How deposits are made

A strong tenet of basic geology is that ore deposits are produced with the help of large amounts of heat. On Mars, heat can come from molten rock moving under the ground and from crater impacts. Liquid rock under the ground is called magma. When magma sits in underground chambers, slowly cooling over thousands of years, heavier elements sink. These elements, including copper, chromium, iron, and nickel become concentrated at the bottom. When the mass of magma has cooled down and has mostly frozen or crystallized into a solid, a small amount of liquid remains. This liquid bears important substances such as lead, silver, tin, bismuth, antimony. When magma is hot, many elements are free to move. As cooling proceeds, the elements bind with each other to form chemical compounds or minerals. Because some elements do not fit easily into minerals, they exist freely after nearly all the other elements have formed minerals. The remaining elements are called incompatible elements. Some of them are quite useful to humans: niobium, a metal used in producing superconductors and specialty steels, lanthanum and neodymium, europium for television monitors and energy-efficient LED light bulbs Sometimes minerals are so hot in the magma chamber that they are in the form of a gas. Others are mixed with water and sulfur. The gases and mineral-rich solutions eventually work their way into cracks and become valuable minerals veins. Ore minerals, including the incompatible elements, remain dissolved in the hot solution, then crystallize out when the solution cools. Deposits created by means of these hot solutions are called hydrothermal deposits. Some of the world's most significant deposits of gold, silver, lead, mercury, zinc, and tungsten started out this way. Nearly all the mines in the northern Black Hills of South Dakota came to be because of hot water desposits of valuable minerals. Cracks often form when a mass of magma cools because magma contracts when it cools. Cracks occur both in the frozen magma mass and in the surrounding rocks, so ore is deposited in any kind of the rock that happens to be nearby, but the ore minerals first had to be concentrated by way of a hot, molten mass of magma.