exploration Inc. and the Balochistan Development Authority signed a joint venture mineral exploration agreement in 1993 to initiate an advanced program for porphyry copper mineralisation in the Chagai belt. A geochemical survey collected ~5000 -80# and BLEG (bulk leached extractable gold) stream sediment samples (following an orientation survey at Saindak) over an area of ~13 000 km2 along the Chagai belt from 1993 to 1995. Interpretation of geochemical anomalies supported by geological mapping, satellite imageries and ground follow-up work resulted in the delineation of 10 prospective areas including Ziarat Pir Sultan, Dasht-e-Kain, Basilani, Gwanshero, Kirtaka, Machi, Ting-Darguan, Koh-e-Sultan, Durban Chah and Reko Diq. Between 1996 and 1998, BHP geologists completed detailed prospect scale geological mapping, rock-chip geochemical sampling, ground magnetic and induced polarization (IP) surveys, followed by ~20 000 m of RC percussion and diamond drilling at Reko Diq (Oczlon et al., 1996; Maryono et al., 1998). This resulted in the discovery of a supergene copper enrichment blanket at Tanjeel (originally named H44), and a series of hypogene porphyry systems including H8, H9, H13, H27, H35, H36, H79 and the giant H14-H15 porphyry deposits at Reko Diq. From 1997 to 1998 exploration was extended beyond the Reko Diq complex and discovered several other sub-economic porphyry centers in Koh-e-Dalil, Sam Koh, Bukit Pasir and Parrah Koh areas (Perelló et al., 2008).
BHP Minerals entered into a joint venture with Mincor Resources of Australia in 1999 and formed the 100% owned Tethyan Copper Company (TCC) to continue regional exploration and drilling at the Tanjeel supergene copper deposit. In the year 2000, TCC completed the initial resource drilling program and defined a potentially leachable resource of 94 Mt @ 0.73% Cu (Perelló et al., 2008). Simultaneously, geological mapping, geochemical sampling, ground magnetic and Induced Polarization (IP) surveys by TTC in the exploration licenses NW of Reko Diq and identified a cluster of porphyry systems in the Bukit Pasir and Sor Baroot areas. All these targets were drilled from 2003 to 2006, for a total of ~48 000 m of RC and diamond drilling including 24 000 m of infill resource drilling at Tanjeel. In 2006, TCC released a total Indicated Minerall Resource of 214 Mt @ 0.60% leachable supergene copper mineralisation at Tanjeel (Perelló et al., 2008). Later in the same year, 2006, Antofagasta Minerals S.A. and Barrick Gold Corporation acquired 100% of TCC and a 75% interest in the Reko Diq and regional licenses. Subsequently, TCC accelerated the exploration program and the drilling campaign from 2006 to 2008 completed ~150 000 m of infill resource drilling and a comprehensive feasibility study of the western Reko Diq centres defining a global resource of 5.9 Gt 0.41 % Cu and 0.22 g/t Au. This exploration summary is drawn from Razique (2013).
The current (2018) Reko Diq project comprises a cluster of 18 known porphyry centres within a ~300° trending 10 x 3 km corridor, within the similarly elongated and marginally more extensive composite Reko Diq Igneous Complex. This corridor is bounded by the Drana Koh fault system to the north and Tuzgi fault to the south. Mineralisation is associated with the following intrusive events that constitute that complex:
• Early Miocene, ~23.3 Ma Tanjeel Porphyry System, overlian by a supergene blanket;
• Mid Miocene, ~18 to 14 Ma, the W, E and H35 deposits, distributed parallel to the main trend in the central to northern section of western half of the cluster, also known as the Northern Porphyry Cluster;
• Mid to Late Miocene, 12.9 to 11.9 Ma deposits of the Western Porphyry Cu-Au Systems, a series of medium-K calc-alkaline granodiorite and quartz-diorite intrusions with which the four adjacent and overlapping H79, H15, H14 and H13 mineralised centres are associated. These centres account for the bulk of the known potentially economic resources at Reko Diq. They are concentrated along the western margin of the overall cluster, aligned NNE, normal to the main trend;
• Late Miocene, ~11 to 10 Ma, the Southern Porphyry Cluster, including 4N, H2, 12, H8, 58, H7, H27, H9, H3, H36 and PK, predominantly concentrated along the southern margin of the area defined by the cluster, parallel to the overall ~300° trend.
The Reko Diq igneous complex is composed of Miocene diorite, quartz-diorite, quartz diorite to granodiorite and hornblende-diorite porphyries intruding Miocene age volcanic sequences of Andesite and tuff, felsic volcanic rocks and volcaniclastic rocks and pyroclastic breccias.
