Editing Sierra Leone — Colonial Geological Surveys 1947–1956

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In the Kambui Hills near Hangha, the Kambui Schists, striking consistently NNE and dipping steeply W, consist of hornblendic rocks which are shown to be of igneous parentage, 6,000 ft. thick, enclosing a belt of biotite-gneiss 200300 ft. thick. The gneiss belt is traversed by a zone of shearing which embraces irregular lenses of dunite up to 250 ft. thick, in which chrome-spinel bands and ore-bodies, though steeply inclined, generally fail to conform with the regional foliation. Since the chromite bands exhibit rudimentary gravity-stratification, they are taken to indicate that both spinel and olivine accumulated as horizontal layers by gravitative settling. Chain-structure is typical of the lower-grade chromite bands. The ultrabasic bodies are considered to have been emplaced after the metamorphism of the schists, by " cold intrusion ", during which serpentine, brucite and chrome-clinochlore were formed on a limited scale. The shear zones enclosing the ultrabasics were subsequently invaded by pegmatite fluids, which promoted the metasomatic development of anthophyllite (in water-deficient regions), chrome-tremolite, talc, chrome-muscovite and phlogopite in addition to quartz and felspar. Leaching of chromite occurred at this stage but no hydrothermal formation of spinel.
 
In the Kambui Hills near Hangha, the Kambui Schists, striking consistently NNE and dipping steeply W, consist of hornblendic rocks which are shown to be of igneous parentage, 6,000 ft. thick, enclosing a belt of biotite-gneiss 200300 ft. thick. The gneiss belt is traversed by a zone of shearing which embraces irregular lenses of dunite up to 250 ft. thick, in which chrome-spinel bands and ore-bodies, though steeply inclined, generally fail to conform with the regional foliation. Since the chromite bands exhibit rudimentary gravity-stratification, they are taken to indicate that both spinel and olivine accumulated as horizontal layers by gravitative settling. Chain-structure is typical of the lower-grade chromite bands. The ultrabasic bodies are considered to have been emplaced after the metamorphism of the schists, by " cold intrusion ", during which serpentine, brucite and chrome-clinochlore were formed on a limited scale. The shear zones enclosing the ultrabasics were subsequently invaded by pegmatite fluids, which promoted the metasomatic development of anthophyllite (in water-deficient regions), chrome-tremolite, talc, chrome-muscovite and phlogopite in addition to quartz and felspar. Leaching of chromite occurred at this stage but no hydrothermal formation of spinel.
  
The Hangha chromite is Cr<sub>59</sub>Al<sub>34 </sub>(Mg<sub>65</sub>) on Thayer's (1946) shortened formula. Conversion of phlogopite to vermiculite represents a later stage, related to weathering; this also applies to alteration of felspars to kaolinite and metahalloysite. Chromite and, to a less extent, anthophyllite resist lateritic weathering much more effectively than the silicates associated with them.
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The Hangha chromite is Cr<sub>59</sub>Al<sub>34 </sub>(Mg<sub>65</sub>) on Thayer's (1946) shortened formula.' Conversion of phlogopite to vermiculite represents a later stage, related to weathering; this also applies to alteration of felspars to kaolinite and metahalloysite. Chromite and, to a less extent, anthophyllite resist lateritic weathering much more effectively than the silicates associated with them.
  
 
The Kambui Schist area is surrounded by Basement granite, and with this the late pegmatites may be associated. The complex alterations of the chromiferous ultrabasics at Hangha may be regarded as an early phase of granitisation; at Bendu-Yawie, near the Gori Hills, a lens of chromite totally enclosed in later granite represents a more advanced stage. Lenses of chromite in granite-gneiss at Jaluahun are interpreted in the same way; abundant mariposite intimately associated with the chromite suggests accession of potash. [''Authors' abstract''.] The existence of chromite in substantial quantity in the south-eastern part of the Sierra Leone Protectorate was first discovered in 1929 by Dr. N. R. Junner and Mr. J. D. Pollett in the course of Geological Survey fieldwork. The initial discoveries were at Jaluahun, near Senduma, 12 miles south of Blama, and in the Kambui Hills west of N'Gerihun, 5 miles north-east of the Sierra Leone Government Railway at Hangha. Other deposits were subsequently found by the Geological Survey on the Moa River between Potoru and Zimi, and on the eastern slope of the Gori Hills near Bandajuma.
 
The Kambui Schist area is surrounded by Basement granite, and with this the late pegmatites may be associated. The complex alterations of the chromiferous ultrabasics at Hangha may be regarded as an early phase of granitisation; at Bendu-Yawie, near the Gori Hills, a lens of chromite totally enclosed in later granite represents a more advanced stage. Lenses of chromite in granite-gneiss at Jaluahun are interpreted in the same way; abundant mariposite intimately associated with the chromite suggests accession of potash. [''Authors' abstract''.] The existence of chromite in substantial quantity in the south-eastern part of the Sierra Leone Protectorate was first discovered in 1929 by Dr. N. R. Junner and Mr. J. D. Pollett in the course of Geological Survey fieldwork. The initial discoveries were at Jaluahun, near Senduma, 12 miles south of Blama, and in the Kambui Hills west of N'Gerihun, 5 miles north-east of the Sierra Leone Government Railway at Hangha. Other deposits were subsequently found by the Geological Survey on the Moa River between Potoru and Zimi, and on the eastern slope of the Gori Hills near Bandajuma.

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