Formation Of Ore
A cubic mile of average rock contains approximately one trillion dollars worth of metal; yet no one is mining ordinary rock. The expense involved in processing ordinary rock is just too great. Fortunately, ore provides a less expensive option. Ore is formed by geologic processes which concentrate metals to tens or even thousands of times their average crustal abundances. Even then, a mine may not prove profitable unless a host of other geologic and nongeologic conditions are met.
Geologic factors include the deposit's size, depth, and amenability to processing. Small amounts of arsenic, for example, may poison the deal through the added expense of its removal. Higher amounts could mean a profitable arsenic mine. Profitability also ties the definition of ore to a host of nongeologic conditions including demand for the metal, geographic location of the deposit, local labor conditions, local energy costs, governmental regulations, and many other economic factors.
Besides metals, ore commonly contains minerals of no particular value. Gold, for example, occurs in veins composed mostly of quartz. Although not of economic value, gangue minerals can yield valuable information on the origin of the deposit. Quartz, for example, reveals the temperature at which the ore formed, information that could be useful in directing the search for more gold along the vein. In addition, it is doubtful that the gold mineralization would have been noticed had not quartz caught the eye of a geologist who knew that precious metals sometimes occur in veins of quartz.
Ore deposits are relatively rare and tend to be distributed in an irregular fashion around the globe, but there is nothing unusual about the manner in which they form. They develop, in fact, from the same geologic processes that form ordinary igneous, sedimentary, and metamorphic rocks.