The main mineralised centres at Reko Diq (e.g., H9, H11, H14-H15, H79 and Koh-i-Dalil) are generally hosted cylindrical intrusions, which in plan view are typically >0.2 km2, the largest of which is Tanjeel (0.68 km2 and H14-H15 (0.75 km2). Miniature examples, with surface diameters of 100 to 200 m are also common (e.g., H7, H35 and H13).
The Chagai Porphyry Copper Belt
The Reko Diq porphyry Cu-Au deposit cluster lies within the Chagai Porphyry Copper Belt, which extends for ~300 km, from Saindak near the Iran border in the west, to Dasht-e-Kain to the east of Reko Diq. It comprises 48 porphyry Cu-(Au, Mo) systems formed in consecutive magmatic events in the,
• Mid to Late Eocene from 43 to 37 Ma; e.g., the Ziarate (43.1±1.1 Ma) and the Ganshero (36.1±1.1Ma; Breitzman, 1979) prospects in the eastern Chagai belt;
• Late Oligocene and Early Miocene from 24 to 22 Ma; e.g., the Saindak deposit in the western Chagai belt and the Tanjeel deposit in the Reko Diq area, and a slightly younger, early Miocene magmatic event from 18 to 16 Ma; e.g., the Sor Baroot (16.9±0.9 Ma), northeastern Koh-e-Dalil (18.66±0.2 Ma) and the Ting Darguan prospects (Perelló et al., 2008; Richards et al., 2012);
• Mid Miocene from 14 to 10 Ma, with associated porphyry Cu-type hydrothermal alteration, particularly in the Reko Diq area (as detailed above) but is also found elsewhere along the Chagai belt (Perelló et al., 2008);
• Late Miocene to Early Pliocene from 6 to ~4 Ma, represented by quartz-alunite epithermal and porphyry Cu-type alteration centres such as that found at Koh-i-Sultan.
Together these events formed a series of magmatic arcs, one of which is the ~500 km long and up to 140 km wide arcuate Neogene Baluchistan arc composed of calc-alkaline plutonic, volcanic and sedimentary rocks, part of the continental-scale Tethyan belt that spans eastern Europe and Asia. The Baluchistan arc contains three major, widely spaced, calc-alkaline volcanic centres in Pakistan and Iran. It is located some 400 km north of the east-west trending Makran trench and subduction zone (which lies beneath the Arabian Sea), and with the Hamun-i-Mashkel forearc depression and Makran ranges, comprise the 1000 km long Makran arc-trench system. This arc-trench system formed during northward subduction of the oceanic slab on the leading edge of the Arabian plate beneath the Eurasian plate in southeastern Iran and western Pakistan. It is bounded to the east by the major regional sinistral, north-south to NNE-SSW Chaman and Ornach-Nal transform fault complex, accommodating the relative motion of the juxtaposed Indian plate which has undergone continent-continent collision with the Eurasian plate east of those structures. The intersection between the east-west Makran arc-trench system and the north-south transform fault complex marks the triple junction between the Arabian, Eurasian and Indian plates. The Baluchistan/Makran magmatic arc is bounded to the west by the active zone of continent-continent collision between the Arabian and Eurasian plates that parallels the Persian Gulf.
The Baluchistan/Makran magmatic arc is separated from the Makran trench by the >350 km wide Makran ranges and accretionary prism which includes a series of kilometre-thick, folded and faulted Oligocene and younger turbidite sequences. In addition, the modern Makran accretionary prism has accumulated since the late Miocene and continues to grow seaward.
Six of the 48 porphyry systems of Chagai Porphyry Copper Belt (Saindak, H8, H13, H14-H15, H35 and Tanjeel) have sufficient copper grades and tonnage to be regarded as potentially economic deposits. Clustering of three or more separate systems is a common feature of the belt, as observed at Saindak, Sor Baroot, Bukit Pasir, Koh-i-Dalil, and Ting-Dargun, although by far the largest concentration of principal centres and several additional smaller occurrences, is at Reko Diq.
In the eastern part of the Chagai Hills belt, porphyry copper intrusions characteristically occur at the edges of the major batholithic bodies of the Chagai intrusions or within a few km of their contact with surrounding Late Cretaceous volcanic rocks of the Sinjrani Group. In the western part of the belt, the majority of the deposits and prospects are hosted by clastic and volcanic rocks of Paleocene and younger age, particularly at Reko Diq. A key feature of the clusters is the proximity of the porphyry centres, with separations averaging ~1 km at Reko Diq and ~500 m at Ting-Dargun. In addition, clusters of two or three porphyry copper-bearing stocks are spaced at <300 m centres at the H8, H36 (Southern Porphyry Cluster), H14-H15 and H79 (Western Porphyry Cu-Au System), all of which are at Reko Diq, where alteration halos typically coalesce to form a single hydrothermal centre, most evident at H14-H15.
