Native Gold

a mineral, the natural solid solution of silver (from trace amounts to 43 percent) in gold; impurities are common (from trace amounts to 0.9 percent) of copper, iron, and, less often, bismuth, mercury, platinum, manganese, and other metals. Varieties are known with an enhanced content of up to 20 percent copper (copper gold, cuproaurite), up to 4 percent bismuth (bismuth gold, bismuthaurite), platinoids (platinum and iridium gold, porpezite Au, Pd; rhodite Au, Rh), and natural amalgams (Au, Hg).

Native gold crystallizes in a cubic system as octahedra, rhombic dodecahedra, cubes, and crystals of more complex shape; often these crystals are deformed, much elongated, forming small “wires” and “fibers” orparallelly compressed octahedral planes. Dendritic shapes are characteristic of native gold, particularly if it is of poor quality. Widely distributed are filamentous and irregular clump-shaped, “hooked,” separations of native gold; their surfaces often hold the imprints of the crystals of other minerals, aggregates of which included accumulations of native gold. Etching reveals the crystalline-grain structure of the gold particles.

Native gold is bright yellow or, depending on the impurities, pale yellow, reddish, or greenish. It does not cleave. It is soft (it can be scratched by a needle); its hardness on the mineralogical scale is 2.5 and its density is 1,560–1,970 kg per cu m. Native gold is divided, according to the size of the particles, into finely-dispersed material (under 1–5 microns), dustlike (5–50 microns), fine (0.05–2 mm), and coarse (over 2 mm). Native gold accumulations of over 5 g, readily distinguished by their coarseness, are called nuggets. The biggest’ nuggets found have not been preserved; those found in Australia-the Holtermann (with rock residues, 285 kg) and the Welcome Stranger (71 kg)-were re melted. In the USSR the eastern (Urals, Lena) and other regions are rich innuggets (the largest nugget found in the Urals weighs 36.2 kg). Valuable nuggets are preserved by the respective state.

Native gold is the principal form in which gold occurs naturally; it is concentrated in hydrothermal deposits and unevenly distributed in fissured, veined quartz and sulfides (pyrites, arsenopyrites, pyrrhotite). It is finely dispersed in substantially sulfide ores. When the ore oxidizes at the earth’s surface, the fine native gold partially dissolves and is redeposited; in a number of cases it enriches the upper parts of the ore bodies. The processes of their destruction result in the liberation of particles of native gold and the accumulation of these particles in placers; moving in water currents along with other clastic material, the particles are rolled, rounded, deformed, and partly recrystallized. Electrochemical corrosion results in a thin shell of high-quality gold, which leads to a general increase in the native gold assay in placers.