Mode of occurrence: how mineral deposits form

Detail

Why the mode of occurrence matters

• Mineral: a natural substance with a fixed chemical make-up and a definite inner structure.
• Ore: a mix of a mineral with other rock, where the mineral is rich enough to mine at a profit.
• Mode of occurrence means the way a mineral deposit forms and the rock it sits in. It decides:
• where a mineral is found (old igneous or metamorphic rocks, sedimentary basins, plateau tops, beaches, the sea floor)

• how it is mined (open pit, quarry or underground shaft)

• Geographers group deposits into five modes: 1. veins and lodes 2. beds or layers 3. residual deposits 4. placer deposits 5. ocean waters and beds

Mode	Host rock or setting	How it forms	Typical minerals	Indian example
Veins and lodes	Igneous and metamorphic rocks	Hot molten or gaseous matter fills cracks, then hardens	Tin, copper, zinc, lead, gold	Khetri copper; Aravalli lead-zinc; Kolar gold (Karnataka)
Beds or layers	Sedimentary rocks	Layers are laid down and concentrated; heat, pressure or evaporation	Coal, some iron ore, gypsum, potash, salt	Damodar coal; Rajasthan gypsum; Gujarat salt
Residual	Weathered surface rock on plateaus	Soluble parts wash away and the ore is left behind	Bauxite, some iron and manganese ores	Eastern Ghats of Odisha; Amarkantak plateau
Placer	Valley sands, gravels and beaches	Running water or waves sort heavy grains	Gold, silver, tin, platinum, diamonds; beach-sand minerals	Chavara (Kerala); Manavalakurichi (Tamil Nadu)
Ocean waters and beds	Seawater; deep sea floor	Dissolved salts; nodules grow slowly on the sea floor	Salt, magnesium, bromine; manganese nodules	Gujarat salt pans; Central Indian Ocean Basin

(i) Veins and lodes

• Veins and lodes: mineral deposits that fill cracks, crevices, faults and joints in igneous and metamorphic rocks.
• Fault: a break in rock where one block has moved past the other.

• Joint: a crack in rock where the blocks have not moved.

• How they form:

• Deep inside the earth, minerals are in a molten (liquid) or gaseous state.
• The pressure pushes them upward through the openings in the rock.

• As they rise they cool, and they solidify (turn hard) inside the cracks.

• Size rule: small deposits are veins and large ones are lodes.

• Minerals: mostly metallic, namely tin, copper, zinc and lead, and also gold in quartz.
• India:
• Khetri copper belt, Rajasthan (Aravalli range)
• Aravalli lead-zinc deposits of Rajasthan (Zawar area, near Udaipur)

• Kolar gold fields, Karnataka: gold sits in quartz reefs (long, vein-like bands of quartz) inside old Dharwar rocks

• Mining link: veins go deep and are narrow, so they are usually worked by underground shaft mines.

(ii) Beds or layers in sedimentary rocks

• Bedded mineral deposits: minerals laid down, collected and concentrated in flat (horizontal) layers, called strata, of sedimentary rock.
• Sedimentary rock: rock made of material that settled in layers, usually under water.

(a) Formed by long heat and pressure

• Coal:
• Plants were buried in swamps.
• Over millions of years, heat and pressure squeezed out water and gases.
• The carbon left behind formed coal seams (layers of coal).

• India's main seams are in the Gondwana basins of the Damodar, Mahanadi and Godavari valleys.

• Some iron ores: banded iron formations (BIF) are thin, alternating layers of iron oxide and silica. They were laid down in ancient seas and later hardened by heat and pressure. They are the source of the rich haematite of the Odisha–Jharkhand belt.

(b) Formed by evaporation: evaporites

• Evaporites: minerals left behind when water evaporates. They form mainly in arid (dry) regions.
• How they form:
• Salty water collects in a closed lake, a lagoon or a tidal flat.
• Strong sun and dry wind evaporate the water.

• The dissolved salts become too concentrated to stay dissolved. They come out as solid layers in a fixed order.

• Minerals: gypsum, potash salt and sodium salt.

• India:
• Gypsum: Rajasthan (Bikaner, Barmer, Nagaur and Jaisalmer districts of the Thar)

• Salt: Gujarat (Rann of Kutch and the Saurashtra coast) and Rajasthan (Sambhar Lake and the Didwana and Pachpadra brines)

• Current ranking: Rajasthan is India's third-largest salt producer, after Gujarat (first) and Tamil Nadu (second). Rajasthan made 21.58 lakh tonnes out of India's 268.87 lakh tonnes in 2014-15 [7].