The stratigraphic succession exposed in the Chagai Hills comprises >10 000 m of volcanic, volcano-sedimentary and sedimentary rocks.
• The Sinjrani Group, the oldest unit of Late Cretaceous age, contains >2500 m (locally to as much as 10 000 m) of predominantly massive lava flows, lapilli tuff and fragmental volcanic rocks, with local flysch-like siliceous shale, calcilutitic limestone, and reddish shale and sandstone, together with minor felsic volcanic horizons. The lava flows are generally massive, and pillowed in the lower sections, dominated by fine-grained porphyritic basalt and andesite. Their base is not exposed although they are believed to be deposited on ophiolitic oceanic crust (Perello et al., 2008 and references cited therein).
• The Humai Formation, which is ~2000 m thick and overlies the Sinjrani Group, is composed of calcareous and clastic sedimentary rocks, which includes the ~300 m thick, massive, latest Cretaceous (Maastrichtian) biohermal Humai Limestone.
• The Juzzak (Paleocene), Saindak (Eocene) and Amalaf (Oligocene?) Formations which are together >4000 m thick, overlie the Humai Formation in the Chagai Hills. They comprise a succession of shallow-marine to fluviatile shale, sandstone, conglomerate and shaly limestone. The Juzzak and Saindak Formations include lava flows that consist of massive, amygdaloidal, porphyritic andesite and basalt, while the volcanic rocks of the overlying Amalaf Formation consist of volcanic breccia and tuff, with locally interbedded massive, porphyritic flows of dominantly andesitic composition.
The remaining Late Oligocene to Early Miocene stratigraphy is dominated by subaerial volcanic and red-bed clastic strata, including a lower section of cross-bedded, reddish conglomerate, sandstone and mudstone, with locally interbedded thin evaporitic dolomitic and gypsiferous material, overlain by an upper suite of predominantly andesitic volcanic rocks exposed at Saindak, Reko Diq, and north of Yak Mach in the Chagai Hills.
The volcanic sequence at Reko Diq, assigned to the Reko Diq Formation, is >400 m thick and predominantly consists of fine to medium grained and porphyritic andesitic lava flows, interbedded with autoclastic volcanic breccia and pyroclastic debris of lapilli and breccia size. These rocks conformably and gradationally overlie red beds regionally correlated with the Juzzak or Saindak formations, assigned by Perello et al. (2008) to the Oligocene Dalbadin Formation via a transitional zone containing interfingered lava flows and breccia with conglomerate, sandstone and siltstone. The Reko Diq Formation is taken to be late Oligocene to early Miocene age (K-Ar and U-Pb zircon dates between 24 and 23 Ma (unpub. BHP reports).
Late Miocene volcanic sequences are absent in the Chagai Hills. Pliocene to Pleistocene units comprise volcanic rocks of the Baluchistan volcanic arc and a series of poorly consolidated clastic sedimentary units of the Kamerod Formation, which include buff silt, sand and fan gravel with interbedded ashfall tuff in places. The fans form terraces that constitute the regional surface of the Baluchistan plateau. The volcanic arc includes the WNW trending, 35 km long, 2500 m high, Koh-i-Sultan (0.2 to 5.9±2.8 Ma) composite stratovolcano, as well as a number of lava domes (e.g., Alam Reg - 2 Ma basaltic flows), isolated vents, and stratovolcanoes (Dam Koh), volcanic plugs (Koh-i-Dalil - 2.0±0.8 Ma), and remnant eroded volcanic necks. The larger stratovolcanoes are dominated by andesitic to dacitic block-and-ash flows, tuff, and laharic and debris deposits. Andesitic lava flows amount to <10% of the pile.
A series or arcuate, convex to the south, fault systems that vary from ESE to the west, through east-west to ENE to the east, delimit the main morphostructural blocks of the region, each with its own characteristic internal structural grain. Each structure persisits over lengths of >100 km. The Chagai Hills also include a large antiformal structure folding Sinjrani Group volcanic and younger strata, the core of which exposes the main batholithic masses of the Chagai intrusions. The faults are characterised by predominantly reverse motion and are responsible for the observed juxtaposition of the Mesozoic and Paleogene strata and intrusive rocks over Neogene units. These structures are also interpreted to have controlled primary sedimentary basin geometry.
