The geologist or prospector must have a knowledge on the ore types are most likely to carry gold; otherwise the time employed will be unfruitful. In most deposits is possible to identify the rock; volcanic, sedimentary, metamorphic, or even the river streams that transport gold. The valuable gold ores are brought in by different system of transportation such as percolating waters and gasses from cooling systems. The first one is probably the most important because they are charged with different heavy metals or base metals. The second one is other important source of gold ores and is characterized by special fluids locked into the rock in chemical combination. These fluids are extremely important in the formation of valuable ore bodies. They pass through different rock formations on their way to the surface dissolving several minerals and carry them in solution for large or short distances. These mineral loaded waters reach a space in the rock where they deposit their valuable charge. These spaces are open and formed by earth movements or large open spaces formed by dissolution of other minerals. In open fissures, vein deposits can be identified. Fluids may leave their metallic minerals into porous formation like granite to form disseminated deposits.
Gold is not very soluble and high pressures and under the presence of some chemical agents is possible to be transported in solution. Percolating fluids are mainly acid ground waters that penetrates rocks, dissolved the matrix and transport metals to different parts. These metals could be lead or copper. For these reason some polymetallic deposits contains important amounts of gold.
When we have to talk about gold mining, everybody thinks on free gold. This idea started at the first days of gold deposit processing. The gold from original veins was gradually disintegrated through the time. Gold being heavy and resistant to weathering has been concentrated in sand, gravel and different rock formations. Probably placer deposits have been mined for most miners since the first days on mining.