This tour, which was developed, organised and managed by Mike Porter of Porter GeoConsultancy Pty Ltd (PGC), as a joint venture with the Australian Mineral Foundation Inc. (AMF), included:
MODULE 2 - WEST AFRICA
Tues 31 October to Thurs 9 November 2000
For information on the remainder of the tour, see: Module 1
MODULE 2 - WEST AFRICA
Super Porphyry Cu and Au|
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This MODULE commenced in Accra, Ghana, on the evening of Monday 30 October, 2000 with an Introductory Meeting and Dinner. Module 2 included detailed briefings and visits to the following gold mines and projects,
as well as expert workshop/briefings, in the order detailed below:
Damang - Ghana
The Damang gold
mine is located some 25 km North-east of the gold mining town of Tarkwa
in south-western Ghana, and 200 km to the west of the capital Accra. The
ore is hosted by the Lower Proterozoic Tarkwaian clastic fluviatile sediments
that overlie the older, metamorphosed Lower Proterozoic greywackes and
mafic meta-volcanics of the Birimian Series. Both suites are part of the
more extensive Man-Leo Shield which comprises Archaean and Proterozoic
rocks that were once part of a larger craton in West Africa and Brazil.
The deposit lies on the eastern margin of one of a number of NE trending
belts of Birimian volcanics that are un-conformably overlain by synclinal
belts of Tarkwaian sediments. Both the Birimian and Tarkwaian have been
progressively deformed, culminating in overturning and over-thrusting on
both edges of the belt.
Within the district
the Tarkwaian sediments are characterised by extensive conformable blankets
of mineralised conglomerate ("bankets"), believed to represent metamorphically
modified palaeo-placer accumulations (typified by the Tarkwa mine as described
below). The Damang deposit is unusual for the Tarkwaian in that it comprises
a free milling hydrothermal quartz-vein stockwork. Similarly Birimian lode
gold deposits (typified by Obuasi as described below) are not developed
to any significant extent in the Tarkwa district, and are geochemically
very distinct from the mineralisation in the Tarkwaian rocks.
at Damang occurs in the core and on the eastern limb of the Damang Anticline,
a long attenuated structure, bounded on both sides by strike faults. Gently
east dipping to flat lying lenticular quartz veins make up ladder like
sets of east dipping mineralised zones. These veins have selvedges of pyrite
and pyrrhotite, while disseminated sulphides are found through the hosts
in the more heavily veined areas. The hosts are steeply east-dipping quartzite
(ex-sandstone) and conglomerate, although in the hangingwall iron rich
laminated staurolite-tourmaline meta-sediments and overlying finer grained
meta-sandstones are also mineralised. Better grades are found in the vein
system where it is developed within the conglomerate units that are the
hosts to the stratabound mineralisation (bankets) found elsewhere in the
The Damang mine
was commissioned in 1997 after an expenditure of USD 135 million. In the
12 month period to June 1999 it produced 8.38 t (0.27 Moz) of gold from
3.73 mt of ore averaging 2.37 g/t Au. The recovery was 95% and cash cost
USD 200/oz. In March 1999 the proven+probable reserve was 33.8 mt @ 2 g/t
Au at a 1.6 g/t Au cut-off, while the resource was 60 mt @ 2.09 g/t Au
at a 1 g/t Au cutoff for 125 t (4.03 Moz) of contained Au. The mine is
operated by Abosso Goldfields Ltd which is owned 90% by Ranger Minerals
Ltd and 10% by the Government of Ghana.
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Tarkwa - Ghana
The Tarkwa mine
is the principal of a number of operations developed within the conglomerates
of the Tarkwaian Sequence over the southern Ashanti Belt in south-western
Ghana. The mine is some 25 km to the south-west of the Damang deposit described
above. In the Tarkwa District the Tarkwaian clastic fluviatile sediments
overlie the older Lower Proterozoic meta-mafic volcanics of the Birimian
Series. These sediments truncate 1890 to 2061 Ma granites which intrude
the Birimian, but are younger than 1650 Ma mafic volcanics. The Tarkwaian
Sequence occupies a belt some 250 km long and up to 16 km wide over the
Ashanti Belt volcanics. The sequence is generally comprises a basal 250
to 650 m thick suite of greenish-grey feldspathic quartzite, grits, breccias
and conglomerates, followed by the Banket Series which is 120 to 600 m
thick, composed of sandstones, quartzites, grits, breccias and conglomerates.
