| 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.
North Afghanistan
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.
East Afghanistan
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 magamatism 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.
. . . the Kabul Block is particularly
prospective for sediment-hosted copper in its basement
sediments . . .
. . . this diverse geological foundation has resulted
in a significant mineral heritage with over 1400 mineral
occurrences recorded to date . . . |
