Page:South African Geology - Schwarz - 1912.djvu/127

 substances in solution seeps into it through the pores in the rocks composing the walls, pressure is relieved, the substances are deposited. Worthless minerals, such as quartz, calcite, fluorspar and barytes, are called gangue, the valuable substances, metals like gold, or metallic minerals such as the sulphides of copper, silver, and so forth are called ores. The gangue and ores are deposited on the walls of the fissure, and the material accumulates till the whole fissure is filled in. Subsequently the fissure may open once more, and another deposit of minerals will take place, and so on, so that in many cases there are six or eight separate infillings of one and the same fissure. The fissure thus filled in is called an ore vein, or, if quartz is the gangue, a quartz vein.

If the vein is now exposed by the removal of the rocks above, the portion which is exposed to the free circulation of water from the surface, which contains oxygen, becomes altered. The sulphides are changed to oxides and carbonates. Also, this oxidized zone usually contains a secondary deposit of the ore by reason that the portion that has been removed by denudation has yielded a certain amount of its mineral contents to the water on the surface, and this sinking in has enriched the top of the vein. Surface enrichment may cause the top 100 ft. or so of a vein to carry ten or a hundred times as much ore as the unaltered vein. At the extreme surface the vein is usually heavily charged with iron oxide, forming a black scoriaceous mass; this is the gossan of the Cornish miners.

Ore veins, when they are exposed to denudation, crumble, as all exposed rocks do, and the heavy minerals in them become separated naturally from the lighter