The intrusive rocks in the Chagai belt (Eocene; 55 to 44 Ma - as at least three separate events at 55, 49 and 45 Ma) and Sor Koh (between 23 and 10 Ma) are collectively known as the Chagai Intrusions, and include a large, composite, semicontinuous batholith exposed over an interval of ~150 km along the core of the Chagai Hills, where it consistently intrudes rocks of the Sinjrani Group. Two intrusive phases are informally recognised: i). an early stage dioritic to granodioritic phase, with minor gabbro and quartz monzodiorite, followed by ii). pulses of granodiorite, quartz monzonite and granite. All have medium- to coarse-grained, equigranular textures.
A string of isolated minor plutons and stocks occupy an additional 150 km extension to the west of Chagai Hills batholiths, comprising stocks, dykes, sills, domes and lopoliths varying in size from 1 km to several hundred metres across. These are predominantly of medium- to coarse-grained, inequigranular (seriate) and porphyritic dacite, but also include basaltic andesite and rhyodacite phases.
The porphyry copper mineralisation of the belt is spatially and genetically associated with porphyritic stocks, as at Saindak (Ahmed et al., 1972; Sillitoe and Khan, 1977) and Reko Diq (Schloderer and McInnes, 2006; Razique et al., 2007).
Porphyry copper-bearing stocks are predominantly centred on multiphase, steeply dipping, porphyritic intrusions with both cylindrical and dyke-like geometries, and irregular combinations of both.
REKO DIQ - Geology and Mineralisation
The key mineralised centres at Reko Diq, the temporal and spatial distribution of which are described above, are as follows:
Tanjeel Porphyry Copper System
The Tanjeel deposit is centred on an Early Miocene (23.3 Ma), ~1000 x 500 m diorite porphyry stock and a cluster of quartz-diorite porphyry intrusions cutting Late Oligocene andesitic volcanic rocks. It contains an Indicated Mineral Resource of 214 Mt @ 0.60% Cu of largely supergene enriched mineralisation (Perelló et al., 2008) that is typically associated with supergene clays, residual silica and hypogene sericitic alteration. The supergene copper mineralisation forms an irregular 50 to 100 m thick chalcocite blanket beneath a 40 to 50 m leached cap dominated by hematite. This cap is characterised by a mosaic of hydrothermal quartz, quartz-limonite veins, supergene clays (kaolinite±alunite), Fe-oxides (jarosite+hematite±goethite) and Cu-oxide (chalconthite+chrysocolla) assemblages. Discontinuous, 1 to 2 m thick perched sulphide zones of preserved pyrite±chalcocite carrying up to 0.4% Cu are trapped in the highly siliceous portions of quartz-diorite intrusions, generally near the contact with the underlying chalcocite blanket. The presence of relict pre-chalcocite sulphides and multiple zones of intense hematisation in the leached cap suggest cyclic leaching and enrichment of multiple chalcocite blankets. The leached cap is generally barren of copper, with the exception of the perched sulphide zones. The underlying chalcocite blanket contains ~5 to 10% disseminated and vein pyrite coated by chalcocite with an average grade of 0.6 % Cu. It has a sharp upper contact with the leached cap and an irregular gradational lower contact with hypogene Cu-sulphides containing up to 0.30% Cu (Maryono et al., 2008; after Razique, 2013).
Northern Porphyry Centres
The individual porphyry centres (e.g., H14E, H12, H35, H11 and H4N) are generally restricted to ~200 m thick hornblende-bearing diorite porphyry stocks hosted by andesitic volcanic rocks and microdiorite intrusions. They are typically associated with intense potassic alteration of biotite±K feldspar and magnetite in their centre, surrounded by discontinuous and poorly developed sericitic alteration and extensive propylitic assemblages. Disseminated and veinlet hypogene Cu-sulphides (chalcopyrite±bornite) are associated with intense potassic alteration and quartz-magnetite veins, overprinted in turn by chlorite and epidote. The northern porphyry centres generally have an average grade of ~0.35% Cu and 0.20g/t Au with the exception of H35 that is intersected by late-stage NE and NW trending quartz veins contributing to higher gold grades, with an Inferred Mineral Resource of ~45 Mt @ 0.40% Cu and 0.61g/t Au (Perelló et al., 2008; after Razique, 2013).
Western Porphyry Centres
The four western porphyry Cu-Au centres (H79, H15, H14 and H13) are spatially and temporally associated with Miocene porphyry stocks and dykes that intruded the Late Oligocene and Early Miocene (28 to 22 Ma) volcanic and sedimentary rocks of the Reko Diq Formation and underlying Oligocene red sedimentary rocks of the Dalbandin Formation. Paleocene sedimentary rocks of the Juzzak Formation and Eocene rocks of the Saindak Formation found elsewhere in the district are interpreted to be present at depth (Siddiqui, 1996, 2004; Perelló et al., 2008).
