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Explanatory Notes for the Geological Map of Southwest Ghana 1:100 000

Sheets Wiawso (0603D), Asafo (0603C), Kukuom (0603B), Goaso (0603A), Sunyani (0703D) and Berekum (0703C)

Ed.: Wolfgang Hirdes; Richard Senger; Joseph Adjei

1994. 139 pages, 32 figures, 18 tables, 6 maps, 17x24cm, 760 g
Language: English

(Geologisches Jahrbuch Reihe B, Band B 83)

ISBN 978-3-510-96262-4, paperback, price: 24.00 €

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Contents

Contents top ↑

1 Introduction 9
1.1 General remarks and objectives 9
1.2 Project realisation 10
1.3 Physiography, climate, accessibility 12
1.4 Previous mapping work in the project area 14
2 General Geology of Birimian and Tarkwaian Rocks in Ghana and Structural
Aspects 16
2.1 Regional setting 16
2.2 Metavolcanic rocks of the Birimian 17
2.2.1 Regional disposition and Ethology 17
2.2.2 Geochemistry 20
2.3 Metasedimentary rocks of the Birimian 25

2.3.1 Volcaniclastic rocks 26
2.3.2 Wackes 27
2.3.3 Argillites 28
2.3.4 Chemical sediments 30
2.4 Facies relationships of the various lithological units 30
2.5 Granitoids 34
2.6 Sediments of the Tarkwaian Group 39
2.7 Structure 40
2.8 Sequence of geological events in the Early Proterozoic in Ghana 46
2.9 A proposed modal for the evolution of the Birimian in Ghana 46
3 Individual Rock Units of the Project Area 49
3.1 Supracrustal rocks 49
3.1.1 Volcanic rocks of the Birimian Supergroup 49
3.1.1.1 Basalts (bvm) 50
3.1.1.2 Andesites, dacites (bvi) 52
3.1.1.3 Rhyodacites, rhyolites (bvf) 55
3.1.2 Sedimentary facies units of the Birimian Supergroup 57
3.1.2.1 Volcaniclastic facies (bsv) 60
3.1.2.2 Volcaniclastic/argillite facies (bsva) 60
3.1.2.3 Argillite/volcaniclastic facies (bsav) 61
3.1.2.4 Argillite facies (bsa) 62
3.1.2.5 Wacke facies (bsw) 63

3.1.2.6 Argillite/wacke facies (bsaw) 66
3.1.2.7 Facies of chemical sediments and/or chemically alterad rocks
(bsch) 66
3.1.3 Metamorphosed Birimian rocks (other than regional lower greenschist
facies) 71
3.1.3.1 Metasediments (upper greenschist to amphibolite facies), mostly
strongly foliated (bmf) 72
3.1.3.2 Amphibolites and minar garnet amphibolites (bvam) 78
3.1.3.3 Chlorite-carbonate schists (bvcc) 79

3.1.3.4 Quartz-sericite schists (bvss) 80
3.1.3.5 Contact-metamorphic sediments 80
3.1.4 Sedimentary rocks of the Tarkwaian Group 82
3.1.4.1 Conglomerates, quartzites, arcosas (tcq) 82
3.1.4.2 Tuflaceous argillites (tta) 88
3.2 Intrusive rocks 88
3.2.1 Belt granitoids 88
3.2.1.1 Gabbros (gvg) 89
3.2.1.2 Belt granitoids (sensu stricto) (gvh) 94
3.2.1.3 Belt granitoids, mica-dominated (gvm) 95
3.2.1.4 K-feldspar-rich belt granitoids (gvk) 96
3.2.1.5 Belt granitoids with NW-SE foliation (gvb) 98
3.2.1.6 Belt granitoids with NE-SW foliation (gvf) 99
3.2.2 Basin granitoids 101
3.2.2.1 Basin granitoids sensu stricto (gsb) 101
3.2.2.2 Leucogranites, pegmatoids (gsl) 105
3.2.3 Dykes and sills (d) 106
3.3 Radiometric age determinations and other isatopic data 106
4 Economic Geology 106
4.1 Gold 106
4.1.1 Gold occurrences in the project area 106
4.1.2 Hard-rock gold analyses 122

4.1.3 Soll geochemistry 122
4.2 Comments on other mineral commodities 126
5 Synthesis and Summary of Geology of the Mapped Project Area 126
6 Comments on Individual 1: 100,000 Field Sheets 128
6.1 Sheet Wiawso (0603D) 129
6.2 Sheet Asato (0603C) 130
6.3 Sheet Kukuom (0603B) 131
6.4 Sheet Goaso (0603A) 132
6.5 Sheet Sunyani (0703D) 133
6.6 Sheet Berekum (0703C) 134
7 References 135