The average crustal abundance of tungsten has been estimated at 1 to 1.3 ppm and so it is considered as one of the rarer elements in the earth’s crust. In the formation of tungsten ore deposits hydrothermal solutions play a critical role. Tungsten becomes concentrated in the residual fluid of crystallising magmas as the tungsten iron, tungstic acid, or sodium tungstate and may be precipitated in wall rock contact zones usually as wolframite or scheelite. The particular mineral formed is controlled mainly by the relative amounts and activity of iron, manganese and calcium present. All members of the wolframite series are called “black ores” in the trade and are the predominant tungsten minerals in the quartz vein deposits that comprise the bulk (60%) of the world’s reserves. Scheelite occurs in skarn rocks formed by high-temperature replacement and recrystallisation of calcareous sediments near or at the contact of intrusive igneous rocks. Scheelite accounts for about 35% of the world’s tungsten deposits.
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