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Hajigak |
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Afghanistan |
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Fe
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Hajigak iron deposits are located in Bamyan province, 130 km west of Kabul in Afghanistan (#Location: 34° 40' 6"N, 68° 3' 25"E).
Afghanistan lies at the junction between the Indo-Australian and Asian crustal plates and is composed of a series of terranes that separated from Gondwana before being accreted onto the southern margin of the Eurasian plate from the Cretaceous (~140 Ma) to recent times. During the Early Cretaceous, there is evidence of collision between one of these terranes, the Farad block, and the Eurasian plate, along the Herat fault zone, followed soon after by collision between the Helmand and Farad block. The rocks that host the Hajigak deposit are now part of the Herat fault zone, and are likely to have originally been part of the Farad and Helmand blocks.
The oldest rocks of the host succession crop out to the north-west of the Hajigak deposit. These are the Mesoproterozoic Jawkol Formation, which comprise grey silicified limestones and dolomites, interbedded with dark grey crystalline schists and light coloured quartzites that exhibit evidence of amphibolite grade metamorphism.
The iron deposit is hosted by the Neoproterozoic Awband Formation that, together with the underlying Kab Formation, constitutes the Qala Series, a 4500 m thick sequence of metavolcanic and metasedimentary rocks. The Kab Formation is composed of dark grey, sandy, sericitic schists, interpreted as metamorphosed clastic and acid volcanic rocks, with minor beds of marble and phyllite. The Awband Formation comprises schists (quartz-sericite, quartz-chlorite sericite, quartz-sericite-chlorite and carbonaceous-sericite) that are interpreted to represent metamorphosed acid and basic tuffites and argillaceous rocks, with minor cherts and marbles. The distinctive Green Schist Formation, which immediately overlies the Awband Formation (and may be part of the latter), is dominantly composed of green chlorite schists and quartz-sericitic schists, locally intruded by granodiorites.
The Upper Devonian rocks of the Hajigak Formation are faulted against the Green Schist unit. Lower Cretaceous and younger rocks crop out to the south west, unconformably overlying the older sequences.
The dominant strike of the Proterozoic and Palaeozoic sequences is between north-east and north-north-east, with a regional dip of ~50° towards the south-southeast or south-east. One major steeply dipping fault juxtaposes the Neoproterozoic and crystalline Mesoproterozoic rocks, while another separates Upper Devonian rocks from the Neoproterozoic Green Schist Formation. Another suite of north-south and northwest-southeast trending faults, some probably thrusts, cut the Neoproterozoic succession, including the iron ore deposits.
The Neoproterozoic rocks of the Qala Series are interpreted to have been deposited in a slowly subsiding marine basin, initially filled with a variable sequence of sandstones and minor volcaniclastic sediments, followed by an increase in volcanic activity and the deposition of iron-rich chemical sediments from exhalative iron-rich fluids. These rocks were buried and subjected to greenschist facies metamorphism.
The Hajigak deposit trends northeast-southwest over a length of >12 km, width of 3 km and comprises 16 separate orebodies, some of which are up to 5 km long, 380 m wide, and extend 550 m down-dip (U.S. Geological Survey Afghanistan Mineral Assessment Team. 2011). The deposit has been grouped into three geographical parts, the western, central and eastern sections, as well as a substantial area of thin fragmental ore deposits that form a further four surficial deposits.
The main hematitic ore is medium- to fine-grained and displays a variety of massive, banded and porous textures, occurring as lenses and sheets within the Awband Formation. Drilling has indicated the thickness of the lenses to be up to 100 m, while the depth of mineralisation is untested beyond 180 m below surface.
Mineralisation occurs as unoxidised primary and semi-oxidised forms. The primary mineralisation occurs below depths of 100 m and consists of magnetite and pyrite, with up to 5% chalcopyrite and pyrrhotite. The oxidised zone, extending up to 130 m below the current surface, consists mainly of magnetite, martite and hydrogoethite. Two other oxidised mineralisation types, hydrogoethite/hematite/semi-martite and carbonate/semi-martite, are found sporadically in small amounts within the deposit. Alteration that may be related to the mineralising event has been observed within the host rocks, and includes sericitisation, silicification and carbonisation.
Soviet resource estimates from testing in the early 1960's were:
Measured + indicated resources of oxidised ore: ~90 Mt @ 62% Fe
Measured + indicated resources of primary ore: ~16 Mt @ 60% Fe
Inferred resources of both primary and oxidised ore: ~315 Mt of unspecified grade
Potential resources: ~1.4 Gt.
The similar Chuy, Khaish, Khakriz, Kharzar and Zerak sediment hosted iron deposits are adjacent and to the east of the main Hajigak iron deposit (U.S. Geological Survey Afghanistan Mineral Assessment Team. 2011).
This description is summarised from the Afghanistan Project report on the British Geological Survey website.
The most recent source geological information used to prepare this decription was dated: 2011.
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.
Hajigak
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