Occasionally large accumulations of native gold (metallic gold) occur, but usually gold is founded as very small grains. These grains occur between mineral grain boundries as inclusions within minerals. Gold is widely distributed in the Earth's at around of 0.004 g/t. Hydrothermal ore deposits of gold occur in metamorphic rocks and igneous rocks; alluvial deposits and placer deposits originate from these sources.
Groundwater solutions (meteoric water), and/or solutions emanating from a cooling magma (hydrothermal fluids), and/or fluids ejected from compressing sediments (pore water) penetrate along fractures and tiny pore spaces between mineral grains in the rock. Under certain conditions these solutions, which contain different compounds, may dissolve, deposit other minerals, or alter the rock-forming minerals. Leaching, or the decomposition, dissolution and removal of soluble minerals from the rocks may be caused by hydrothermal or meteoric solutions. The dissolved mineral products may be dispersed or redeposited elsewhere as the same or different minerals, leaving often greater concentrations. Relative content of gold may be increased by the removal of originally associated minerals, leaving an enriched residual gold deposit.
The primary source of gold is usually igneous rocks. Pressure and temperature within the earth increase gradually with distance below the surface. At depths of several tens of miles the material which makes up the earth's crust (depending upon its composition) may become partially molten. This molten material is called magma, and is normally less dense than the overlying solid rock. As the magma is subjected to tectonic forces, it may be squeezed upward along weak zones in the crust. This process by which magma penetrates the crustal rock is called Intrusion. The intrusive magma which has cooled and solidified is known as igneous rock. Common examples of igneous rocks are granite, granodiorite, and diorite.
A gold deposit usually has a secondary form of enrichment that can be either chemical or physical processes like erosion or solution or more generally metamorphism, which concentrates the gold in sulfide minerals or quartz. Decomposition and disintegration of rocks at or near the surface by physical and chemical processes is called weathering. The products of weathering are normally carried off by erosion. Water percolating into the ground dissolves some minerals from the rocks and forms acidic or basic solutions which further attack the rock and break it down chemically.
The distribution of gold seems to validate the theory that gold was carried toward the earth's surface from great depths by geologic activity, perhaps with other metals as a solid solution within molten rock. After this solid solution cooled, its gold content was spread through such a great volume of rock that large fragments were unusual; this theory explains why much of the world's gold is in small, often microscopic particles. The theory also explains why small amounts of gold are widespread in all igneous rocks.
Native Gold | Electrum | Calaverite | Unusual gold occurrences | Valuation of Gold Deposits | Average Gold Content | Exactitude to Sampling Gold Ores | How to Buy Gold Deposits without Fraude | Calculation of the Average Gold Grade | Volume of Valuable Gold Ore | Errors Associated to Sampling Gold Ores | Determination of Tonnages in Gold Deposits | Visual Classification of Gold Ores | Classification of Gold Ores in a Deposit | Structural Characterization of Gold Deposits | Secondary Alteration of Gold Deposits | Precipitation of Base Metals in Gold Deposits | Solubility of Gold in a Mineralized Deposit | Size of Gold Deposits | Depth of Gold Deposits |