Native gold occurs in part in situ in its original deposits, in part in beds of sand gravel which have been formed by the destruction of the former. Gold of the first class is called reef-gold, of the second alluvial gold. Basically, reef-gold is generally found disseminated in quartz, accompanied by brown hematite in the upper portions (gossans) of the deposits, and by sulphides in the lower portions, especially pyrite, arsenopyrite, galena and sphalerite. It also occurs in very small quantities in many deposits that pyrite or blende.
Alluvial gold occurs as dust, grains, scales and larger pieces called nuggets in alluvial deposits, which from beds of sand, gravel or breccia either upon the surface, shallow placers, or the greater or less depths beneath it, deep placers, and which have been produced by the destruction of gold-bearing deposits. It also occurs in the sands of many rivers and streams, having been washed out of the rocks on their banks. Amongst the foreign substances with alluvial gold is mixed with quartz, clay, mica, chlorite, serpentine, ilmenite, magnetite, chromite, garnet, spinel, zircon, and platinum, one of the most important alluvial deposits were found in the western flanks of the Sierra Nevada in California, where approximately they were 650 feet in thickness and filled the valley of Pliocene rivers.
The leads sometimes consist of white quartz with coarse gold almost free of sulphides, sometimes of much slate with quartz when they are very productive. Also, the upper wall is usually quartzite, the lower slate, both walls occasionally slate, but cases of quartzite forming both walls is not common. The contrast was found in some places in Australia between the continuance of hard and soft veins. The leads in which the quartz is enclosed in a soft friable rock are no common and do not seem to extend far longitudinally, while those in which the vein stone is very hard are very regular and of long continuation and contain gold equally distributed. As a general rule it is found that the variation in richness in different points of the length of vein is great, so that lodes rich on the whole are very poor in some parts.
Other important consideration is the fact that when gold occurs in a hard white quartz free of sulphides, it is mostly in visible flakes and granules, but such veins, though affording fine specimens are not often regularly and remunerative productive. Some of the most steadily remunerative veins are only of moderate size. It was formerly believed that veins of auriferous quartz become gradually less productive at increasing depth, but more extensive experience has indicated this situation is very variable. Of the minerals enumerated, pyrite and arsenopyrite are found in large quantities in the quartz gangue, but other sulphides such as chalcopyrite, galena or sphalerite are present in variable quantities. It is to be remarked that the affinity of these minerals is not similar and the gold is contained in the gangue along the shoot of these minerals, but where the gangue is far apart from the metalliferous indications, it is generally of low grade. On this theory, it is not difficult to account for flat leaders and the running out of the quartz stretches, now the caps of the reefs being often richer than the rest of the gangue, or to account for gold grade of reefs when they suddenly become contracted for a certain length, conformable to the greater or less opportunity offered to the gold to precipitate according to the physical conditions of the place.