Earth’s crust has a wealth of mineral
resources within different types of its rocks, Geologists and Explorers make a
great effort to understand and discover these resources.
Mining companies come to invest in
extraction these minerals from their host rocks, and because such things as extraction costs, labor
costs, and energy costs of extraction are expensive, they have to
know if these deposits are economic or not.
So, What are the Factors that determine
whether a Deposit can be Mined?
1- Mineral Concentration
A deposit of minerals varies considerably in time and place. In general, the higher the concentration of the substance, the more economical it is to mine.
Since economics is what controls the grade or concentration of the substance in a deposit that makes the deposit profitable to mine, different substances require different concentrations to be profitable. But, the concentration that can be economically mined changes due to economic conditions such as demand for the substance and the cost of extraction.
The copper concentration in copper ore deposits has shown changes throughout history. From 1880 to about 1960 the grade of copper ore showed a steady decrease from about 3% to less than 1%, mainly due to increased efficiency of mining. From about 1960 to 1980 the grade increased to over 1% due to increasing costs of energy and an abundant supply produced by cheaper labor in other countries.
Gold prices vary on a daily basis. When gold prices are high, old abandoned mines re-open, when the price drops, gold mines close. The cost of labor is currently so high in the U.S. that few gold mines can operate profitably, but in third world countries where labor costs are lower, gold mines that have ore concentrations well below those found in the U.S. can operate with a profit.
So, for every substance we can determine the concentration necessary in a mineral deposit for profitable mining. By dividing this economical concentration by the average crustal abundance for that substance, we can determine a value called the concentration factor. The table below lists average crustal abundances and concentration factors for some of the important materials that are commonly sought. For example, Al, which has an average crustal abundance of 8%, has a concentration factor of 3 to 4. This means that an economic deposit of Aluminum must contain between 3 and 4 times the average crustal abundance that is between 24 and 32% Aluminum, to be economical.
2- Grade and Tonnage
For a deposit to be mineable it must contain more than a given concentration of the valuable commodity, and more than a given tonnage of this commodity.
Most deposits that are both big, close to the surface and high-grade have been mined out and what remains are small rich deposits and much bigger low-grade deposits in more remote regions or at greater depth in the crust.
Some metals are abundant in the Earth’s crust and they are present in high concentrations in ores. As a consequence their price is relatively low. Other metals are present in far lower concentrations and their price is much higher.
In any deposit the ore type varies, from small areas of rich, high-grade ore to larger areas with lower grades, or a mixture of high and low grade ore.
The cut-off grade “Important Parameter”
What is left in the ground after mining is material, geologically very similar to the material that has been mined, but simply containing a lower concentration of the ore metal, a concentration that is below a certain threshold.
To include sub-ore in the material being mined would lead to the operation becoming unprofitable: the cost of mining would exceed the value of the recovered metal.
But what would happen if the metal price improves? It is evident that if the price increases, the cut-off grade decreases because lower-grade material can then be mined at a profit. As a consequence, the amount of mineable material in the deposit increases.
In many cases, the ore contains amounts of valuable metals in concentrations that are below the normal cut-off grade, but if they are extracted as a by-product during the recovery of the major ore metals they contribute significantly to the viability of the operation.
Common examples of such “bonus metals” include gold or silver in copper ores, and platinum metals in Ni ores. Another topical example is the rare- earth elements which were initially recovered as a by-product during mining of the Bayan Obo iron deposit in China.
In contrast, the presence of small amounts of other metals can complicate the extraction process and decrease the value of the ore. Examples of “toxic” or unwanted metals include phosphorous in iron ore and arsenic in basemetal sulfide ores.
3- Nature of the Deposit
Characteristics to be considered include the type of mineral, the grain size, and the texture of the ore, all of which influence the cost of mining and the extraction of the valuable commodity.
The lowest extraction costs are for ores in which the extracted element is only mechanically bound into its gangue (e.g. free-milling gold ores or placer deposits); higher extraction costs are associated with ores in which the element is chemically bound to sulfur or oxide (most base-metal ores) because it takes more energy to break such chemical bonds than to mechanically liberate a particle.
The highest extraction costs are for ores in which the element is chemically bound to silicates because these bonds are much stronger than metal sulfur bonds. Each type of ore has its advantages and disadvantages.
The grain size and the hardness of the ore influence the cost of grinding it to the fine powder that is fed into the refinery or smelter. Three Zn-Pb (Cu) deposits in Australia provide a striking example. All have similar ore grades but the Broken Hill deposit has been metamorphosed to granulite facies and its coarse ore is very easy to process; Mt Isa is less metamorphosed and its finer-grained ore is less attractive; and the virtually unmetamorphosed McArthur River ore is so fine that the ore metals cannot be extracted from waste minerals by simple crushing.
Friable and soft sedimentary ores are easier to mine and process than ores in hard magmatic rocks. And finally a continuous and compact ore body is far easier to mine than an ore body that is disrupted by faulting or other geological factors.
Two platinum deposits in southern Africa provide an interesting example. Those in the Bushveld Complex in South Africa are near-continuous reefs that make the mining operation predictable and efficient, but deposits in another intrusion, the Great Dyke in Zimbabwe, although of similar grade to the Bushveld deposits, are so irregular and disrupted by faulting that mining had proved very difficult.
4- Location of the Deposit
Its value, and its very viability, decreases if it is far from centres of industry or population, or in a harsh climate, or in a politically unstable region. All these factors increase the cost of mining or of bringing the metals to market; or they render the operation of a mine too dangerous or risky.
The depth of a deposit has a major influence on the cost of mining. A shallow deposit can be exploited in an open-pit mine, which is far cheaper than the alternative, an underground mine, that must be developed if the deposit is deeper.