supergene and residual deposits Mineralization formed deep in the Earth's crust can be exposed to the surface environment by uplift and erosion of its host rocks. The high oxygen content of the Earth's atmosphere and the presence of water result in chemical reactions that break down ore minerals that are not chemically stable under surface conditions. Sulphide minerals are particularly susceptible to chemical weathering, which results in the release of their contained metals and sulphur. The oxidation of the sulphur produces large quantities of acid which then promotes further breakdown of other more stable minerals. In the air-saturated zone near the surface, valuable metals can be dissolved into groundwater because of oxidizing conditions in this region of the soil. When the groundwater percolates down to the water-table, conditions become more reducing and the metals can no longer remain dissolved and are precipitated as supergene minerals. Over time, large accumulations of supergene minerals can form and constitute a supergene deposit. In some instances sparse mineralization that is uneconomical to mine can be concentrated by supergene processes into mineable ore; supergene deposits are commonly underlain by such primary mineralization. Common supergene ore minerals include those of copper (chalcocite, covellite, native copper), lead (anglesite), and zinc (smithsonite). Carbon dioxide can also combine with metals to form spectacular encrustations of colourful minerals such as azurite and malachite, both carbonates of copper. Other metals that are concentrated in supergene deposits include gold, silver, nickel, manganese, and uranium. Surface exposures of supergene deposits are unusually rich in iron hydroxide minerals, imparting a red coloration to the surface that contrasts strongly with surrounding unmineralized rocks. These exposures, commonly referred to as
gossans, have been useful indicators of subsurface mineralization to prospectors and exploration geologists.
Supergene deposits are formed by the weathering of earlier mineralization, but there are other types of ore deposit that are formed by the weathering of ordinary unmineralized rocks. Residual deposits form when intense weathering removes much of a rock's components and concentrates a few of them to very high levels. Bauxites are ores of aluminium generated by this process. Bauxites form in areas of well-drained terrain that has low to moderate relief and has been geologically stable for long periods of time. A tropical climate and intense vegetation favour their formation over suitable host rocks. In Jamaica, extensive bauxite deposits are found overlying limestone and dolomite. These resulted from the dissolution of the carbonate component of the host rock by organic acids that were generated by decaying vegetation. The remaining clay component from the limestone can be further altered by these acids and, over time, it is converted to aluminium oxyhydroxide minerals such as gibbsite, diaspore, and boehmite. In some other locations, thick accumulations of bauxite were formed by the weathering of volcanic rocks. A major source of iron, called
iron laterite, is a type of residual deposit generated by the intense weathering of iron-rich rocks such as mafic and ultramafic volcanic rocks. During weathering, acid solutions remove much of the rock-forming elements and, because mafic and ultramafic rocks are poor in aluminum, the remaining laterite is highly enriched in iron oxyhydroxide minerals such as haematite and goethite. If an ultramafic rock rich in nickel undergoes such a process, a nickel laterite is formed. These nickel laterites are important resources of this metal in the South Pacific and South America. Other residual deposits are sources of chromium, titanium, rare-earth elements, and even gold.
Bruce W. Mountain
