The removal of tin, copper and other impurities from ferrotungsten by grinding and chemical treatment has made possible the use of impure ores in the production of high-grade ferrotungsten in the electric furnace.
GEOLOGICAL DISTRIBUTION
Tungsten, even more than tin, is found almost exclusively with granitic rocks. In a few places tungsten ores are found in volcanic, sedimentary or metamorphic rocks, but as is postulated with certain tin deposits, many of these deposits may be explained on the supposition that they are not far vertically above underlying granite.
Among the deposits themselves there is a considerable variety of types, and they may be classed as follows: Segregation deposits, pegmatite dikes, veins, replacement deposits, contact metamorphic deposits and placers.
Segregation deposits are few and of little importance and constitute those deposits in which wolframite is segregated in granite, like biotite or hornblende. A closely related type is the occurrence of tungsten minerals in aplitic granite, and this grades almost insensibly into the second type, the pegmatites.
The pegmatites are also of comparatively small importance, but do yield certain quantities of tungsten minerals. The pegmatites also grade into the next type, the veins, which have heretofore furnished the greater part of the tungsten minerals of the world. Closely connected with the veins are the fourth type, replacement deposits, in which the country rocks alongside the veins, though the veins may be very small, are replaced by various minerals, including those of tungsten. The only known large examples are the wolframite deposits near Lead, South Dakota, which were considerable producers under the high prices of the Great War.
Among the replacement deposits are to be noted also such deposits as those in the Deep Creek Mountains, Utah, in which solutions following cracks in monzonite have replaced the rock with a mass of feldspar, quartz, tourmaline, apatite, scheelite, wolframite (very little), bismuth, copper, and molybdenum minerals, which under other conditions would be unhesitatingly called pegmatite.
Closely related to the replacement deposits are the contact metamorphic deposits, the fifth type. These only recently have begun to be of commercial importance, but promise to be among the greatest, if not the greatest, producers of this country and possibly of other countries. They are of the familiar kind—limestones or limey rocks which have been invaded by granites bringing large quantities of watery or gaseous solutions of silicon, iron, aluminum and magnesium, with less chlorine, fluorine, potash and sulphur, and, in this case, tungsten. In the Great Basin broad areas of limestone, extending from northwestern Utah to the Sierra Nevadas and around their southern extremity, have been thus intruded and metamorphosed. Some large deposits of scheelite have already been exploited in the region and others remain to be worked. Similar deposits occur in Korea, Japan and Tasmania, and probably exist, though as yet undiscovered, in China, the Malay Peninsula and other countries. The tungsten mineral in such deposits is invariably scheelite.
Placers, the sixth type, are formed from all grades of deposits, but their value depends largely on local conditions. They are both residual and fluviatile deposits and have been large producers of tungsten minerals, especially of wolframite. A very large proportion of the Burmese output has been from semi-residual and stream placers, and the Chinese output in 1918, the largest ever made by any country in one year, was almost wholly from placers.