Each of the porphyry centres contains compositionally and texturally discrete calc-alkaline porphyry intrusions. The more altered rocks tend to be granodioritic and locally granitic, whilst less altered intrusions have quartz diorite compositions and the least altered are, overall, metaluminous, medium-K quartz-diorite to granodiorite (SiO2 ~56 to 63 wt.%; Richards et al., 2012; Razique, 2013). These less altered, predominantly quartz diorite porphyry intrusions are somewhat enriched in incompatible elements, depleted in compatible elements, and characterized by an La/Yb ratio of between 10 and 25 (Richards et al., 2012; Razique, 2013).
With the exception of the hornblende diorite at the H79, the common texture, mineralogy and temporal relationships of the porphyry intrusions at H13, H14 and H15 allow the recognition of four distinct intrusive suites at each centre, from oldest to youngest, i). medium-grained porphyritic (PFB1), ii). coarse-grained porphyritic (PFB2), iii). coarse-grained porphyritic with aphanitic groundmass (PFB3) and iv). coarse-grained porphyritic with phaneritic groundmass (PFB4). These intrusive rocks generally have porphyritic textures with phenocrysts of plagioclase and variable biotite, igneous quartz and amphibole embedded in a fine-grained microcrystalline to aphanitic groundmass.
The hornblende porphyry at H79 is altered to hydrothermal biotite-K feldspar-magnetite. At H15, H14 and H13, the early medium-grained and intermineral coarse-grained quartz diorite to granodiorite porphyry (PFB1 and PFB2) are intensely altered to hydrothermal biotite-K feldspar-magnetite, forming a potassic alteration assemblage. This assemblage is variously overprinted by a sericite and chlorite alteration, with the latter being more intense at H15 than in either H14 or H13 (Perelló et al., 2008; Razique, 2013). The pervasive alteration of the PFB1 and PFB2 porphyry produced a range of SiO2 contents that are largely between 55.7 and 63.4 wt.%, but are locally higher (Razique, 2013).
The two late- to post-mineral coarse-grained porphyritic intrusive suites with aphanitic groundmasses (PFB3 and PFB4) are less altered and have limited ranges of SiO2 between 59.0 and 61.8 wt.%, consistent with diorite compositions (Razique, 2013). At H15, PFB3 is characterised by pervasive chlorite-sericite-clay minerals at shallow levels and a deeper core of weak chlorite-sericite-biotite. At H14, the PFB3 suite is similarly altered to chlorite-sericite-clay, but to a much lesser extent (Perelló et al., 2008; Razique, 2013). The late barren porphyry suite (PFB4) is most common at H14, but only found over a small interval in drill core at H15. Characteristically, it has a well preserved primary mineralogy and coarse-grained porphyritic textures but is locally overprinted by propylitic alteration assemblages. Ubiquitous quartz phenocrysts and the dominant diorite compositions to the least altered rocks suggests that all of the intrusive suites had a primary quartz diorite composition. The individual porphyry centres may be summarised as follows:
H79 porphyry centre
This porphyry centre is characterised by hornblende diorite porphyry intrusions occurring as a swarm of narrow (50 x 100 m thick), NE-trending (50°) dykes which contain common angular to subangular, centimetre-scale xenoliths of volcanic and sedimentary country rocks. These hornblende diorite dykes are overprinted by intense potassic alteration and fringed by propylitic assemblages, both of which are overprinted by sericite-chlorite alteration (Perelló et al., 2008; Razique 2013).
H15 porphyry centre
The H15 mineralised system is centred on an 800 m diameter early quartz diorite stock (PFB1) intruded by NE and NW trending, 25 to 50 m thick and 100 to 200 m long dykes of intermineral quartz diorite porphyry (PFB2). The early quartz diorite porphyry has xenolith rich contact zones, whilst the intermineral quartz diorite porphyry intrusions have locally preserved centimetre-scale chilled margins. Both the early and intermineral (PFB1 and PFB2) porphyries have been overprinted by pervasive potassic, sericite-chlorite and sericitic alteration assemblages (Perelló et al., 2008; Razique, 2013) as described below, and both are cut by multistage quartz sulphide veins. Small (~30 x ~20 m), late mineral quartz diorite porphyry (PFB3) intrude the older porphyries, associated hydrothermal alteration, veins and mineralised rock. A small dyke that is petrologically similar to PFB4 at H14 has been encountered in drilling.
