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