12 enriched water flowing across warm, shallow shelf environments or mixing with warm ocean currents. The precipitation of inorganic phosphate minerals is accompanied by the deposition of the skeletal remains of the aquatic life that thrived in the phosphate-rich areas and of inorganic debris from the continent, including quartz and clay particles. In addition, calcite may precipitate from the rising currents. A special category among the sedimentary type of deposits is formed by phosphates originating from the weathering of phosphatic sedimentary limestone. The genetic origin of these deposits is much the same as that of the ‘normal’ sedimentary phosphates. The most important sedimentary phosphate deposits are located in the US, China and in a belt south and east of the Mediterranean (Morocco to Jordan). In general, sedimentary phosphate ore is friable, sometimes even unconsolidated, as for example the Tertiary age ore located in south-east US. Crushing and milling, if necessary, are therefore relatively easy and cheap. Igneous phosphates Igneous phosphates find their origin in magmatic activity. They occur as apatite-enriched masses, sheets or veins in so-called alkaline intrusive complexes. Alkaline complexes are relatively small, round intrusive bodies of a specific silica-poor composition. Major phosphate containing alkaline complexes are to be found in Russia, South Africa and Brazil. Some magmatic phosphates are associated with magnetite iron ore and in other cases with rare earth elements, as in the unexploited carbonatite of Cargill, Ontario, Canada. The phosphate enriched iron ores of Kiruna, Sweden are a special category, although they are not directly the result of magmatic activity but were probably formed by the extrusion of hot iron and phosphorus-rich fluids on the seabed. When associated with magnetite, igneous phosphate may become a by-product of iron mining. Igneous phosphate rock is much harder than its sedimentary counterparts and, as a result, may be more expensive to exploit in terms of mining and processing. However, the resulting product will be of higher quality, as the apatite minerals are purer and easier to separate. Guano All large guano deposits were formed by the accumulation of sea bird droppings. The present composition of guano deposits, for the most part calcium phosphate, may vary considerably, mainly as a result of leaching by surface water. The major guano deposits occur in Oceania. However, guano reserves are dwindling and do not play a significant role in the phosphate market. Consequently, this type of ore will not be given further consideration. Composition of phosphate ores The main phosphate minerals in phosphate rock are francolite, fluorapatite, dahllite and hydroxyapatite. As the chemical composition of these minerals may vary, they are better represented by a Pagina 21

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