Dharwar system

Also called: Dharwar rocks · Topic: Minerals, Rocks and the Rock Cycle · NCERT: Class 12, Ch 5 "Mineral and Energy Resources"

Meaning

The Dharwar system is a group of India's first sedimentary rocks, about 2.5–1.8 billion years old. They were made from worn-down Archaean rocks and later strongly changed by heat and pressure, so today they are highly metamorphosed (changed by heat and pressure) schists, quartzites and banded iron formations. It is the most important rock system for India's economy. Most of the country's iron ore, manganese, gold, copper and uranium comes from these rocks.

Explanation

How the Dharwar rocks formed

• Step 1: the old crust wore down.
• The Archaean rocks (granites and gneisses) were India's first solid crust.

• Weathering and erosion broke them into small pieces.

• Step 2: the pieces settled in water.

• Rivers carried the pieces into seas and basins.

• Layer by layer, they hardened into India's first sedimentary rocks.

• Step 3: heat and pressure changed them.

• Over a very long time, these layers were buried, heated and squeezed.

• So they are now very highly metamorphosed, more than any later Precambrian system.

• Result: rocks rich in metal.

• Billions of years of heating, squeezing and folding gathered metals into rich ore bodies.

Main rock types

• Schist: a metamorphic rock that splits into thin, shiny flakes because its mica crystals are lined up.
• Quartzite: a very hard rock made when sandstone is heated and pressed.
• BIF (banded iron formation): a rock made of thin, alternating layers of iron oxide and silica.
• It formed when iron dissolved in the early oceans reacted with oxygen.

• BIFs are the main source of high-grade iron ore.

• Greenstone belt: a long strip of old volcanic and sedimentary rocks, turned green by metamorphism. Gold is often found in these belts.

Key features

• No fossils. Life had not yet formed hard parts, so these rocks contain no fossils, even though they are sedimentary in origin.
• Part of the Precambrian (Purana) group. "Precambrian" means older than about 541 million years. The group, from oldest to youngest, is Archaean → Dharwar → Cuddapah → Vindhyan.
• They sit on the Archaean basement. The Dharwar rocks lie on top of the older Archaean rocks, which formed from the first cooling crust.

Feature	Dharwar system
Age	about 2.5–1.8 billion years
Origin	sedimentary (from eroded Archaean rocks)
Metamorphism	very high
Fossils	none
Main rocks	schist, quartzite, BIF
Key minerals	iron, manganese, gold, copper, uranium

Where it occurs

• Type area: the system was first studied around Dharwar and Shimoga (Shivamogga) in Karnataka. "Type area" means the place where a rock system was first described and named.
• Other regions: the Aravallis (Rajasthan) and the Chota Nagpur Plateau (Jharkhand).
• Iron ore:
• Bababudan and Kudremukh, Karnataka
• Bailadila, Chhattisgarh

• Noamundi, Jharkhand

• Gold: Hutti, Raichur district, Karnataka (still working).

• Hutti Gold Mines Limited (HGML) runs mines at Hutti and Hira-Buddini in Raichur district [3].
• The gold lies in the greenstone belts of the Eastern Dharwar Craton: the Hutti, Kushtagi and Deodurg–Raichur schist belts [3][4].

• Gold is also found in the Chitradurga, Dharwad, Gadag, Gulbarga, Hassan, Haveri, Kolar and Tumkur districts of Karnataka [3].

• Gold: Kolar Gold Fields (KGF), Kolar district, Karnataka (now closed).

• Bharat Gold Mines Limited (BGML) ran the mines. BGML was set up in April 1972 [7].

• Mining stopped making money. All production stopped from October 2000, and the company was formally closed on 1 March 2001 [7].

• Aravallis:

• They are among the oldest fold mountains in the world, about 2.5 billion years old (NCERT).
• They are made up of the Aravalli and Delhi supergroups. A supergroup is a large set of related rock layers.

• Rajasthan's non-ferrous minerals (copper, lead, zinc) come from these old peninsular rocks. Non-ferrous means "not iron".

• Industry built on these rocks: the steel belt of Chota Nagpur–Odisha–Chhattisgarh grew up around these ores.

Don't confuse with

• Archaean system: the Archaean rocks are older (more than about 2.5 billion years). They are igneous and metamorphic (granite, gneiss, charnockite) and formed from the first cooling crust. The Dharwar rocks are sedimentary in origin, made from eroded Archaean rocks. Both are fossil-free.
• Dharwar craton: a craton is the old, stable core of a continent. The Dharwar craton is an Archaean "granite–greenstone" block whose history goes back about 3.4–3.5 billion years [5][6]. It is a block of crust, not the same thing as the 2.5–1.8-billion-year-old Dharwar rock system.
• Cuddapah system: these are younger Proterozoic sediments with little metamorphism, lying above the Eparchaean unconformity (a surface marking a long time gap). They give limestone, barytes, asbestos and uranium (Tummalapalle) [2]. The Dharwar rocks are very highly metamorphosed and give mainly metallic ores.
• Gondwana system: much younger rocks that hold most of India's coal. The metals and mica of the Peninsula come from the Precambrian rocks, including the Dharwar, not from the Gondwana.

Prelims Hooks

• Order, oldest to youngest: Archaean → Dharwar → Cuddapah → Vindhyan → Gondwana → Deccan Traps → Tertiary → Quaternary.
• Common trap: Dharwar rocks are sedimentary in origin, yet they are fossil-free and highly metamorphosed.
• Least to most metamorphosed: Vindhyan < Cuddapah < Dharwar.
• Iron ore mine–state pairs: Bababudan and Kudremukh (Karnataka), Bailadila (Chhattisgarh), Noamundi (Jharkhand).
• Gold: Hutti and Hira-Buddini in Raichur district, Karnataka, are still working and run by HGML [3]. Kolar (KGF) was closed on 1 March 2001 [7].
• Aravallis (about 2.5 billion years old) are among the world's oldest fold mountains and are made of Dharwar-age Precambrian rocks (Aravalli and Delhi supergroups).

Mains Points

• Why the Peninsula is rich in minerals (GS-I/III):
• The shield is old and stable. Its Archaean–Dharwar rocks have been heated, squeezed and folded for billions of years.
• This gathered metals into rich ore bodies: iron, manganese, gold, copper and uranium in Karnataka, Odisha, Jharkhand, Chhattisgarh and Rajasthan.

• It decides where industry is located, for example the steel belt of Chota Nagpur–Odisha–Chhattisgarh.

• Rock type decides the resource:

• Highly metamorphosed Dharwar rocks give metallic ores.

• The barely changed Cuddapah and Vindhyan sediments give mainly non-metallic materials such as limestone for cement and building stone.

• Running out of resources (GS-III):

• Kolar closed in 2001 when mining stopped making money [7]. Gold in the Dharwar rocks lies deep and is low-grade, so it now costs more to mine.
• Mining continues at Hutti [3], but this shows that even rich, old rock systems face depletion. India needs to explore more and use its minerals with care.

Related concepts

• Indian rock systems
• Archaean system
• Cuddapah system
• Vindhyan system

Read more

• India's rock systems I: Archaean to Vindhyan (the Precambrian shield)