The outcrop of a mineral vein is much altered by weathering; the depth to which this alteration penetrates is determined by the level of the ground water. For example, in the deeper portion of many gold-bearing veins the gold is contained in crystals of pyrite, while above the groundwater level, in the shell of weathering, the gold is scattered in minute threads and grains of native metal through a mass of more or less shattered quartz, which is stained rusty red or brown, and the pyrite has disappeared. Pyrite, when exposed to air and water, is slowly converted into the soluble ferrous sulphate (FeS04), which in turn is oxidized into limonite, with liberation of sulphuric acid. Iron is an important constituent of most ore bodies, and its concentration and deposition below the surface as limonite or haematite forms the gossan, or iron hat of mining phraseology.

In many veins the process of weathering results in the formation of a zone of secondarily enriched sulphides. The unaltered ores in the depths of the vein are sulphides, but from the surface to the ground-water level they are oxidized, and below the zone of oxidation is found that of the secondary sulphides, which, when present, is apt to be much richer than the deeper portions of the vein, because it represents an additional stage of concentration. The metals are dissolved in the oxidized portion of the vein by percolating waters, carried downward and substituted for part of the iron in the original sulphides below.