Geology of Afghanistan
Afghanistan has some of the most complex and varied geology in the world. The oldest rocks are Archean and they are succeeded by rocks from the Proterozoic and every Phanerozoic system up to the present day. The country also has a long and complicated tectonic history, partly related to its position at the western end of the Himalaya. This diverse geological foundation has resulted in a significant mineral heritage with over 1400 mineral occurrences recorded to date. Historical mining concentrated mostly on precious stone production, with some of the oldest known mines in the world believed to have been established in Afghanistan to produce lapis lazuli for the Egyptian Pharaohs. More recent exploration in the 1960s and 70s resulted in the discovery of significant resources of metallic minerals, including copper, iron and gold, and non-metallic minerals, including halite, talc and mica. The bedrock geology of Afghanistan can be thought of as a jigsaw of crustal blocks separated by fault zones, each with a different geological history and mineral prospectivity. This jigsaw has been put together by a series of tectonic events dating from the Jurassic.
The Tadjik block of northern Afghanistan formed the southern margin of the Eurasian continental plate during Permo-Triassic times. The Palaeozoic basement was intruded by Triassic granitoids as a result of subduction related to the first stages of the closure of the Tethys Ocean during the Cimmeride Orogeny. Subsequent to this, a Jurassic clastic sequence was deposited, which changes upwards to Cretaceous carbonate platform sedimentation. This area is now the prime target for hydrocarbon exploration, although the exposed granitoids in the northeast of the block are prospective particularly for precious (and base) metal mineralisation, and further exploration of the occurrences identified to date is warranted.
The Cimmeride Orogeny
During the Triassic, parts of the northern edge of the Gondwanaland supercontinent broke away and began drifting north, before colliding with the Tadjik block, resulting in the Cimmeride Orogeny. The orogeny is marked by two distinct collisions which brought first the Farad block against the Tadjik block, followed closely by the Helmand block against the Farad block. The Herat Fault system marks the suture line of this first collision, which was finished by the beginning of the Cretaceous, and the Panjao Suture marks the line of the second collision that was complete by early Cretaceous times. Both suture zones are ophiolite bearing, and the Herat Fault system in particular has had a long history of sedimentation and igneous activity up to the present. The Farad block was subsequently overlain by Upper Jurassic-Cretaceous sediments and the Helmand block by Cretaceous sediments only. During this period the Pamir and West Nuristan blocks of northeast Afghanistan were also accreted onto Eurasia. These four blocks, together with the Tadjik block, are collectively known as the Afghan Block. Due to processes discussed below, the southeastern margin of this Block is considered prospective for precious and base metal mineralisation, as well as rare metals in the numerous pegmatite fields.
The Himalayan Orogeny
Following a brief period of quiescence, tectonic activity began once again as India drifted north, away from Gondwanaland and towards the enlarged Eurasian plate with the Afghan block at its southern margin. The first evidence of this is preserved as the Kandahar volcanics, which marked the beginning of the development of a volcanic arc on the margins of the Eurasian plate. These were intruded by subduction-related, 'I-type' granitoids in the Helmand and West Nuristan blocks (during the Cretaceous to early Tertiary). This geological setting is highly prospective for a number of different mineralisation styles, and the large number of mineral discoveries to date only reinforces the potential of the east-central Afghanistan region. Igneous activity was not confined to this region, with younger (Oligocene) alkaline intrusions and basaltic extrusions in the Farad Block and the sedimentary basins within the Herat Fault Zone. The chemistry of these rocks suggests derivation from a mantle source beneath a zone of continental extension (within an overall setting of dextral transtension). Oligocene granitoids were also intruded into the thickened continental crust of northeast Afghanistan. By the start of the Tertiary, the widespread marine sedimentation that had preceded the Himalayan Orogeny had become restricted to the Tadjik Block and by Neogene times even this had become localised as the collision of India began to raise the area above sea level. Himalayan deformation of the Afghan Block resulted in the reactivation of many of the internal block boundaries including the Herat Fault system (as discussed above, but not active since the Miocene) and the Chaman Fault system (which marks the southeast edge of the Afghan Block and is still active to the present day). Folding and thrusting of the Mesozoic sediments also led to basin inversion and imbrication with the Palaeozoic basement.
To the east of the Afghan Block is a complex collage of tectonic units that marks the collision zone with the Indian plate. During the Cretaceous period, the East Nuristan volcanic arc was accreted to the margin of Eurasia (although magmatism continued into the Eocene). This was followed by the docking of the Kabul Block. The Kabul Block is somewhat of an enigma in Afghan geology. It includes, to the west and east, the Kabul and Khost ophiolites respectively, but is itself formed of Lower Palaeozoic basement overlain by Mesozoic sediments. It is now believed that the Block was a sliver of continental crust, separated from the Indian and Afghan blocks by oceanic crust that got caught up in the collision and was accreted to the edge of the Afghan Block before final collision with India. The Kabul Block is particularly prospective for sediment-hosted copper in its basement sediments and chromite in the ophiolites. The final block in the Afghanistan jigsaw is the Katawaz Basin in Southeast Afghanistan. This is interpreted as a flexural basin on the western margin of the Indian Plate where subsidence synchronous with sedimentation resulted in the deposition of more than 10 km of Tertiary sediments before shortening and inversion in the late Tertiary as India finally collided with Afghanistan. Sedimentation across the country since this time has been continental, with large areas of Quaternary deposits particularly across the very north and south of the country.