Early fine-grained biotite-magnetite alteration occurs in the deeper andesitic volcanic and clastic sedimentary rocks peripheral to the main ore zone, whilst coarser grained early dark-mica and magnetite is commonly associated with deep-barren potassic alteration in the early PFB1 and PFB2 porphyry intrusions. The early fine-grained biotite-magnetite alteration occurs as very fine-grained biotite with associated magnetite and is locally characterised by pervasive dark green chloritisation interpreted to be due to hornfelsing during contact metamorphism. Early biotite-magnetite in the central granodiorite (PFB1) and quartz-diorite (PFB2) porphyry intrusions occurs as disseminated µm size flakes to millimetre scale irregular patches and discontinuous veins of early dark-mica invading the matrix and weak cleavage planes of plagioclase feldspars.
The main ore-stage mineralisation is associated with potassic alteration that occurs as a broad, 1000 x 800 m zone in the central part of the H15 complex, with the bulk of Cu-sulphides accompanied by an intense potassic assemblage and early quartz±magnetite±K feldspar veins in the porphyry intrusions and adjacent host rocks. This ore-stage potassic alteration is characterised by a variably developed biotite + K feldspar + magnetite + quartz assemblage with associated quartz ±magnetite ±K feldspar veins. Much of the high-grade Cu-Fe-sulphide mineralisation is accompanied by intense potassic alteration and quartz stockworks in the porphyry intrusions and adjacent country rocks (Perelló et al., 2008; Razique 2013). Potassic alteration can be differentiated as either biotite- or K feldspar-rich end-member assemblages (Razique 2013).
Biotite-rich potassic alteration occurs as an intense shreddy biotite + magnetite ±K feldspar assemblage. Shreddy biotite replaces phenocrysts of biotite ±amphiboles and occurs as concentrations of complexly intergrown clusters obliterating all traces of the replaced phenocrysts, and when intense, is found as randomly oriented clusters in the matrix. The magnetite is granular, <1 mm disseminated crystals in the matrix, with strings of micro-veinlets containing quartz ±K feldspars ±sulphides. Biotite-rich assemblages are the dominant style of potassic alteration in the early PFB1, syn-mineral PFB2 and in a large volume of andesite and volcanogenic sedimentary host rocks surrounding the H15 porphyry centre. It is accompanied by quartz ±magnetite ±K feldspar A-type veins (c.f., Gustafson and Hunt, 1975), disseminated and vein-hosted chalcopyrite + bornite ±pyrite containing up to ~1.5 % Cu and 1.0 g/t Au in the H15 complex (Razique 2013).
K feldspar-rich potassic alteration comprises K feldspar + quartz + biotite + magnetite ±anhydrite and is predominantly found in the PFB1 granodiorite porphyry, suggesting a strong host rock control on the style of potassic alteration. It is also locally observed in the PFB2 quartz-diorite and adjacent andesitic volcanic rocks where these rocks are in contact with PFB1 granodiorite porphyry. K feldspar replaces plagioclase, occurring in selvages and suture zones of A and B-type quartz veins (c.f., Gustafson and Hunt, 1975). The porphyry groundmass is commonly replaced by fine-grained pervasive K feldspar and mosaics of hydrothermal quartz with locally preserved pseudomorphs of hydrothermal biotite replacing mafic minerals. Magnetite is predominantly disseminated, but also occurs as granular and hair-line micro-veinlets. The K feldspar-rich potassic alteration hosts fine-grained, intergrown chalcopyrite + bornite ±pyrite (in a 6:3:1 ratio respectively) ±molybdenite accounting for the highest grade Cu-Au mineralisation in the H15 deposits (Razique 2013). Coarser grained molybdenite + chalcopyrite generally occurs in the centre line of quartz-sulphide B-type veins. The intensity of Cu-Fe-sulphide mineralisation increases to as much as ~4.0 vol.% Cu where early quartz A-veins and potassic alteration are more intense in the porphyry intrusions, while the adjacent country rocks may contain up to 2.0% Cu and 1.5 g/t Au (Perelló et al., 2008).
The main ore-stage potassic alteration is cut by, and A and B-type veins are truncated by, 1 to 20 mm thick chalcopyrite-pyrite ±quartz D-veins with up to 5 cm wide halos of sericite-chlorite (clay) alteration.
Deep in the alteration system, the potassic alteration is overprinted by a poorly developed sodic-calcic assemblage. This assemblage is characterised by Na and Ca-rich products replacing K and Fe-rich minerals (Carten, 1981; Dilles and Einaudi, 1992). Sodic-calcic alteration comprising albite+epidote ±actinolite ±chlorite has been encountered at depths of >1000 m in the H15-H14 porphyry complex. Albite replaces primary K feldspars and occurs as cream to white Na-rich plagioclase and as mm scale suture zones and selvages within and fringing quartz veins (Razique 2013).