The Banket Series contains all of the economic gold mineralisation of the
Tarkwa District, concentrated in three to four conglomerate bands, each
of 1 to 10 m in thickness, concentrated in the lower 40 to 75 m. Each conglomerate
is separated by quartzite and grit. The conglomerates are oligomictic with
90% of the pebbles being (vein) quartz, with the remainder being quartzite
and schist. The matrix is principally quartz grains with sericite, hematite
and magnetite. Accessories are tourmaline, zircon, rutile, garnet chloritoid,
epidote, leucoxene and pyrite. Traces of bornite and chalcopyrite have
also been reported. The Banket Series is overlain by the 100 to 650 m thick
Tarkwa Phyllite and the 1350 m thick Huni Sandstone.
The Banket Series
conglomerate units comprise the Sub-basal Reef which is discontinuously
developed, up to 40 m thick and only carries low gold values. It is overlain
by the main producing unit, the Basal Reef which is 2 to 10 m thick and
has been historically mined over widths of 1.2 to 7 m with 3.5 to 14 g/t
Au, averaging about 6 g/t Au. The Basal Reef is in turn composed of a number
of lesser conglomerate bands. At Tarkwa the highest grade Au is found in
the bottom 20 cm of the lower most conglomeratic horizon of the Basal Reef,
associated with well sorted, well packed, hematitic conglomerates which
are usually less than 2 m thick. It is overlain in turn by the Middle Reef
which is 1.5 to 15 m thick with grades generally varying between 1.5 and
2 g/t Au. The uppermost conglomerate band is the Breccia Reef which is
2 to 10, to a maximum of 20 m thick, with low gold values that rarely exceed
1.5 g/t Au.
Gold grade appears
to be inversely proportional to the thickness and directly proportional
to the hematite content and possibly the diameter of conglomerate pebbles.
The intercalated quartzites between the conglomerate bands also contain
gold and hematite, although usually <0.5 g/t. The individual payable
conglomerates are variable, being present as discontinuous lenses which
are of the order of 600 to 1000 m long and 100 to 150 m wide or less. In
polished sections of the Tarkwaian ore, gold is present as 10 to 15 µm,
generally equi-dimensional grains with irregular outlines. These grains
occur as clusters in the vicinity of hematite; lodged within areas containing
hematite; in the quartz and sericite matrix of the conglomerate; and near
the margins of quartz pebbles. Much of the hematite has been recrystallised
as it is euhedral and often porphyroblastic, comprising from 2 to 60% of
the matrix to the reef conglomerates.
Tarkwa District mines have produced around 250 t (8 Moz) of gold since
1880. The Tarkwa mine was taken over by Gold Fields Ltd in 1993 and a new
open pit and underground operation brought into production in May 1998.
The total resource in the open pit operation is estimated to be 420 t (13.5
Moz) of gold. Production in the twelve months to June 30 1999 was 5 mt
of ore averaging 1.3 g/t Au for 6.4 t Au.
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Obuasi - Ghana
The Obuasi gold
deposit is located on the western margin of the largest of the ten NNE
trending linear belts of Lower Proterozoic greenstones (dated at 2166±66
Ma) found in Ghana and neighbouring West Africa. Each of these belts is
predominantly composed of metamorphosed tholeiitic basalts and lesser andesites
with intercalated graphitic phyllites belonging to the Birimian System
and is separated from the next belt by sequences of black and grey phyllites,
schists and meta-greywackes with subordinate volcanics. Individual belts
are 15 to 40 km in width. The largest, the Ashanti Belt which hosts the
Obuasi deposit is 250 km long and passes under younger Voltaian sediments
to the north. The Birimian has been intruded by Eburnian (1.8 Ga) syn-
and post-tectonic granitoids. The structure if the Birimian is characterised
by isoclinal folds with near vertical axial planes; locally developed open
symmetric folds in the volcanic belts; axial plane cleavage parallel to
bedding throughout the steeply inclined sediments; and by a weak secondary
cleavage oblique or perpendicular to the first. Three phases of fold deformation
are recognised in Ghana. In the Ashanti Belt high angle reverse faults
or upthrusts are found in mines. Both margins of most of the volcanic belts
are defined by shears.
The Obuasi deposits
are found along the western margin of the Ashanti Belt, at the sheared
and overthrust contact between the more competent volcanics to the east
and the more ductile sediments to the west. The main rock types in the
mine area are siltstones, phyllites, meta-greywackes, schists and meta-volcanics.
The major structural trend hosting gold mineralisation at Obuasi extends
over a length of 24 km. The most prominent structures are the flat dipping
Cote D'or Fissure and the steeply dipping Obuasi Fissure which intersect
to form the southerly pitching Main Reef Fissure.
