Geoffrey Loh; Wolfgang Hirdes:

Explanatory Notes for the Geological Map of Southwest Ghana 1:100,000 Sekondi (0402A) and Axim (0403B) Sheets

Contributors: Chris Anani; Don W. Davis; Ulrich K. Vetter

1999. 149 pages, 19 tables, 2 maps, 50 color figs., 17x24cm, 510 g
Language: English

(Geologisches Jahrbuch Reihe B, Band B 93)

ISBN 978-3-510-95841-2, paperback, price: 37.00 €

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geological mapGhanaSekondiAxim


Abstract top ↑

This volume presents the results of geological mapping and research work performed in southwestern Ghana.
The studied area covers about 2000 km2 and comprises the land surface south of latitude 5° N and between longitude 1°30' W and 2°30' W. It is prospective for gold and manganese and includes supracrustal and intrusive rocks of the Birimian Supergroup, subordinately sedimentary rocks of the Tarkwaian Group, as well as minor, late-kinematic basin-type granitoids - all of Paleoproterozoic age. Furthermore, Paleozoic and Cretaceous sediments are present locally. The two geological sheets at the scale of 1:100,0000 (Axim and Sekondi) and cover the southern tip of the Ashanti volcanic belt which is the prime gold producer of Ghana, the southernmost part of the Kumasi sedimentary basin and migmatitic gneisses representing a comparatively deep crustal level of the Cape Coast sedimentary basin.

Most of the investigated terrane, however, is occupied by three major synvolcanic/belt-type granitoid plutons from which precise U-Pb zircon crystallization ages of 2172 + 2 Ma and 2174 + 2 Ma are available. Regional foliation trends NE-SW or NNE-SSW, being the result of Eburnean NW-SE compression of the terrane. Three major high-strain zones occur, one of which represents the southern continuation of the important gold-bearing Prestea-Obuasi-Konongo high-strain zone. 26 gold occurrences and four manganese occurrences - abandoned European as well as active and abandoned native ones - were inventarized and their characteristics and features are presented in tabular form.

The two most important regional controls for gold deposition seem to be the high-strain zone in and adjacent to the belt/basin transition zone on the western flank of the Ashanti belt, in accordance with conceptions previously established by GSD and BGR, as well as sheared granitoid/greenstone contacts.

Chapter 1 of these Explanatory Notes is introductory, whereas Chapter 2 summarizes the present state of knowledge of Birimian and Tarkwaian rocks in Ghana and the neighbouring countries. Chapter 3 describes the geology, petrography and some geochemistry of all the different lithological units distinguished on the 1:100,000 geological maps. Chapter 4 outlines isotopic constraints and results of precise radiometric dating carried out on zircon, monazite and titanite. Chapter 5 lists and briefly describes abandoned gold mines, prospects, and occurrences encountered in the project area, as well as manganese and kaolin occurrences. Chapter 6 summarizes the geological data obtained, emphazising features of importance for further geological investigation and/or exploration.

The explanatory notes are accompanied by two full-color map sheets

Table of Contents top ↑

1 Introduction 7
1.1 General remarks and objectives 7
1.2 Project implementation 8
1.3 Physiography, climate, accessibility 11
1.4 Previous mapping work in the project area 12
2 General Geology of Birimian and Tarkwaian Rocks 14
2.1 Birimian Supergroup 14
2.2 Tarkwaian Group 19
2.3 Sequence of geological events in the Paleoproterozoic of Ghana 20
2.4 Remarks on the Ashanti belt 21
3 Individual Rock Units of the Project Area 22
3.1 Supracrustal rocks 22
3.1.1 Volcanic rocks of the Birimian Supergroup 22 Basalts (bvm) 24 Andesites, minor dacites (bvi) 28 Rhyodacites, rhyolites (bvf) 31
3.1.2 Sedimentary facies units of the Birimian Supergroup 34 Volcaniclastic facies (bsv) 35 Argillite/volcaniclastic facies (bsav) 38 Argillite facies (bsa) 38 Wacke facies (bsw) 41 Facies of chemical sediments and/or chemically altered
rock (bsch), including banded manganese formation (bsm) 42
3.1.3 Metamorphosed Birimian rocks (other than regional lower
greenschist facies) 44 Quartz-sericite schist (bvss) 44 Actinolite schist, chlorite-carbonate schist (bvcc) 45 Amphibolite (bvam) 45 Contact-metamorphic sediment 47 Felsic to intermediate gneiss (bmg) 49
3.1.4 Sedimentary rocks of the Tarkwaian Group (tcq) 50
3.1.5 Phanerozoic sedimentary rocks 54 Sekondian Group (s1-s6) 54 Apollonian Group (cu) 57
3.2 Intrusive rocks 60
3.2.1 Belt granitoids, including porphyries
(gvh, gvhd, gvht, gvhr, gvhg, gvp, gvpt) 60
3.2.2 Gabbro, norite (gvg, gvgf) 89
3.2.3 Ultramafic rocks of belt affiliation (gvpx) 95
3.2.4 Basin granitoid (gsb) 101
3.2.5 Dykes and sills (d) 102
4 Isotopic Constraints and Radiometric Dating 106
4.1 Previous radiometric dating in the area 106
4.2 Sampling and analytical methods 107
4.3 Results 109
4.4 Discussion and interpretation of radiometric age data 111
5 Economic Geology 112
5.1 Gold occurrences in the project area 112
5.2 Manganese occurrences in the project area 117
5.3 Comments on other mineral commodities 118
6 Synthesis and Summary of Geology of the Mapped Project Area 119
7 References 125
Appendix: List of studied localities 131