Many mineral deposits are virtually unreachable beneath oceans, while others are in desert, jungle, or arctic areas, remote from power and transportation required for economic mining. This search requires significant investments in research, time and money. The mineral exploration process is intricate and multi-faceted, tying cutting-edge technologies with old-fashioned determination. Advances in technology and the earth sciences - particularly geology, physics and chemistry - have greatly improved the methods used to explore for mineral resources. Ore (solid rock or mineral from which metals or valuable minerals may be extracted) deposits are not easy to find and many of the ones exposed on surface have already been found. Sophisticated techniques now help locate deposits hidden far beneath the surface.
First, geologists use geology maps to look for favorable areas to explore. Geologists use the physical and chemical characteristics of the rocks they are looking for to zero in on prospective areas. They formulate a concept through comparison with known deposits and their knowledge of geological processes and mineral distribution patterns.
Once favorable geology is established, remote sensing, airborne and ground geophysics and geochemistry are used. The information gathered in geological, geophysical and geochemical studies is used to delineate areas for more intense investigation until a significant target is identified.
Samples for testing are obtained through trenching - if the minerals are reasonably close to the surface - or drilling - if the target area is deep underground.
If significant mineralization is present, additional drilling is used to delineate the size and parameters of the deposit. These samples are analyzed for their mineral content.
If there is enough metal in the deposit to be produced economically, ore samples are taken and examined for the metallurgical quality of the metal in order to determine the appropriate processing technique required to remove it.
To extract the ore a mine must be permitted and constructed; the type of mine built depends on the topography of the region and the physical characteristics of the deposit. If the ore is near the surface Open Pit Mining may be practical in the initial stages. The ore is mined in four steps: drilling, blasting, loading and hauling. A pattern of holes is drilled in the pit and filled with explosives; the explosives are detonated in order to break up the ground, so it can be loaded by large shovels or front-end loaders into haul trucks.
If the ore is deep below the surface or if it cannot be mined economically by open pit methods then it is mined from underground - Underground Mining. In this case, a tunnel called an adit (horizontal) or a shaft (vertical), is dug into the earth. Short tunnels leading from the adit or shaft, called stopes, are dug to access the ore. The surface containing the ore, called a face, is drilled and loaded with explosives. Following blasting, the broken ore is loaded into trucks and taken to the surface. Once mining is completed in a particular stope, it is backfilled with a cement compound.
The haul trucks transport the ore from the open pit or the underground mine to an onsite mill where it is crushed into smaller loose rock. Low-grade gold ore goes from the crusher straight into the separation process, whereas most other ores go from the crusher into a series of grinding mills where steel balls grind the ore into a fine powder.
Onsite separation processes then remove the mineral particles from the masses of host rock waste. Different processes have been designed for different minerals and for different ore types.
The process for low-grade gold ore is relatively simple; a cyanide solution is applied to a heap (the leaching circuit) of the crushed ore, dissolving the gold, which is then collected. How high-grade gold ore powder is separated from host rock, depends on which type of ore it is, since different ore types require different processes for optimal recovery of the gold. For example:
- Oxide ore goes directly to the leaching circuit, where cyanide dissolves the gold.
- Refractory ore, which contains carbon, is roasted at 1000 degrees Fahrenheit, burning off the sulphide and carbon, then goes to the leaching circuit.
- Sulphide refractory ore, which does not contain carbon, is oxidized in an autoclave in order to separate the sulphide safely, and in an environmentally friendly manner, from the ore, which then enters the leaching circuit.
The gold is then extracted from the cyanide solution and deposited onto activated carbon, from which the gold is then chemically stripped. The gold is then melted onsite in a furnace (a smelter) and poured to create dore bars, which are about 90% pure gold. The gold dore bars are usually shipped offsite to a refinery where they undergo further processing to strip out the remaining impurities from the gold to become 99.99% pure gold.
Most base metals, however, are found in a complex chemical form and often deposited in sulphide compounds and therefore cannot be economically converted into metal onsite at the mine. In such cases, the fine ore powder produced from milling is separated by flotation: The finely ground ore is mixed with water to form a slurry. Certain chemicals or reagents that coat only the desired mineral components are added in small amounts. Rising air bubbles capture the coated mineral particles and float them to the surface where they are skimmed off to a separate circuit; thereby leaving the waste rock (tailing) in the slurry. The concentrated metal (concentrate) is then shipped offsite to smelting and refining facilities.
After a number of years, the ore reserves in a mine will be exhausted. To restore the landscape for future uses, reclamation is implemented to return the land as much as possible, to its previous natural state. Reclamation can include such things as planting trees and grass, and returning wildlife to the area.
The essential purpose of exploration is to find mineral deposits and prove their economic potential. The discovery becomes ore when the mineral assemblage can be developed, mined and processed at a profit. Whether a profit can be realized depends on many technical and economic factors. While the characteristics of an ore body, such as tonnage and grade, may be relatively static, economic factors such as metal prices and operating costs are constantly changing. As a result, what constitutes ore varies over time. Producers are extremely sensitive to these changes and continually review and update ore reserve estimates.