OXIDATION AND REDUCTION ORES The free circulation in the zone of percolation of waters charged with oxygen and carbon dioxide makes it, in effect, one of oxida tion, in which rocks and ore deposits become disintegrated and their component minerals decomposed with removal of the soluble constituents. The chief chemical processes involved are hydration, oxidation and carbonation. Silicates are broken down, the alkalies and alkaline earths being removed as soluble bicarbonates, whilst silica is set free. Sulphides give place to soluble sulphates or to sulphuric acid.
The soluble compounds are carried down to the belt of satura tion. A portion is lost in the waters escaping in the zone of dis charge. The remainder is deposited as the descending acid waters become neutralized by the alkaline waters of the lower zone.
The changes produced in ore deposits by the process described above are most striking, and of the greatest economic importance. Those metals that form soluble compounds are removed from the zone of weathering, leaving in most cases an impoverished residual material. On the other hand, the precipitation of new metallic compounds or of the metals themselves at lower levels leads to enrichment at those levels. As a rule oxy-salts, halogen salts and native metals are precipitated above the water-table, while second ary sulphides are deposited below it.
The dictum of the mining camp that lodes become richer in depth, is therefore only true as regards their upper portions. In progressing from the outcrop downwards, a barren or lean por tion of the lode is first passed through. As the water-table is ap proached, the metal content increases, attaining a maximum in a zone of secondary sulphides below it ; the grade then falls until a minimum is reached in the unenriched zone of lean primary sulphides.
The zone of secondary enrichment is of prime importance for the mining of copper and silver, the bulk of these metals being won from it. Copper mines, rich in their upper levels, become poor in depth and ultimately unpayable when the lean primary ores are reached, unless the deposits are sufficiently large to be worked at a very low cost. The primary ore is almost invariably
a cupriferous iron pyrites. On oxidation the copper goes into solu tion either as sulphate or as a bicarbonate, and is removed, leaving behind the iron oxides. Consequently a prominent feature of copper lodes is the existence at or near the surface of a gossan or iron-cap (chapeau de fer), from which the valuable copper content has been removed. In wet climates the gossan consists almost entirely of hydrated oxides of iron. In drier climates, the anhydrous oxide (haematite) occurs; and in still drier climates (for instance the rainless regions of Arizona and Chile) gossans contain chlorides and sulphates of copper, which in wet climates are unable to exist in the solid state.
The copper minerals especially characteristic of the upper por tion of the zone of secondary enrichment are the hydrated carbon ates—malachite [OH] 2 ) and ( 2CuCO,Cu [OH] 2), chessylite, the hydrated silicates—chrysocolla and an oxy-chloride—atacamite together with native copper. Lower down, the sulphides—chalcocite bornite and covellite (CuS), are found.