Au (Gold)

Due to its relative chemical inertness, gold is usually discovered as a native metal or alloy. Occasionally, large accumulations of native gold (also known as nuggets) occur, but usually gold occurs as minute grains.  These grains occur between mineral grain boundaries or as inclusions within minerals. Common gold associations are quartz, often as veins and sulphide minerals. The most common sulphide associations are pyrite, chalcopyrite, galena, sphalerite, arsenopyrite, stibnite and pyrrhotite.  Rarer mineral associations are petzite, calaverite, sylvanite, muthmannite, nagyagite and krennerite.

Gold is widely distributed in the earth's crust at a background level of 0.03 g/1000kg (0.03ppm).  Hydrothermal ore deposits of gold occur in metapmorphic rocks and igneous rocks; alluvial deposits and placer deposits originate from these sources. The primary source of gold is usually igneous rocks or surface concentrations. A deposit usually needs some form of secondary enrichment to form an economically viable ore deposit: either chemical or physical processes like erosion or solution or more generally, metamorphism, which concentrates the gold in sulphide minerals or quartz.

There are several primary deposit types, common ones are termed 'reef' or 'vein'.  Primary deposits can be weathered and eroded, with most of the gold being transported into stream beds whereby it congregates with other heavy minerals to form placer deposits. In all of these deposits the gold is in its native form. Another important ore type is in sedimentary black shale and limestone deposits containing finely disseminated gold and other platinum group metals.

Gold ore grades may be as little as 0.5 g/1000 kg (0.5 ppm) on average in large easily mined deposits, typical ore grades in open-pit mines are 1 - 5 g/1000 kg (1-5 ppm), ore grades in underground or hard rock mines are usually at least 3 g/1000 kg (3 ppm) on average. Ore grades of 30 g/1000 kg (30 ppm) are usually needed before gold will be visible to the naked eye, therefore even in gold mines you will often not see any gold.