Railway
Metallic minerals that are found as alluvial deposits in the sands of river valleys and flood plains are known as:
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Answer: Option D — Placer deposits
Detailed Solution
Placer deposits are concentrations of heavy, valuable minerals that have been eroded from their original rocks, carried by water and deposited in the sands and gravels of river beds, valleys, flood plains and beaches.
How placer deposits form:
Rocks containing valuable minerals are broken down by weathering and erosion.
Running water carries the fragments downstream. Heavy minerals, being denser than ordinary sand, settle quickly where the water slows down, such as on the inner bends of rivers, in pools below rapids and behind obstacles.
Over time, these heavy particles are concentrated in certain layers of sand and gravel.
Similar sorting by waves and currents creates beach placers along coasts.
Minerals that form placers:
Only minerals that are heavy, chemically stable and resistant to wear form placers. These include:
Gold (the most famous placer mineral).
Platinum.
Tin (as the mineral cassiterite).
Diamonds and other gemstones.
Heavy beach minerals such as monazite, zircon and garnet.
Examples:
- The California Gold Rush (1848) and the Klondike Gold Rush in Canada (1896) began with discoveries of placer gold.
- Tin placers in Malaysia, Thailand and Indonesia have been major world sources.
- In India, people pan for gold in the sands of the Subarnarekha river (whose name means "streak of gold") in Jharkhand and the rivers of the north-east.
- Beach sands of Kerala and Tamil Nadu are rich in heavy minerals.
Other modes of occurrence of minerals:
Veins and lodes: minerals deposited in cracks, crevices, faults or joints in igneous and metamorphic rocks. Small occurrences are called veins and large ones lodes. Many metallic minerals, such as tin, copper, zinc and lead, occur in this way.
Beds or layers: minerals formed by deposition and accumulation in sedimentary rocks, such as coal, gypsum and potash salts.
Residual deposits: formed by decomposition of surface rocks, leaving behind the insoluble minerals, such as bauxite.
Why not the other options?
- ✗Vein deposits fill cracks in rocks.
- ✗Lode deposits are large veins.
- ✗Residual deposits remain after weathering removes soluble material.
Quick tip
Remember: Placer = heavy minerals (gold, tin, platinum) in river sands; veins and lodes = in rock cracks; residual = bauxite.
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Minerals & Energy Resources — Preparation
This question tests your understanding of Minerals & Energy Resources, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers India's mineral resources (iron ore, manganese, bauxite, copper, mica and others) and energy resources (coal, petroleum, natural gas, nuclear power and renewable energy), with their main producing regions, mines, oil fields and power plants. Questions often ask where a mineral is found, which state is the leading producer, or where an important power plant or oil field is located. For example, Jharia is known for coal, Digboi in Assam is the oldest oil field in India, and Tarapur in Maharashtra is the site of India's first nuclear power plant. Practise the minerals and energy resources questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.