Read the sources given below and answer the questions that follows:

Minerals are usually found in “ores". The term ore is used to describe an accumulation of any mineral mixed with other elements. The mineral content of the ore must be in sufficient concentration to make its extraction commercially viable. The type of formation or structure in which they are found determines the relative ease with which mineral ores may be mined. This also determines the cost of extraction. It is, therefore, important for us to understand the main types of formations in which minerals occur.

Minerals generally occur in these forms:

In Igneous and Metamorphic rocks minerals may occur in the cracks, crevices, faults or joints. The smaller occurrences are called veins and the larger are called lodes. In most cases, they are formed ‘when minerals in liquid/molten and gaseous forms are forced upward through cavities towards the

earth's surface. They cool and solidify as they rise. Major metallic minerals like tin, copper, zinc and lead etc. are obtained from veins and lodes.

In Sedimentary rocks a number of minerals occur in beds or layers. They have been formed as a result

of deposition, accumulation and concentration in horizontal strata. Coal and some forms of iron ore

have been concentrated as a result of long periods under great heat and pressure. Another group of sedimentary minerals include gypsum, potash salt and sodium salt. These are formed as a result of evaporation especially in arid regions.

Another mode of formation involves the decomposition of surface rocks and the removal

of soluble constituents, leaving a residual mass of weathered material containing ores. Bauxite is

formed this way.

Certain minerals may occur as alluvial deposits in sands of valley floors and the base of hills. These

deposits are called ‘placer deposits’ and generally contain minerals, which are not corroded by water.

Gold, silver, tin and platinum are most important among such minerals.

The ocean waters contain vast quantities of minerals, but most of these are too widely diffused to be of economic significance. However, common salt, magnesium and bromine are largely derived from ocean waters. The ocean beds, too, are rich in manganese nodules.

The placer deposits generally contain minerals, which are not corroded by:

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Detailed Solution

The placer deposits generally contain minerals which are not corroded by water.
Placer deposits are the aggregation of precious minerals due to gravity separation. Weathering processes break the heavy and stable minerals free from their matrix; they slowly wash downslope and into water streams, which swiftly wash away the lighter flux material. As a result, heavy minerals concentrate in streams, beaches, and residual gravels, forming ore deposits. Minerals that produce placer deposits have a higher density and are chemically resistant to weathering.

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