annexc - Newmont Mining Corporation

Annex C3 – Geology and Geochemistry Supplemental Information C3 - 2
DISCUSSION OF GOLD MINERALISATION PROCESSES
Gold is principally associated with belts of Proterozoic Birimian and Tarkwaian rocks. The
bulk of the gold, about 80 percent, comes from primary lode occurrences within Birimian
rocks (Figure 3-5), which together with their associated intrusives, occur over
approximately one third of the country (Wright and Hastings 1985). The remainder of the
gold produced comes mainly from the blanket conglomerate deposits (possible paleo
placers) of the Tarkwaian System and from placer mines exploiting material derived from
both the Birimian and Tarkwaian source rocks.
The Birimian Gold Belts are a set of six parallel, evenly spaced, northeast trending volcanic
ridges over 300 km long (see for example the Ashanti Belt, Figure 3-5). Steeply dipping,
regional shears bound these ridges, which are separated by basins containing
metamorphosed pyroclastic and metasedimentary rocks (Figure 3-5).
Within the Birimian, primary gold is associated with a typical submarine exhalative volcanosedimentary
facies developed along narrow laterally continuous zones at the boundary
between volcanic belts and the sedimentary basins. This primary, syn-sedimentary gold was
subjected to migration and concentration in northeast trending regional shear zones during
metamorphism and the emplacement of the Birimian intrusives, as well as during major
structural events.
LOCAL GEOLOGIC SETTING DISCUSSION
The Akyem deposit is localized in the hanging wall (upper side or plate) of a regional fault
that trends to the northeast (N70 o E) parallel to regional structures, and dips about 60
degrees to the southeast (Figures 3-6 and 3-7), parallel to the foliation developed in the
Birimian host rock. The planar fault structure (thrust fault) (Feybesse 1999; Lescuyer 1999)
is intensely sheared and exhibits a mylonitic to cataclastic fragmental texture consisting of
lens of metavolcanic fragments in a matrix of sheared and plastically deformed graphitic
material. The presence of graphitic rubble zones suggests reactivation of the fault zone over
time. The fault occurs in fine-grained, gray-green massive to locally sheared Birimian mafic
metavolcanic rocks exhibiting chlorite and carbonate alteration that locally contain euhedral
magnetite. Immediately overlying the metavolcanics is a distinctive gray-green-pink unit
containing blue quartz phenocrysts in a mylonitic matrix (quartz epiclastic rock). The upper
and lower contacts of this unit are typically sheared and brecciated and the upper contact of
this unit is locally in sharp contact withoverlying light gray-tan chert (Monthel et al. 2001).
This unit is considered sedimentary and represents a distinctive marker between the
metavolcanic and metasedimentary units of the Birimian.
The overlying metasedimentary rocks are for the most part turbidite sequences consisting
of greywacke, argillites, black carbonaceous siltstone, and fine-grained arkosic sandstones.
This unit grades upwards into a saprolitically weathered zone (deeply weathered bedrock
largely altered to clay) that ranges from 10 to 50 meters thick. The saprolite consists of
lateritic clay and quartz fragments with as much as 25 percent weathered rock remaining
within the saprolite. Near the surface, in the upper 1 to 5 meters, red lateritic clay is
developed as subsoil.
Akyem Gold Mining Project November 2008 FINAL EIS