Ore is present
as both quartz veins and lenses, and as disseminated sulphide ore formed
on the fringes of the shear zones. In the former the gold is free milling,
whereas in the second it is locked in sulphides within mafic rocks and
in schists. In addition a third ore type, oxidised ore derived from the
weathering of the sulphides, is exploited from the surface, while granitoid
stockworks have also been recognised. The sulphide ore is principally composed
of a disseminated assemblage of arsenopyrite, rutile, pyrite, pyrrhotite,
chalcopyrite, stibnite, sphalerite and galena. The gangue comprises quartz,
carbonate, chlorite, sericite and carbonaceous matter. Higher grade gold
is found in rocks with finer arsenopyrite. The gold itself within the disseminated
sulphide ore is present as micron and sub micron grains at crystal surfaces
and boundaries. Within the quartz vein type ore, free gold is visible,
while minor sulphides are also present. Wall rock alteration fringing the
quartz veins includes sulphidation, mainly arsenopyrite, sericitisation
As at December
1998, the total estimated resource was 89.4 mt @ 8.1 g/t Au for 725 t (23.4
Moz) of contained gold, including 63.9 mt @ 10.2 g/t Au underground, 19
mt @ 3.2 g/t Au in open pit and 6.5 mt @ 2.5 g/t Au in tailings. This mine
is the flagship of the Ashanti Goldfields Limited. The operation has a
plant capacity of 10.6 mtpa and uses CIL-bioxidation. Historical production
has been in excess of 550 t (17.3 Moz) of gold from 556 mt grading on average
17.6 g/t recovered gold. In 1998 the underground operation produced 2.24
mt of ore at 8.57 g/t Au, while the surface operations yielded 4.32 mt
@ 3.22 g/t Au. The open pits will be exhausted in 2000 and the underground
production rate increased.
extend over a strike length of 8 km and to a depth of 1500 m, served by
12 shafts and 3 declines. The haulage capacity at the end of 1998 was 6.1
mtpa of ore and waste. Several open pits are distributed over the same
8 km strike length.
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Yamfo-Sefwi - Ghana
project is located on the north-west margin of the 2.2 to 2.1 Ga Birimian
Yamfo-Sefwi belt of volcanics, dominated by tholeiitic meta-volcanics and
meta-sediments, near the sheared contact with the adjacent Sunyani Basin
sediments. The latter are mainly volcaniclastic shales, greywackes and
siltstones. Eburnian co-magmatic granitoids (the Dixcove Suite) have intruded
the volcanics, notably along the contact with the Sunyani Basin sediments.
The gold mineralisation has been emplaced close to the hydrothermally altered
brittle-ductile shear zones at or near the volcanics-sediment contact.
Three deformations are in evidence, namely an early S1 foliation, related
to the shears that form many of the lithological contacts in the region;
a regional F2 folding and shearing reworking earlier D1. The mineralisation
at Yamfo is hosted by shearing of this phase. The third phase is a late
brittle episode associated with hydraulic brecciation and minor gold. In
the main Centenary Zone there are two mineralised NE trending and SE dipping
shears that are mineralised. Gold is also discontinuously developed between
the shears. The gold is generally hosted by sheared, altered and brecciated
greywacke/siltstone in which the main alteration products are quartz-carbonate
and feldspar-chlorite-pyrite. It is usually fine grained, <25 µm,
and is associated preferentially with carbonate and quartz-carbonate veinlets
and stockworks, together with fine grained pyrite . The mineralised shears
average 20 m in thickness but may be up to 70 m wide. Narrow high grade
shoots of limited extent are indicated within the main ore zones. These
have grades of up to 14 g/t over narrow widths. To the south in the Kenyase
Zone, the sediment-volcanic contact is occupied by a dioritic intrusive
of the Dixcove Suit. Gold is hosted by the sheared contact between the
intrusive and sediments and a mylonite, along with gold bearing quartz
vein stockworks in the hangingwall diorite.
Within the Rank
area, some 25 km to the SSE, the main contact mineralisation is less continuous,
while the hangingwall stockworks are potentially more significant. A second
mineralised shear, to the east of and parallel to the main zone is found
within a granodiorite, with gold being associated with strong hydrothermal
alteration which dip at 70 degrees to the east. Intersections have been
indicated over a two km length, ranging from 10 to 130 m (average 30 m)
wide, with 2 to 3.9 g/t Au.
The project is
currently the property of Normandy Ghana Gold, a wholly owned subsidiary
of Normandy Mining Ltd. In mid 1999, the resource estimated for he project
area was 64.56 mt @ 3 g/t Au for 190 t (6.16 Moz) of gold using a 0.5 g/t
Au cut-off. The project is currently in feasibility, but is as yet un-developed.
