annexc - Newmont Mining Corporation

Annex C3 – Geology and Geochemistry Supplemental Information C3 - 1
DISCUSSION OF REGIONAL GEOLOGIC TERRAINS IN GHANA
A primary feature of the geology of Ghana is the parallel alignment of narrow (15 to 40
kilometres (km) wide), somewhat evenly spaced, northeasterly trending belts of folded
Birimian metavolcanic rocks (Figure 3-5). Between these belts are broad (approximately
60 to 90 km wide) sedimentary “basins” consisting predominantly of Birimian turbidity
sedimentary rock sequences (Ghana Minerals Commission and WGM 1998; Ireland et al.
2001).
Across southwestern Ghana, these belts can be divided geologically into four distinct
terrains based on the major rock type or systems that outcrop (Figure 3-5). These
systems include:
1. An early Proterozoic terrain (Birimian System) with older metavolcanics (intrusive
dikes and lava flows) of basaltic and andesitic composition. These metavolcanic units
exhibit pillow structures indicating subaqueous eruption of the original basaltic lava.
These metavolcanics underly younger sedimentary sequences comprised of
greywacke, turbidities, volcaniclastics, and argillites. Together these units form five
sub-parallel belts of volcanic rock separated by broad “basins” of sedimentary rocks.
Although difficult to determine because of folding and faulting, the total thickness of
the Birimian in Ghana may be as much as 10,000 to 15,000 meters. The contact
between the sediments and volcanics is poorly exposed (Ghana Minerals
Commission and WGM 1998). The Birimian system hosts most of the country’s
gold mineral deposits and outcrops over as much as one-third of the southwestern
part of the country.
2. Tarkwaian System is a distinctive sequence of clastic sediments, which were
deposited in sedimentary basins that form the core of many of the Birimian volcanic
belts (i.e., 250 km long Ashanti Belt).
3. Voltaian Basin, in which are preserved the late Precambrian to Paleozoic sediments
that mantle the craton.
4. Intrusive rocks (primarily granitoids) that have been emplaced in Birimian and
Tarkwaian rocks, some of which (Belt-type) are intrusive equivalents of the Birimian
Volcanics.
The Birimian and Tarkwaian sequence exhibits a strong northeast-southwest oriented
structural fabric that is a manifestation of both the axial planes of major tight isoclinal folds
(anticlinal sedimentary basins and synclinal volcanic belts) and major fault systems. The
rocks typically dip more than 60°. Regional fault systems often exhibit a complex history of
thrusting, normal and strike slip motion. Metamorphism in the Birimian is “low-grade”
greenschist facies except near intrusive contacts where amphibolite assemblages occur in
the metasediments.
Akyem Gold Mining Project November 2008 FINAL EIS