2. Extractive industries: ores and metals

If we consider the abundance of elements in the earth's crust, we can see that most elements of economic interest are extremely diluted in rocks (table 1 ), compared with the usual content of products used in metallurgy (table 2 ).

Between these two extremes lie the grades found in mining deposits. These are veritable geological "monsters", the result of an accumulation caused by natural concentrating phenomena affecting the earth's crust, often due to repetitive variations in the physical and chemical conditions to which the rocks are subjected. The result is high concentrations of metals in small volumes. Table 3 shows the values corresponding to mining cut-off grades, below which the...