• History link: in the Dandi March (1930), Gandhi made salt from seawater at Dandi on the Gujarat coast. This showed how easily salt can be won by evaporation along the coast (NCERT Class 12 question).
• Mining link: flat, shallow layers suit open-pit (open-cast) mining. Most Indian coal is mined this way. Deep seams need shafts.

(iii) Residual deposits

• Residual mineral deposits: ore that stays behind at the surface after weathering has removed the other parts of the rock.
• How they form:
• Cause: a hot, wet climate with alternating wet and dry seasons. This brings strong chemical weathering (rock breaking down when water and air react with its minerals).
• Process: rainwater leaches the rock (dissolves and carries away its parts). Silica, lime, soda and potash wash downward.

• Result: the parts that do not dissolve, the oxides of aluminium, iron and manganese, stay at the top. The weathered layer becomes richer in ore. This is called residual enrichment.

• Bauxite: the ore of aluminium. It forms from aluminium-silicate rocks (such as khondalite and basalt) under laterite caps on peninsular plateau tops.

• Laterite: a red, iron- and aluminium-rich crust formed by heavy leaching. The name comes from the Latin later, meaning brick.

• Main Indian areas:

• Eastern Ghats of Odisha, e.g. Panchpatmali in Koraput district
• Andhra Pradesh

• the Amarkantak–Maikal plateau (Madhya Pradesh–Chhattisgarh). Amarkantak is also where the Narmada rises.

• Current data:

• Odisha has about 51% of India's bauxite resources, followed by Andhra Pradesh (16%), Gujarat (9%), Jharkhand (6%) and Maharashtra (5%) (IMYB 2019) [5].
• These are the East Coast bauxite deposits. In Odisha the bauxite lies with khondalite rocks of the Eastern Ghats Mobile Belt [5].

• Odisha produced about 76% of India's bauxite in 2020-21 [6].

• Some iron ores (lateritic iron) and manganese ores (in parts of Odisha, Karnataka and Goa) also form this way.

(iv) Placer deposits

• Placer deposits: heavy, durable mineral grains that collect in the sands and gravels of valley floors and beaches. They are dropped there by running water or waves.
• How they form:
• Source rocks (often veins) weather and break up.
• Rivers or waves carry the loose grains.

• Light grains are washed away. Heavy grains settle where the water slows down: inside river bends, at the foot of rapids, and along beaches.

• Conditions: the minerals must be heavy and not corroded (worn away chemically) by water.

• Minerals: gold, silver, tin, platinum and diamonds.
• India's marine placers: beach-sand heavy minerals
• The group of seven: ilmenite, leucoxene (brown ilmenite), rutile, zircon, sillimanite, garnet and monazite [2][4].
• Ilmenite (FeO.TiO₂) and rutile (TiO₂) are the two main ores of titanium [2].
• Extent: these sands occur along almost the whole coast, from the Saurashtra coast (Gujarat) in the west to the Digha coast (West Bengal) in the east [2].
• Key deposits: Chavara (Kerala), Manavalakurichi (Tamil Nadu) and Gopalpur (Odisha) [2][4].
• Monazite (the source of rare earths and thorium): 12.73 million tonnes of resources as of March 2021, in 130 deposits [3].
  • Beach placers: Kerala, Tamil Nadu, Odisha, Andhra Pradesh, Maharashtra and Gujarat.
  • Inland alluvium: parts of Jharkhand, West Bengal and Tamil Nadu [3].

• Why the Kerala coast is rich: rivers carry weathered grains from the old crystalline rocks of the Western Ghats to the sea. Monsoon waves then sort the grains and pile the heavy ones on the beach.

• World example and plate tectonics (NCERT Class 11):

• Rich gold placers lie on the Ghana coast (West Africa), yet the nearby land has no gold-bearing source rock.
• The source veins are in Brazil (South America).
• Alfred Wegener used this as evidence for continental drift: Africa and South America were once joined.

(v) Ocean waters and ocean beds

• Ocean mineral resources: minerals taken from seawater or from the sea floor.
• Seawater:
• It holds huge amounts of minerals, but most are too diffuse (too thinly spread) to extract at a profit.
• Exceptions: common salt, magnesium and bromine.

• India: salt pans of Gujarat and Tamil Nadu; bromine from the brines (very salty water) of the Rann of Kutch.