The potassic alteration is largely overprinted and surrounded by transitional sericite-chlorite (clay) alteration, passing outward into extensive sericitic (phyllic) alteration containing <0.15% Cu and 0.1g/t Au. Propylitic alteration is extensively developed in peripheral volcanic and underlying sedimentary rocks to the north, east and west of the main H15 intrusive complex (Razique 2013).
Transitional sericite-chlorite (clay) alteration is a distinct assemblage, occurring in the transition between the central potassic core and outer sericitic (phyllic) zones and is important part of the Reko Diq H15 porphyry Cu-Au (Mo) deposits. In hand specimen, it is recognised as a pale-green assemblage of fine-grained greenish soapy sericite/muscovite, random flakes of green chlorite and subordinate grey-green illitic clay, with variable amounts of albite, calcite and rutile (Perelló et al., 2008). Fine-grained flakes of sericite/muscovite replace plagioclase, overgrow secondary K feldspars and occur as random clusters in the rock matrix. Sericite is also generally abundant in selvages to mm to cm scale D-type sulphide veins (c.f., Gustafson and Hunt, 1975). Chlorite is mainly found in concentrations and irregular patches replacing magmatic hornblende and/or biotite and/or hydrothermal biotite. Clay minerals are mainly greyish green, fine-grained micaceous illite replacing plagioclase feldspars. Magnetite has commonly been converted to martite and locally transformed to specular hematite. Sulphides in the sericite-chlorite (clay) alteration zone mainly occurs in cm scale chalcopyrite-pyrite D-type veins as well as disseminated grains within vein halos contributing around 0.40 % Cu and 0.20 g/t Au at H15 (Perelló et al. 2008; Razique 2013).
Sericitic (phyllic) alteration occurs as broad halos overprinting a sizeable volume of volcanic and sedimentary rocks surrounding both the H15 and H14 porphyry centres. It typically consists of pale-white, texturally destructive aggregates of quartz, fine grained sericite/muscovite, clay minerals (kaolinite ±montmorillonite), minor tourmaline, anhydrite, abundant (up to 5 vol.%) pyrite and traces of chalcopyrite (Perelló et al. 2008). It is pervasive in close proximity to the porphyry system and contains discontinuous veins and zones of quartz-anhydrite and late-stage 1 to 20 mm thick pyrite ±chalcopyrite D-veins containing <0.2% Cu and 0.1g/t Au, and includes a distinct assemblage of covellite ±bornite + chalcopyrite + pyrite. This alteration-sulphide assemblage is commonly found in narrow sub-vertical structural zones, but also laterally follows gently dipping 1 to 20 m thick stratigraphic units of sandstone, siltstone, conglomerate and volcanic host rocks on the flanks of H15. These zones are generally characterised by abundant (up to 8 vol.%) 300 to 400 µm, disseminations and micro-veinlets of pyrite ±chalcopyrite (in a ratio of 7:3 respectively) crystals intergrown with bornite and/or covellite leading to higher Cu-grades (~1.0% Cu) at H15.
Propylitic alteration, comprising chlorite + epidote with accessory pyrite and carbonate, is extensively developed in the peripheral volcanic and sedimentary host rocks around the combined H15-H14 complex. It extends to depth and occurs as a large halo surrounding and overlapping the potassic alteration in the deeper core of the two complexes. The deep potassic alteration in the PFB1 and PFB2 porphyry intrusions are commonly overprinted by fine-grained disseminated (~2 vol.%) chlorite + epidote replacing biotite and/or amphiboles. The intermediate to felsic volcanic country rocks locally exhibit intensely developed (~5 vol.%) mm-scale irregular specks of epidote and chlorite that appear to be randomly distributed clusters replacing amphiboles and plagioclase. Chlorite occurs as dark flakes overprinting pale-brown biotite and/or amphiboles. Propylitic altered rocks generally lacks quartz-veins and sulphides, with the exception of late-pyrite veins, and contains no significant Cu sulphide mineralisation (Razique 2013).
H14 porphyry centre
Mineralisation at H14 is centred on ~100 x 250 m diameter early quartz diorite porphyry stocks (PFB1) that are intruded by numerous NE trending, 25 to 50 m wide and 100 to 200 m long stocks to dykes of quartz diorite porphyry (PFB2). Mineralised quartz sulphide veins are accompanied by pervasive potassic and sericitic alteration of the PFB1 and PFB2 porphyry suites. Narrow (10 to 50 m2) late mineral quartz diorite porphyry (PFB3) intrude the early porphyry phases. Pyrite or chalcopyrite ±quartz-sericite veins that are 2 to 20 mm thick postdate emplacement of the late mineral PFB3 porphyry intrusions. Late barren quartz diorite porphyry (PFB4) dykes intrude all of the previous phases and the hydrothermal mineralisation and are characterised by coarse grained porphyritic textures with local sericitic and propylitic alteration assemblages.
