Natural Occurrence of Gold
The natural occurrence of gold is related to siliceous or quartz veins, filling up fissures of the earth’s crust, and in placers or alluvial deposits left by streams that have disintegrated rocks containing auriferous veins. Gold is rarely found pure in nature, silver being the most common impurity, while it is also associated, either to a fine state of mechanical mixture or in some instances in chemical association with pyrite, tellurides, copper minerals, or black sands. In some locations such Australia, Peru or Colombia, gold occurs free and in combination with tellurium and lead and pyrite in a matrix of quartz. There are numerous other minerals in which gold is present in small quantities, the ores of silver, antimony and bismuth appear to be a common source of gold.
With the years, it was possible to use several terms to make reference to native gold, depending upon the outward form in which it is in naturally found. In this way, Flake gold is a type usually found on the faces of foliation and cleavages planes of the mineral in which it is contained, generally in kaolin and biabase schists. Flake gold is exceedingly thin and floats on water, and very little of it may be extracted from its matrix by ordinary battery treatment. in some deposits gold may be found in the form of an extremely fine yellow powder, contained in microscopic fissures of its matrix, and doubtless produced by the decomposition of highly auriferous sulphides and tellurides.
Rough or coarse Gold is usually found in reefs, being irregular in form and having no specific dimensions. Dendritic gold is considered a beautiful fern like form of the metal incipiently crystallized in cleavages. Crystalline gold presents crystals in the form of single octahedrons. Sponge gold is found in masses of oxidized zones where at certain depths tellurides are found. In structure it is very much akin to the ordinary marine sponge, consisting of irregular strings and cellular masses of brilliant gold, which form has doubtless been produced by the oxidation and removal of the tellurium with which it was originally combined.
Free gold or visible gold is found in quartzose rock and alluvial sands and it is present as disseminated in a finely divided state throughout the matrix, or in small grains or pieces of substantial weight unevenly distributed. These particles are usually called nuggets and occasionally are found in large and big size. Nowadays, it is difficult to find extremely large specimens, but during the gold rush, it was found to find them. The free or metallic gold is always associated with impurities, the nature and extent of these materials is variable. In the States, specifically in California, free gold particles have reported 88% of gold, 11% of silver and 1% of base metals. In Oregon, free gold particles have reported 87% of gold, 12% of silver and 1% of base metals.