Understanding Our Planet Through Chemistry
Analytical chemistry plays a key role in our continuing quest to understand how ore deposits form and in the practical exploration for ore deposits. If you pick up an ordinary rock that builds the crust of the Earth and determine its chemical composition, for every billion atoms, 1 to 10,000 atoms will be metallic elements such as gold, silver, platinum, mercury, copper, cobalt, nickel, chromium, lead, zinc, molybdenum, tin, and tungsten. Natural processes in the Earth's crust have the remarkable ability to concentrate and purify certain rare metallic elements to form unusual deposits of minerals that contain 1,000 to 10,000 times the amounts found in ordinary rocks.
With today's modern mining and extraction technology, it has become possible to mine very low-grade deposits. For example, gold can be economically recovered from rocks that contain less than one tenth of an ounce of gold per ton of rock. But gold continues to be expensive because of the cost in locating the deposit, mining the rock, and extracting the small amount of gold in each ton of rock. All of the inorganic raw materials used to manufacture the products of today's technological society have to be either mined or recycled.
Almost every process that takes place in the Earth's crust, whether from the action of molten rock, heat and pressure at depth, hot springs or steam, running water, weather, or biological activity can contribute to the formation of an ore deposit. Geologists use the principles of chemistry to try to understand how these processes scavenge elements from ordinary rock, transport them, and concentrate them to form an ore deposit. Geologists have developed models that describe the physical characteristics and chemical composition of each ore deposit type and how they relate to the geologic environment in which they form similar to the way biologists describe how an organism fits into a particular environmental niche.
In North America and many other parts of the world, almost all of the rich ore deposits exposed at the surface have already been discovered. Most of the ore yet to be found is not visible to the human eye. Therefore, geologists have had to improve their understanding and develop more sophisticated ways to detect where ore deposits can occur.
Two main approaches are used to detect deposits hidden below the surface. One uses the ore-deposit model, and the other is based on the detection of a dispersion halo that extends for some distance from the deposit (preview discussion of dispersion halos in Chapter IIIa).
The following analogy shows how geologists use ore-deposit models. If all but the tip of the tail of an elephant was buried by a landslide, a biologist could recognize from the skin, hair, and shape of the appendage that the tail belonged to a mammal. With advanced testing of tissue samples, a biologist could prove that the tail belongs to an elephant and could easily predict that the body should be buried about 1 meter below the tip of the tail.
Most ore-deposit models are not as advanced as biologists models for elephants, but a few are nearly so. Several copper and molybdenum porphyry deposits, located as deep as 2,000 to 4,000 feet below the surface, have been discovered based on small surface exposures measuring several feet across. These exposures were of breccia pipes (vertical pipe-shaped bodies of pulverized rock), which are known to extend thousands of feet above the main body of porphyry deposits. Because not all porphyries contain deposits of economic metals, geologists can collect and analyze field samples to determine what metals the porphry will contain, and if it is worth drilling.