Background concentrations vs. mineralisation

Several estimations of the abundance of elements in Earth’s crust have been made. The abundances of elements in the crust vary because of the heterogeneity of geological processes and the nature of the elements themselves.

The diagram in Figure 1 illustrates the elemental abundance of the upper continental crust and shows that oxygen and silicon are the most abundant elements. Silicon and oxygen bond together to form silicate and form silicate minerals. 92% of all minerals on Earth are silicate minerals, which makes them the most abundant mineral group. Common ore minerals containing oxide and sulphide are relatively rare minerals in the crust. The diagram also shows that sodium, aluminium, silicon, potassium, calcium, and iron, are abundant elements. They all occur in percentage levels and together with oxygen and silicon, form most of the rock-forming minerals in the upper continental crust.

The rarest elements in the upper continental crust are marked yellow in the diagram. These elements are “iron-loving” (siderophile) elements, which make their abundance higher in the earth’s core and lower in the continental crust because the earth’s core mainly consists of iron and nickel. From the diagram we can read that gold, platinum, and palladium are among these metals and we call them precious metals.

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Figure 1: Elemental distribution of the crust.

Average elemental abundances of the continental crust show that many metals that we need for the fossil-free energy transition occur in very low concentrations in the bedrock. Natural enrichment processes must have occurred for these metals to be concentrated to a level where we can extract the metal economically.

The table below compares iron, copper, lithium, and gold. It shows that the average abundance of iron in the continental crust is ca. 5.6%. However, the typical iron concentrations in an iron mineralisation are 20-60%, which requires a geological process to enrich the crust in a certain site between 4 and 10 times in respect to the average crustal abundance of iron. For copper, a natural enrichment of more than 100 times is required to produce typical mineralisation of with a copper concentration slightly under 1%. The average abundance of lithium in the crust is only 20 parts per million but a typical lithium mineralisation has a concentration of around 1% lithium and higher. This requires a geological process to enrich a site with lithium about 500 times. Gold is an extreme example, it requires a typical geological enrichment of more than 1000 times to enrich gold from average crustal levels in the parts per billion (ppb), to the parts per million (ppm) level in a gold mineralisation.

Table 1: Table to show the crustal abundance, ore grade, enrichment level and mineral phase of certain metals.

An important implication on the grade of a mined commodity is the amount of rock that needs to be excavated and how much waste it produces. In terms of an iron ore containing 65% of the iron-bearing mineral magnetite, one metric tonne of iron ore produces 350 kg of waste rock if 100% of magnetite can be recovered.

For gold with an ore grade of 1 ppm, only 1 gram of gold produces 999,99 kg waste rock. This is the reason why the mining industry produces the largest amount of waste among all industries in the world. Many geological exploration projects reported results on mineral resources that have elevated metal content, however it is not economically feasible to extract them now under current legislation and technology. It is very important that improving the effectiveness of extracting these metals is done, as by-products to manage our natural resources in the most responsible way possible to reduce the amount of mine waste.