The alteration patterns observed in the H14 mineralised centre are similar to those of the H15 of the complex. These include early fine-grained biotite-magnetite assemblages in deep andesitic volcanic and clastic sedimentary rocks peripheral to the ore zone, with early dark-mica and fine-grained magnetite most common in the deeper porphyry intrusions and adjacent country rocks. A distinct sodic-calcic alteration suite overprints the latter at a depth of ~980 m below surface. A well-preserved 800 x 800 m ore-stage potassic alteration zone occurs in the centre of the complex with intense hydrothermal quartz±K feldspar±magnetite veins carrying much of the high-grade Cu-Fe-sulphide mineralisation. The potassic core is concentrically surrounded by a mixed potassic-sericitic alteration, transitional sericitic-chlorite (clay) and outer sericitic (phyllic) alteration zones containing <0.15% Cu and 0.1g/t Au. A propylitic chlorite-epidote±pyrite assemblage is well developed in the peripheral volcanic and underlying sedimentary rocks to the east and west of the H14 complex.
H13 porphyry centre
H13 is the southernmost centre, and is associated with NW trending, hornblende rich quartz diorite dykes that are intruded by narrow 10 to 20 m thick, isolated dacite porphyry stocks. The quartz diorite porphyry texturally resembles the PFB1 and PFB2 phases in the H15 and H14 centres. A hydrothermal breccia crops out in the central part of the system. The quartz diorite contains xenoliths of the host volcanic and sedimentary rocks and is overprinted by hydrothermal potassic and sericite-chlorite alteration assemblages, and is cut by quartz veins.
Southern Porphyry Centres
The Parrah Koh, H36, H3, H9, H27, H7 and H8 porphyry centres are clustered along a NW trend in the southern part of the Reko Diq complex, apparently related to late Miocene (11 to 10 Ma) diorite porphyry intrusions emplaced into the andesitic volcanic rocks and the underlying sedimentary sequence. Individual porphyry centres generally cover an area of ~500 m2 and tend to have lateral sericitic alteration halos. The mineralised diorite and quartz-diorite porphyry stocks are commonly cut by ~100 m thick late-stage hornblende-dacite porphyry bodies which form a barren core to the Parrah Koh, H3, H9, H27 and H8 porphyry centres. A 300 m wide, weakly mineralised hydrothermal tourmaline breccia cuts the H8 porphyry system. Mineralisation within the southern porphyry centres occurs as disseminated and veinlet chalcopyrite-pyrite±bornite, typically with associated intense potassic alteration, and quartz and quartz±magnetite veins, and an average grade of ~0.4% Cu and 0.20 g/t Au. Locally, the gold content is augmented by 1 to 20 cm thick, late-stage quartz veins (e.g., H36 and H9; Maryono et al., 1998). The H8 complex contains an Inferred Resource of 335 Mt @ 0.38% Cu and 0.19g/t Au (Perelló et al., 2008; after Razique, 2013).
Reserves and Resources
According to the Tethyan Copper Company (website, viewed February, 2018), Mineral Resources at Reko Diq comprise:
Total Indicated + Inferred Mineral Resource - 5.9 Gt @ 0.41% Cu, 0.22 g/t Au, including an
Economically mineable resource of 2.2 Gt @ 0.53% Cu, 0.30 g/t Au.
In addition to the references listed below, this summary is drawn from Razique, A., 2013 - Magmatic evolution and genesis of the giant Reko Diq H14-H15 porphyry copper gold deposit, District Chagai, Balochistan-Pakistan; M.Sc.Dissertaion to the Faculty of Graduate Studies, Geological Sciences, The University of British Columbia, Vancouver, 337p.
The most recent source geological information used to prepare this summary was dated: 2008.
This description is a summary from published sources, the chief of which are listed below.
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Reko Diq H14-H15
Perello J, Razique A, Schloderer J and Asad-ur-Rehman 2008 - The Chagai Porphyry Copper Belt, Baluchistan Province, Pakistan : in Econ. Geol. 103 pp 1583-1612|
Razique, A., Tosdal, R.M. and Creaser, R.A., 2014 - Temporal Evolution of the Western Porphyry Cu-Au Systems at Reko Diq, Balochistan, Western Pakistan: in Econ. Geol. v.109, pp. 2003-2021.|
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