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The role of silica and iron oxide mobility in the formation of gold-bearing fluvial sediments in the Proterozoic Mporokoso Basin, northern Zambia

Published online by Cambridge University Press:  01 May 2009

C. P. Andrews-Speed
C. P. Andrews-Speed
Affiliation:
Minex Department, Zimco Ltd, P.O. Box 30090, Lusaka, Zambia
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Abstract

Gold-bearing sandstone and conglomerate near the base of the Proterozoic Mporokoso Group were deposited in a braided river system. The detrital sand grade material is mainly of quartz, sericite and haematite, and the pebbles are of vein-quartz, chert, silicic volcanics, quartzose metasediment and jaspilite. The basement rocks presently exposed in the basement are silicic igneous rocks and quartzose metasediments.

The petrography of the fluvial sediments suggests that silica and, to a lesser extent, iron oxide were mobile both in the source-area and in the braided river system. Evidence for silica-mobility includes jaspilite pebbles with spherulites and glaebules of chalcedony, abundant vein-quartz pebbles, intra-basinal sandstone pebbles, and the silicification of volcanic pebbles. The detrital haematite in the fluvial sandstone forms pseudomorphs after magmatic magnetite. Authigenic iron oxide occurs in several forms which suggest that iron oxide was mobile in the source-area and in the fluvial sediments. Uranium is locally abundant in basement and sedimentary rocks, cassiterite is a common heavy mineral in the fluvial sediments, and fluorite has been found in the basement.

These features may be explained by intense weathering which mobilized both silica and iron. The silica was concentrated near the surface to form silcretes in the basement and later in the overlying fluvial sediments. Hydrothermal convection cells driven by the granites may have carried silica, iron, tin, fluorine and uranium towards the surface before and during the erosion of the igneous basement.

Type
Articles
Information
Geological Magazine , Volume 123 , Issue 2 , March 1986 , pp. 143 - 152
Copyright
Copyright © Cambridge University Press 1986

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References

Alley, N. F. 1977. Age and origin of laterite and silcrete duricrusts and their relationship to episodic tectonism in the mid-north of South Australia. Journal of the Geological Society of Australia 24, 107–116.CrossRefGoogle Scholar
Andersen, L. S. & Unrug, R. 1984. Geodynamic evolution of the Bangweulu block, northern Zambia. Precambrian Research 25, 187–212.CrossRefGoogle Scholar
Andrews-Speed, C. P. 1985. Gold-bearing fluvial and associated tidal marine sediments of Proterozoic age in the Mporokoso Basin, northern Zambia. Sedimentary Geology (in press).CrossRefGoogle Scholar
Andrews-Speed, C. P. & Mulumba, B. P. (In prep.). Empirical guides to gold enrichment in Proterozoic quartz-pebble conglomerates in northern Zambia.Google Scholar
Andrews-Speed, C. P. & Unrug, R. 1984. Gold in the sedimentary cover of the Bangweulu block, northern Zambia. In Gold 82: The Geology, Geochemistry and Genesis of Gold Deposits (ed. Foster, R. P.), pp. 221–37. Rotterdam: Balkema.Google Scholar
Brewer, M. S., Halsam, H. W., Darbyshire, P. F. P. & Davis, A. E. 1979. Rb–Sr determinations in the Bangweulu block, Luapula Province, Zambia. Report of the Institute of Geological Sciences 79/5, 11 pp.Google Scholar
Deans, T. 1938. Igneous rocks from the Abercorn and Kasama Districts, Northern Rhodesia. Geological Magazine 75, 547–58.CrossRefGoogle Scholar
Hancock, N. J. 1978. Possible causes of Rotliegend sandstone diagenesis in northern West Germany. Journal of the Geological Society of London 135, 35–40.CrossRefGoogle Scholar
Miall, A. D. 1978. Lithofacies types and vertical profile models in braided river deposits: a summary. In Fluvial Sedimentology (ed. Miall, A. D.), pp. 597–604. Calgary: Canadian Society of Petroleum Geologists.Google Scholar
Parker, A. & Sellwood, B. W. 1983. Facies controls on sandstone diagenesis. In Sediment Diagenesis (ed. Parker, A. and Sellwood, B. W.), pp. 269–88, Dordrecht: D. Reidel.CrossRefGoogle Scholar
Pretorius, D. A. 1984. The source of Witwatersrand gold: conjecture of the unknown (Abstract). In Gold 82: The Geology, Geochemistry and Genesis of Gold Deposits (ed. Foster, R. P.), p. 219. Rotterdam: Balkema.Google Scholar
Pye, K. 1983. Red beds. In Chemical Sediments and Geomorphology (ed. Goudie, A. S. and Pye, K.), pp. 227–63. London: Academic Press.Google Scholar
Selleck, B. W. 1978. Syndepositional brecciation in the Potsdam sandstone of northern New York State. Journal of Sedimentary Geology 48, 1177–84.Google Scholar
Simpson, P. R., Brown, G. C., Plant, J. & Ostle, D. 1979. Uranium mineralization and granite magmatism in the British Isles. Philosophical Transactions of the Royal Society of London, Series A, 291, 385–412.Google Scholar
Smale, D. 1973. Silcretes and associated silica diagenesis in southern Africa and Australia. Journal of Sedimentary Geology 43, 1077–89.Google Scholar
Summerfield, M. A. 1983. Silcrete. In Chemical Sediments and Geomorphology (ed. Goudie, A. S. and Pye, K.), pp. 59–91. London: Academic Press.Google Scholar
Turner, P. 1979. The palaeomagnetic evolution of continental red beds. Geological Magazine 116, 289–301.CrossRefGoogle Scholar
Turner, P. & Ixer, R. 1977. Diagenetic development of unstable and stable magnetization in the St Bees Sandstone (Triassic) of Northern England. Earth and Planetary Science Letters 34, 113–24.CrossRefGoogle Scholar
Unrug, R. 1984. The mid-Proterozoic Mporokoso Group of northern Zambia: stratigraphy, sedimentation and regional position. Precambrian Research 24, 99–121.CrossRefGoogle Scholar
Van Houten, F. B. 1968. Iron oxides in red beds. Bulletin of the Geological Society of America 79, 399–416.CrossRefGoogle Scholar
Viljoen, R. P., Saager, R. & Viljoen, M. J. 1970. Some thoughts on the origin and processes responsible for the concentration of gold in the Early Precambrian of southern Africa. Mineralium Deposita 5, 164–80.CrossRefGoogle Scholar
Walker, T. R. 1974. Formation of red beds in moist tropical climates: a hypothesis. Bulletin of the Geological Society of America 85, 633–8.2.0.CO;2>CrossRefGoogle Scholar
Watts, S. H. 1978. A petrographic study of silcrete from inland Australia. Journal of Sedimentary Geology 48, 987–94.Google Scholar