Mineral deposits, weathering

Valuable mineral deposits which form by residual concentration pertain to iron, manganese, aluminum, nickel, clays, tin and gold. Aluminum comes almost exclusively from residual concentrations of bauxite, which result from the laterihsation of alumina rich igneous rocks like syenites under tropical and subtropical climatic conditions. Laterites also contribute nearly 80% of the world s reserves of nickel at grades better than 1% Ni. They form by residual concentrations as a result of weathering of mafic and ultramafic igneous rocks, which are relatively enriched in nickel. 

As a soil develops, OM decomposes to produce humus, which is black. Additionally, release of iron from minerals by weathering yields various reds and yellows. Both mechanisms yield soil coloring agents. Under oxidizing conditions, where soil is not saturated with water, the iron will be oxidized and thus in the ferric state [Fe(III)]. When the iron and OM are deposited on the surfaces of sand, silt, clay, and peds, they develop a coat that gives them a surface color. However, soil color is not only a surface characteristic but extends through the soil matrix. Under oxidizing conditions, soil has a reddish color. The chroma of this color depends to some extent on the amount of and the particular iron oxide present. 

Most of the heavy metal contaminants associated with mining or agricultural practices are released from less common mineral phases, some of which are primary (i.e., formed initially by an igneous, metamorphic, or sedimentary process) and some of which are secondary (i.e., formed by chemical alteration of the primary minerals). Table 7.4 lists some of the more common contaminant ions and the mineral phases with which they are associated in economic mineral deposits or weathered zones associated with these deposits. 

Aluminum is present in many primary minerals. The weathering of these primary minerals over time results in the deposition of sedimentary clay minerals, such as the aluminosilicates kaolinite and montmorillonite. The weathering of soil results in the more rapid release of silicon, and aluminum precipitates as hydrated aluminum oxides such as gibbsite and boehmite, which are constituents of bauxites and laterites (Bodek et al. 1988). Aluminum is found in the soil complexed with other electron rich species such as fluoride, sulfate, and phosphate. 

The final case of surficial ore genesis dealt with here involves the transport of physically disaggregated residuum from physically and chemically weathered materials by wind and water, resulting in placers, mineral deposits formed at the Earth s surface by mechanical... 

Schwarz, T. Germann, K. (1999) Weathering surfaces, laterite-derived sediments and associated mineral deposits in north-east Africa. In Thiry, M. Simon-Coinf on, R. (Eds) Palaeoweathering, Palaeosutfaces and Related Continental Deposits. International Association of Sediment ologists Special Publication 27. Oxford Blackwell, pp. 367-390. 

Germann, K., Schwarz, T. Wipki, M. (1994) Mineral-deposit formation in Phanerozoic sedimentary basins of northeast Africa - the contribution of weathering. Geologische Rundschau 83, 787-798. 

A few minerals produce acid when they contact water. These minerals can be described as salts of weak bases and strong acids. They chiefly result from weathering and oxidation of the pyrite or marcasite (FeS2) exposed in the mining of mineral deposits and coal. Such acid minerals, which are dominantly Fe sulfates and to a minor extent AP sulfates, typically form from the evaporation of pooled acid-mine waters or of the moisture in unsaturated mine wastes or spoils that contain the sulfides. Acidity is produced when they are dissolved by fresh runoff or recharge. For example... 

Al3+). In nature they occur widely in silicate minerals, although weathering processes give rise to concentrated deposits of other compounds such as halides (e.g. NaCl, CaF2) carbonates (CaC03) and... 

Sediment-laden rivers flowing over flat terrain commonly develop extensive floodplains. At times, floodplains coalesce into broad depositional alluvial plains such as the Llanos of South America. The sediments in those deposits weather chemically. Less stable minerals in the sediment are broken down and alluvial soils develop. Eventually, only the most stable minerals such as quartz remain, and the clays are transformed into cation-deficient varieties. Sediment in such rivers, especially the sand, may go through many cycles of deposition, weathering, and erosion before it is transported out of the system. Compositionally, this sediment resembles that derived from transport-limited erosion. Elemental fractionation between the original bedrock and erosion products still occurs because of the permanent burial of some cation-rich material and the uninterrupted transport of much of the fine-grained suspended sediment out of the system (Johnsson etal., 1988 StaUard, 1985,1988). 

Silicon reactions are central to rock weathering and soil development. Silicon is the soil component lost in greatest amount from rock minerals during weathering, and the transformations of silica into secondary minerals are the major reactions of soil development. The sand fraction of soils is usually >90% quartz (SiC>2), the most prevalent form of Si in soils. Highly weathered soils may contain as little as 20% Si (Table 2.1a). A1 and Fe ore deposits are essentially highly weathered soils from which most of the Si has been lost. 

---