Gold is not typically found in the topsoil that supports plant life, but rather in specific types of geological deposits like rock, gravel, and decomposed bedrock. Prospecting involves understanding how natural forces have concentrated the metal within the Earth’s crust and on its surface. Gold deposits are broadly categorized into primary, residual, and secondary types. This understanding guides the search, moving from the hard rock source where gold first formed to the unconsolidated sediments where it is most easily recovered.
Gold in Primary Deposits
The original source of all gold is found deep within the Earth in hard rock formations known as primary or lode deposits. These deposits form through hydrothermal processes, where superheated, mineral-rich fluids circulate through fractures in the crust. As these fluids cool, they deposit gold and other minerals, most often forming veins of white quartz.
The gold within these veins is locked inside the quartz matrix or associated with sulfide minerals like pyrite and arsenopyrite. These veins frequently appear in metamorphic rocks such as slate, schist, or greenstone belts. Extracting this gold requires hard rock mining, which involves crushing the ore before chemical or physical separation.
Gold in Residual and Eluvial Soils
Residual and eluvial deposits form when the primary hard rock deposit begins to weather in place. Residual soil, or saprolite, is chemically weathered bedrock that retains the structure of the original rock but has become soft and crumbly. The gold, being chemically resistant and heavy, remains concentrated within this decomposed material directly above the original vein.
Eluvial deposits form when gravity and rainwater cause the weathered material to move a short distance downslope from the source, without being fully transported by a stream or river. This material often consists of angular fragments of the original quartz vein and gold mixed with the surrounding soil. This type of deposit can be a rich source for prospectors, representing the first stage of free gold liberation from the bedrock.
Gold in Secondary Deposits
Secondary deposits, also known as placer deposits, are the most common source of gold sought by recreational prospectors because the gold is easily accessible in loose material. These deposits form when water transports the gold liberated from primary and eluvial sources, concentrating it due to its high specific gravity. The density of gold causes it to drop out of suspension much faster than lighter materials like quartz.
The “soil” in these secondary environments is actually a mix of unconsolidated sediments like gravel, sand, silt, and clay, collectively called alluvium. Gold concentrates in natural traps within stream beds, ancient river channels, floodplains, and terraces. Specific locations include the inside bends of rivers, behind large boulders, or in natural depressions in the stream bed that act as “riffles.”
The most productive material, often called “pay dirt,” is found close to the bedrock or within the heavy gravel layer just above it. An indicator of gold presence is the concentration of “black sand,” which is a mixture of heavy minerals such as magnetite and ilmenite that settle out of the water flow. The gold found here is typically worn and rounded from its journey, ranging from microscopic “flour gold” to larger nuggets.