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Chehelkureh

Iran

Main commodities: Cu Pb Zn Ag Au
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The Chehelkureh (or Chehel Kureh) base metal copper, lead, zinc, silver, gold deposit is located in the Nehbandan-Khash zone of eastern Iran, ~70 km NE of the Afghan border, 110 km NNE of Zahedan, and ~1000 km SE of Tehran (#Location: 30° 15' 00"N, 60° 7' 00"E).

The Chehelkureh base metal deposit is located within the Sistan suture zone of eastern Iran (Tirrul et al., 1983), between the Lut and Afghan (or Helmand) microcontinental blocks to the west (in Iran) and east (in Afghanistan) respectively. These two blocks are exposed as Late Cretaceous to Tertiary rocks of continental character, which were separated by Early Cretaceous rifting to create an intervening ocean basin that eventually closed by middle Eocene time through NE-dipping subduction beneath the Afghan/Helmand block and subsequent collision (Tirrul et al., 1983). The regional context of these blocks, is as described below.

Following the amalgamation of Pangea in the Late Palaeozoic, a major triangular embayment remained, which constituted the Palaeotethyan ocean, partially separating what was to become Laurasia to the north and Gondwana to the south. During the late Permian and Triassic, a large sliver of the Gondwana margin of the Palaeotethyan ocean, the Cimmerian microcontinent, was separated and moved north to form the Neotethyan ocean in its wake. As this sliver progressed north, the Palaeotethyan Ocean closed, with its oceanic crust being subducted below Laurasia to the north. After the Cimmerian microcontinent collided with Laurasia, the Neotethyan ocean between Gondwana and Laurasia began to widen during the Cretaceous, and opened to the west to separate Laurasia and Gondwana. During the early Cretaceous, further slivers separated from Gondwana on the southern margin of the Neotethys ocean, including the Lut and Afghan/Helmand microcontinents. Subsequently, during the Cretaceous, as Gondwana (and Laurasia) began to break-up, and the Indian Ocean opened, the Neotethys ocean began to close, with subduction below the microcontinents as each progressively collided with the approaching Eurasian plate to the north. As part of this process the Lut and Afghan/Helmand microcontinents collided and were cratonised onto the Eurasian Plate. The final collision between the Eurasian plate (with the amalgamated Lut and Afghan/Helmand microcontinents on its leading edge, and the Arabian-African plate to the south, formed the major collisional Zagros Fold and Thrust belt and associated magmatic arc, commencing possibly as early as the Late Eocene, with closure during the Miocene (~10 Ma) and deformation continuing to the present (Muttoni et al., 2009; McQuarrie et al., 2003; Hessami et al., 2001).

The Sistan suture zone (Camp and Griffis, 1982) features two distinct geologic terranes, the Neh-Ratuk Complex and Sefidabeh Basin, which are considered to be an accretionary prism and a forearc basin, respectively. The Neh Complex forms the western half of the suture zone, while the Sefidabeh Basin forms the eastern half, enclosing inliers of the Ratuk Complex. Rocks within these terranes comprise thick piles of deep-water marine sediments with ophiolite and ophiolitic mélange. Rocks in the mélange are pre-Cretaceous to Late Cretaceous in age and more abundant in the western (Neh Complex) than in the eastern (Ratuk Complex) range. These rocks trend mostly north-south to NW-SE. They consist of gabbro, peridotite, schist, amphibolite, serpentinite, radiolarite and limestone. Although rocks in the ophiolitic mélange are weakly metamorphosed, some spatially associated rocks are metamorphosed to amphibolite facies. Dykes and lavas from the Chehelkureh ophiolitic mélange are plagioclase-phyric basalts with chemical compositions that indicate mid-ocean ridge and marginal basin tholeiites (Desmons and Beccaluva, 1983) with Zr/Y (2.8-3.4) and Zr/Nb (26-38) ratios that resemble normal MORB (Saunders et al., 1980). The metamorphosed rocks in the Neh Complex comprise schist and amphibolite. Ultrabasic rocks predominate in the ophiolite complex, exposed mostly to the SW, and include peridotite, harzburgite, dunite, wehrlite and serpentinite. The contact between the ultrabasic and adjacent sedimentary rocks is faulted. A K-Ar age of amphibole from gabbro in an ophiolite complex has been dared at 124±11 Ma (Delaloye and Desmons, 1980).

There are no metasedimentary rocks older than Cretaceous in the Sistan suture zone. The Cretaceous facies comprise flysch (turbidite) metasediments and volcanic rocks (Stocklin et al., 1972) up to 3 km thick, tectonised and metamorphosed to zeolite-subgreenschist facies during the Cretaceous to form tightly folded, steeply dipping and faulted slate, phyllite and schist with small lenses of marble. The K-Ar age of the schists near Kuh-e-Malusan is 65.3±1.6 Ma (Delaloye and Desmons, 1980). Paleocene turbidites, comprising shale and sandstone with rare limestone layers, are strongly fractured and weakly metamorphosed, while a thick sequence (up to 1 km) of Eocene abyssal turbidites are widespread. In the latter sequence, a basal conglomerate, which unconformably overlies the older deposits, is overlain by phyllite, which includes lenses of metamorphosed limestone.

