Striking gold requires some geological and historical soil analysis of an area. Certain types of soil and rock formations are more likely to be gold-bearing due to environmental and geological processes. Gold is relatively rare, but it occurs in various types of soils and environments, particularly those that have been shifted or compacted by the action of water, ice, volcanoes and Earth's tectonic plates. Over time, deposits of this precious mineral become concentrated in soils with these common characteristics.
Alluvium is a fertile soil that consists of clay, sand, silt, gravel and organic matter. It is formed by rivers and is most often found in floodplains and river deltas such as those of the Mississippi in the U.S., the Nile in Northern Africa and the Ganges River in India. It is also found in some river banks, particularly where a river empties into a lake. Research published in the Journal of Geochemical Exploration notes that gold is found in alluvial riverbeds, including almost 130 locations in Great Britain and Ireland. These gold deposits may have been carried from other areas and concentrated in the alluvial soil by the fast-moving river waters.
The Canadian Northwest is known for the Klondike gold rush and several gold-bearing areas. A study published in Quaternary International notes that the largest gold deposits in this area are in the pre-glacial gravels of Dawson Range, such as the Klondike Plateau in the Yukon. Glaciation played a pre-historical role in scraping or gathering soil, and even changing the direction of river drainages through the movement of vast melting ice sheets. This action concentrated gold-bearing gravels on the northern and eastern slopes of Dawson Range and along some areas of the Yukon River, leading to men finding gold there during the gold rush.
Shale and Sedimentary Rocks
In northwestern Iran, soil that contains carbonate and black shale formations has been found to be gold-bearing. Research published in the geology journal Mineralium Deposita explores gold mineral deposits in Zarshuran, Iran. This region contains fractured sedimentary rocks that also contain silica, pyrite, coloradoite and other rock formations. The gold is mainly found in trace amounts within arsenical pyrite and other rocks, and must be detected using electron microprobe technology.
Pyrite Rock and Limestone
Soil that contains limestone is rich in calcium and minerals that form both shallow and deep rock formations. Gold-bearing arsenian pyrite rocks are found in limestone soil deposits in several regions of China. The journal Ore Geology Reviews reports that up to 55 tonnes of gold have been extracted from arsenian pyrite in mines in Shuiyindong, China. Most of the gold mined in this area is deemed to be "invisible," because it cannot be seen with conventional microscopy.