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