1. Otjua Tungsten Deposit in Namibia
The Otjua tungsten deposit is situated about 30 km north of Omaruru on the farms Otjua 37 and Schonfeld 92 along the southern side of a major anticlinal structure. The metasedimentary sequence at Otjua comprises biotite schists, calcsilicates and marbles of the Khan, Rossing, Oberwasser and Karibib Formations, which are overturned, strike 103 degrees with a steep northerly dip, and are intruded by Damara granites and pegmatites. Two contrasting styles of tungsten mineralisation have been reported: Scheelite mineralisation associated with calc-silicate hornfels and metasomatic scheelite-skarns replacing marble.
Intensive exploration of the Otjua skarn occurrence started in 1981 followed by a drilling programme and geophysical surveys until 1985. Drilling indicates that the medium grade (0 to 1% scheelite) mineralised idocrase facies skarn extends at least 370 m below surface and is well developed towards the footwall of the carbonate. The economically most important garnet facies skarn has been intersected in drillcore at a depth of 350 m and is composed of garnet, hedenbergitic pyroxene, scapolite, fluorite, scheelite (0 to 3%), plagioclase, apatite, epidote, hornblende, chlorite, carbonate, pyrite, pyrrhotite and chalcopyrite. 92% of all samples from garnet facies skarn have tungsten oxide contents of less than 1% reflecting the relatively low grade of the Otjua deposit with indicated reserves of 5.4 million t of skarn. As indicated by whole rock geochemistry and stable isotope studies, the Otjua skarn mineralisation was derived from the Otjua granite (infiltration metasomatism), whereas the hornfels-hosted scheelite mineralisation is considered to result from syn-sedimentary tungsten concentrations.
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The Otjua tungsten deposit is situated about 30 km north of Omaruru on the farms Otjua 37 and Schonfeld 92 along the southern side of a major anticlinal structure. The metasedimentary sequence at Otjua comprises biotite schists, calcsilicates and marbles of the Khan, Rossing, Oberwasser and Karibib Formations, which are overturned, strike 103 degrees with a steep northerly dip, and are intruded by Damara granites and pegmatites. Two contrasting styles of tungsten mineralisation have been reported: Scheelite mineralisation associated with calc-silicate hornfels and metasomatic scheelite-skarns replacing marble.
Intensive exploration of the Otjua skarn occurrence started in 1981 followed by a drilling programme and geophysical surveys until 1985. Drilling indicates that the medium grade (0 to 1% scheelite) mineralised idocrase facies skarn extends at least 370 m below surface and is well developed towards the footwall of the carbonate. The economically most important garnet facies skarn has been intersected in drillcore at a depth of 350 m and is composed of garnet, hedenbergitic pyroxene, scapolite, fluorite, scheelite (0 to 3%), plagioclase, apatite, epidote, hornblende, chlorite, carbonate, pyrite, pyrrhotite and chalcopyrite. 92% of all samples from garnet facies skarn have tungsten oxide contents of less than 1% reflecting the relatively low grade of the Otjua deposit with indicated reserves of 5.4 million t of skarn. As indicated by whole rock geochemistry and stable isotope studies, the Otjua skarn mineralisation was derived from the Otjua granite (infiltration metasomatism), whereas the hornfels-hosted scheelite mineralisation is considered to result from syn-sedimentary tungsten concentrations.
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