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Accra Workshop - "The Setting & Occurrence of Gold in West Africa"
This full day workshop was presented by experienced, Accra based consultant Bob Griffis and included coverage of the following topics:
Mali Workshop - "The Geology of, & Exploration for, Gold in Mali"
The workshop in Bamako
covered the geology of, & exploration for, gold in Mali in general, and at the Sadiola mine in particular, and was presented by Jon Hill of AngloGold, Mali .
- The Geological and
Tectonic Framework of West Africa.
- The Distribution,
Geological Controls and Characteristics of Gold Mineralisation in West
- Descriptions and
details of important deposits not visited and a comparison with those seen.
- Exploration for Gold
in Ghana, techniques applied, data sets available, particular problems
and benefits, legal framework, etc.
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Sadiola - Mali
The Sadiola Hill
Gold Mine operated by AngloGold Limited is located in Mali, some 510 km
to the north west of the capital Bamako. The deposit falls within the Kenieba-Kedougou
window, which comprises an outlier of the early Proterozoic Birimian Series
on the north-east margin of the Kenema (or Man) Shield. The Birimian Series
is composed of a collage of volcanics to the west, overlain by the younger,
dominantly sedimentary domain of the Kofi Formation to the east. These
younger sediments are cut by the major Senegalo-Malian shear zone which
has a number of associated gold deposits located in its splays.
The gold mineralisation
at Sadiola Hill is associated with the near north-south trending Sadiola
Fracture Zone, a diorite filled fault zone that separates a greywacke and
meta-pelite sequence to the west from an impure marble to the east. The
diorite is discontinuously developed within the fault.
weathering has produced a deep kaolin rich clay zone, which has decarbonated
the marble at and near the surface. Gold has been enhanced in the kaolinitic
oxide zone, particularly in the decarbonated marble. This saprolite zone
contains the bulk of the resource, and all of the current reserve as a
flat lying body which is mostly free digging. It passes down into a soft
sulphidic zone below, and then to untested hard sulphide mineralisation.
In the primary
zone the gold, with a fineness of 850 to 970, is associated with sulphides
which are dominated by arsenopyrite, pyrite and pyrrhotite. The primary
mineralisation is accompanied by calc-silicate, potassic and carbonate
alteration and by silicification.
in 1990 when IAMGold, a Canadian junior secured he mineral rights and entered
into a joint venture in 1992 with Anglo American, whose interest later
passed to AngloGold. A regional geochemical survey in 1987 had rediscovered
the deposit when a clear 7000x3000 m anomaly was delineated, centred on
Sadiola Hill where artisanal workings in saprolitic oxide ore could be
followed for 1200 m along trend. After a capital expenditure of USD 80
million, the first gold was poured in December 1996, with full production
being achieved in 1997. By 1999 the throughput had reached 5.2 mt of ore
and by September 1999 the operation had produced 40 t (1.28 Moz) of gold.
In late 199 the resource totalled 106 mt of ore @ 2.39 g/t Au, using a
1.0 g/t Au cut-off. This corresponds to 254 t (8.2 Moz) of contained gold.
The project is
owned by AngloGold (38%), IAMGold (38%), the Mali Government (18%) and
the International Finance Corp (6%).
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Morila - Mali
The Morila deposit
is located in southern Mali, some 180 km south of the capital Bamako, and
600 km south-east of Sadiola. AngloGold Limited recently purchased 50%
of Randgold Resources (Morila) Limited which holds 80% of Societe des Mines
de Morila SA. The balance is held by the Mali Government. Construction
commenced in late 1999, and the mine is currently in the pre-strip phase
and the first gold is scheduled to be poured in late 2000. Production is
expected to total around 13 t (0.42 Moz) per year, with a mill throughput
of 250 000 t per month, a recovery rate of 90% and cash cost of USD 137
per oz. The total capital expenditure will be USD 85 million.
is present as a quartz-sulphide stockwork and disseminated sulphides hosted
in an intrusive contact zone. Up to 60% of the gold is free and can be
recovered by gravity. The total resource is estimated to be just over 180
t (5.6 Moz) of contained gold, comprising 1 mt @ 6.7 g/t Au as free digging
oxide ore, followed by sulphide ore averaging 4.35 g/t Au
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For more information contact: T M (Mike) Porter, of Porter GeoConsultancy (email@example.com)
This was another of the International Study Tours designed, developed, organised and escorted by T M (Mike) Porter of Porter GeoConsultancy Pty Ltd (PGC) in joint venture with the Australian Mineral Foundation (AMF). While the reputation and support of the AMF contributed to the establishment of the tours, after it ceased trading at the end of 2001, PGC has continued to develop, organise and manage the tour series.
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