Several granitoid stocks and dykes intrude the sedimentary sequence, oriented parallel to the major NW-SE trending fault set, and locally hornfels Eocene turbidites. Plutonic rocks crop out mostly in the triangular block bounded to the west by the north-south, dextral strike-slip Neh fault and to the east by the NNW-SSE trending Chehelkureh Fault (Valeh and Saeedi, 1989). This wedge also includes the Chehelkureh deposit. Intrusive bodies consist of quartz monzodiorite and granodiorite at the Chehelkureh deposit, granodiorite at the Pourchangi ore deposit, and granite near Garagheh. A 40.9±1.1 Ma K-Ar age of biotite in the Garagheh granite reflects late Eocene to Oligocene magmatism (Delaloye and Desmons, 1980). These calc-alkaline intrusions represents a final episode of magmatism prior to collision between the Lut and Afghan/Helmand microcontinents (McCall, 1997). A subduction zone beneath the Afghan/Helmand block is implied by the Paleogene subduction-related arc volcanism present in the Chehelkureh area. The accretionary prism fore-arc basin polarity, the structural vergence, and general younging of the accretionary prism to the SW, as well as the position of the (relatively) high P-T metamorphic rock on the inner side of the prism, are consistent with NE-dipping subduction (Tirrul et al., 1983)

The Chehelkureh deposit is hosted by a sequence of Eocene intercalated greywackes, siltstones and shales, that are likely part of the Neh-Ratuk Complex accretionary prism, folded by a north-south trending structure, the axis of which is displaced by east-west striking faults, with drag folding of the eastern limb. The host Eocene sequence is bounded to the west by Cretaceous ophiolitic mélange and to the east by middle Eocene limestone (Valeh and Saeedi, 1989).

The Chehelkureh deposit comprises numerous lenses and veins in NW-SE trending faults as open-space fillings, that extend over an interval of 1500 m in a 157° direction, displaced by late brittle faults striking roughly east-west. The zone of metallic mineralisation varies from 80 to 280 m in width.

The hydrothermal alteration assemblages associated with mineralisation predominantly consist of quartz, chlorite, Fe-Mg-Ca carbonates, sericite, kaolinite and pyrite, with veins enriched in Cu, Zn, Pb, Ag, Se and Bi. Fluid inclusion evidence and stable isotope geothermometry, based on quartz-carbonate pairs, suggest that metallic mineralisation formed at high temperatures of ~450°C from a moderately saline (up to 15 wt.% NaCl equiv.) fluid. The majority of the fluids from Chehelkureh have relatively high δ18O values (>5‰) and a fairly narrow range of δD values (-31 to -23‰). Stable isotope data signatures indicate contribution of ore fluids from intrusive bodies, especially as δ34S values are consistent with a magmatic origin, consistent with the spatial and temporal relationship of base metal mineralisation to granitoids in the Chehelkureh ore deposit (Maanijou et al., 2012).

There were two stages of primary Cu-Zn-Pb mineralisation: i). metallic mineralisation concentrated along the brittle, finely fractured parts of the beds of sandstone, siltstone and shale. The in-fill mineralisation includes quartz, calcite, dolomite, ankerite, siderite, ilmenite, rutile, molybdenite, pyrrhotite, arsenopyrite, pyrite and chalcopyrite, of which pyrrhotite, pyrite and chalcopyrite are the most common; and ii). mineralisation that formed along fractures that crosscut sandstone, siltstone and shale, displacing them by several millimetres. These fissures are filled with quartz, dolomite, ankerite, siderite, chalcopyrite, sphalerite, pyrite, galena, selenian galena, marcasite, nevskite and paraguanajuatite. Chalcopyrite, galena and pyrite occur in dolomite and quartz veins, while selenian galena usually accompanies carbonatisation and argillic alteration. Locally, fine-grained pyrite, chalcopyrite and pyrrhotite also form veins.

The deposit is estimated to contain 14 Mt @ 1.5% Cu, 0.88% Pb, 1.81% Zn, 22 g/t Ag, 0.14 g/t Au (Maanijou et al., 2012).

The most recent source geological information used to prepare this summary was dated: 2012.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Chehelkureh

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Maanijou M, Rasa I and Lentz D R,  2012 - Petrology, Geochemistry, and Stable Isotope Studies of the Chehelkureh Cu-Zn-Pb Deposit, Zahedan, Iran: in    Econ. Geol.   v.107 p683-712


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