• Ocean beds: polymetallic (manganese) nodules

• Polymetallic nodules: potato-sized lumps on the deep sea floor. They grow very slowly and are rich in manganese, nickel, copper and cobalt.
• In 1987, India became the first country to get pioneer investor status. The UN gave it an exclusive area in the Central Indian Ocean Basin (CIOB) [9].
• In 2002, India signed a 15-year exploration contract with the International Seabed Authority (ISA). India keeps 75,000 sq km in the CIOB [8].
• Estimated resource: 380 million tonnes of nodules. They contain 92.59 Mt of manganese, 4.7 Mt of nickel, 4.29 Mt of copper and 0.55 Mt of cobalt [8].
• In 2016, India signed a second 15-year ISA contract. It covers polymetallic sulphides in 10,000 sq km along the Central Indian Ridge and the Southwest Indian Ridge [8].
• All three metals (nickel, copper and cobalt) are on India's critical minerals list (2023). This makes the CIOB nodules important for the future.

Mining method follows the form of the deposit

Method	When it is used	Example
Open-pit (open-cast) mine	Ore is near the surface, in thick or flat bodies; the cover rock is stripped off	Coal of Jharia and Talcher; bauxite of Panchpatmali
Quarry	Building stone or other rock is cut from an open face at the surface	Marble of Makrana (Rajasthan); limestone; sandstone
Underground shaft mine	Ore lies deep, in narrow veins or lodes; vertical shafts and side tunnels reach it	Kolar gold fields; Khetri copper

• Rule: the depth and form (vein, layer, crust or loose sand) of a deposit decide the method.
• Placers are dredged or scooped up.
• Residual crusts are mined in open pits.
• Veins need shafts.

Prelims Hooks

• Veins and lodes sit in igneous and metamorphic rocks. Bedded deposits and evaporites sit in sedimentary rocks. This is a common trap.
• Small = vein, large = lode. Both give tin, copper, zinc and lead (Khetri copper, Rajasthan).
• Gypsum, potash and sodium salt are evaporites and form in arid regions. Gypsum mainly comes from Rajasthan.
• Salt producers, highest first: Gujarat > Tamil Nadu > Rajasthan [7].
• Bauxite is a residual deposit under laterite caps. Odisha has about 51% of resources (2019) [5] and made about 76% of output (2020-21) [6].
• Placer minerals are heavy and not corroded by water: gold, silver, tin, platinum, diamonds. Monazite, the thorium and rare-earth source, is a beach placer [3].
• Beach-sand pairs: Chavara – Kerala; Manavalakurichi – Tamil Nadu; Gopalpur – Odisha [2].
• Ilmenite and rutile are titanium ores [2].
• Ghana gold placers with source veins in Brazil were Wegener's evidence for continental drift.
• Seawater gives salt, magnesium and bromine. India's CIOB nodule area is 75,000 sq km, under an ISA contract from 2002, and India became a pioneer investor in 1987 [8][9].

Mains Points

• Geology shapes distribution.
• Old Peninsular igneous and metamorphic rocks hold vein metals (Aravalli, Dharwar).
• Gondwana sedimentary basins hold coal.
• Laterite-capped Eastern Ghats plateaus hold bauxite.
• The coastline holds placers.

• So mineral wealth is concentrated in the plateau states of Odisha, Jharkhand and Chhattisgarh, which are also tribal and forested areas. This raises questions of displacement and benefit-sharing (GS-I and GS-III).

• Mode decides mining method and its environmental cost.

• Open-pit mining of shallow beds and residual crusts strips forests and topsoil. Bauxite hilltops are watersheds and sacred sites, e.g. the Niyamgiri (Odisha) dispute.
• Shaft mining of veins is safer for land but more dangerous for workers.

• Beach-sand dredging affects coastal erosion and fishing livelihoods.

• Strategic value of placers and ocean minerals.

• Monazite (12.73 Mt) [3] and titanium sands support atomic energy and the aerospace sector.
• CIOB nodules (380 Mt, rich in Ni, Cu and Co) [8] can reduce import dependence for critical minerals used in batteries and the energy transition.

• Deep-sea mining also needs strong environmental safeguards.

• Evaporite and marine resources support livelihoods. Salt production in Gujarat, Tamil Nadu and Rajasthan [7] supports salt-pan workers in harsh, dry conditions. Their welfare and working conditions are a regional development issue.