Ore Deposits
You may be able to go outside and easily find a rock in the ground, but chances are this rock is not worth any money. Mining and exploration companies spend millions of dollars looking for valuable mineral deposits in the earth's surface. What makes these deposits different than the common rocks you might find in your backyard? These valuable deposits are called ore deposits and are the subject of this lesson.
Ore deposits are solid, naturally occurring mineral deposits that can be extracted from the earth for an economic profit. What makes ore deposits unique is the economic benefit that can be gained by extracting these minerals. There are several factors that influence the value of individual ore deposits.
While we must extract all of our metallic resources from the earth's crust, the crust is actually quite poor in metals compared to deeper parts of the earth. For instance, when we look at the crust as a whole, silver only makes up eight parts per billion of the earth's crust. For this reason, it is not economical to mine any random part of the earth and try to extract silver.
Production mines exist in locations where an ore deposit is concentrated in much greater levels than typical crustal abundances. This concentration level is also known as a concentration factor, which is the ore concentration needed to be an economically viable deposit divided by the average crustal abundance. For instance, iron makes up approximately 6% of the earth's crust but must be present at about 40% to be an economically viable ore deposit. This equals a concentration factor of about six to seven for iron to be a valuable ore deposit.
Part of the cost of extracting these valuable deposits is that they are surrounded by hard silicate minerals that are not valuable and termed gangue minerals. The process of removing gangue from the valuable ore can be quite difficult and time-consuming depending on the size of the ore minerals. These unwanted minerals form vast tailing deposits at mines, which are sometimes later the target of further mineral extraction.
