Geology of Ghana - Summary Notes

Course Information

  • Course: Geology of Ghana MR/ES 259
  • Lecturer: Dr. Asare Asante-Annor
  • Institution: University of Mines and Technology (UMAT), Tarkwa
  • Assessment Overview:
    • Class attendance: 10 marks
    • Continuous Assessment (Assignments + Tests): 30 marks
    • Final Examination: 60 marks

Geological Framework Overview

  • Ghana's Geology: Primarily within the Pre-Cambrian Guinean Shield, comprising roughly 2/3 of the country's landmass.
  • Key Formations:
    • Birimian System: Characterized by alternating NE-SW trending volcanic belts and sedimentary basins. It is the most significant system for gold mineralization.
    • Tarkwaian System: Comprises molasse-type sediments. It unconformably overlies the Birimian and is known for its \text{Banket} conglomerates which host significant detrital gold.
  • Important Stratigraphic Units:
    • Voltaian System: A large interior sedimentary basin covering approximately 40\% of Ghana. It ranges from Late Proterozoic to Paleozoic in age.
    • Accraian Series: Early Devonian seafloor deposits found specifically in the Accra area.

Stratigraphy and Rock Types

  • Birimian Stratigraphy: Subdivided into Meta-sedimentary (55\%) and Meta-volcanic rocks (20\%).
    • Meta-volcanics: Mainly basaltic to andesitic lavas and pyroclastics.
    • Meta-sediments: Include phyllites, tuffs, and greywackes.
  • Key Deposits and Minerals: Significant for Gold, Manganese (e.g., Nsuta), Bauxite (e.g., Awaso), and Iron minerals.
  • Granitoids: Categorized into four main types:
    • Cape Coast (G1): Syn-tectonic, often associated with sedimentary basins.
    • Dixcove (G2): Late-tectonic, typically found within volcanic belts.
    • Winneba and Bongo Types: Specific regional variations with distinct mineralogical compositions.

Processes of Mineral Formation

  • Mineral Formation: Primarily occurs through the crystallization of magma or hydrothermal solutions (hot, mineral-rich fluids) moving through fractures.
  • Metamorphism Types:
    • Contact: High temperature, low pressure, occurs near igneous intrusions.
    • Dynamic: Associated with high pressure and faulting/shearing.
    • Regional: Large-scale changes due to tectonic plate movements, affecting temperature and pressure simultaneously.

Geological Provinces of Ghana

  1. Western Unit (Shield Area): Covers 45\% of Ghana; includes the Birimian and Tarkwaian systems.
  2. Southeastern Unit: Includes the Dahomeyan System (oldest rocks, mostly gneisses) and the Togo Series (quartzites and phyllites).
  3. Central Unit: Primarily the Voltaian System sedimentary rocks.
  4. Coastal Basins: Includes the Tano, Keta, and Accraian basins, significant for hydrocarbon exploration.
  5. Tertiary to Recent Deposits: Localized surface deposits and weathering crusts.

Rock Cycle Highlights

  • Metasediments and Metavolcanics: These are original sedimentary or volcanic rocks that have been structurally and chemically altered. Understanding their protolith (original rock) is vital for mapping.
  • Weathering and Erosion: Ghana's tropical climate promotes intense chemical weathering, leading to the formation of thick regolith and laterites.

Key Concepts

  • Erosion vs Weathering: Weathering is the in-situ breakdown; erosion is the transportation of the material.
  • Igneous Rock Textures:
    • Phaneritic: Coarse-grained, indicating slow cooling (e.g., Granite).
    • Aphanitic: Fine-grained, indicating rapid cooling (e.g., Basalt).
    • Porphyritic: Large crystals (phenocrysts) in a fine-grained matrix, showing a two-stage cooling history.
  • Granulite Facies: Represent the highest grade of regional metamorphism in the Deep Crust.

Economic Geology

  • Mineral Resources:
    • Gold: Predominantly in Birimian shear zones and Tarkwaian conglomerates.
    • Diamonds: Found in Birimian alluvial deposits (e.g., Akwatia).
  • Hydrogeological Potential: The Voltaian and crystalline basement rocks are critical aquifers for rural water supply.
  • Hydrocarbons: Active exploration occurs in the Tano (Western) and Keta (Eastern) offshore basins.