Introduction
The greenstone belt consists of two sections, referred to here as East Kahama and West Kahama. The topography is that of a plain with low rolling
hills. The western and northeastern sections are made up mainly of acidic
volcanic and chemical sedimentary rocks, cherts and BIF forming long and
pronounced ridges.
Geology.
The kahama greenstone belt (Fig.25) is an extension of rocks in the Mabale (and Geita) and Rwamagaza belts, as shown by the airborne magnetic
features (Geosurvey International GmbH, 1981). The belt curves around a
large granite intrusion in Lunguya, northwest of Kahama. Generally, the
geology of this area is composed predominantly of mafic and felsic volcanic
rocks, BIF and clastic sedimentary rocks, all intruded by granites, weakly
metamorphosed and regionally deformed. BIF is nearly absent in the west
Kahama sector. On the regional scale, what was once likely to be a
continuous area of greenstone rocks was disrupted by the intrusion of a late
granite (Bukoli Pluton) to form the three areas present today with
Bulyanhulu area to the east, Rwamagaza to the northwest and ikina to the
west (Fig.26).
Mineralisation.
Significant gold mineralisation has been discovered at Bulyanhulu (now an
underground mine), Golden Ridge and Jubilee Reef. Other reported gold
prospects include Mega, Nungwiza, Isonda, Ikina, Ngulu, Nyakafuru, kanegele
and Miyabi ( a small detached greenstone / BIF hill southwest of Kanegele).
Gold mineralization occurs in all stratigraphic and lithological units with
the exception of the granitoids. The Bulyanhulu type gold mineralisation is
hosted by shear zones, which occur in mafic metavolcanic rocks. The Isonda
type is associated with a bedding parallel shear and / or breccia zone
between oxide facies BIF and overlying lapilli tuff. Higher gold values
occur in locally graphitic shear zones and breccias, which contain fresh
pyrite and boxworks along microfractures. The Nungwiza type is a typical
gold quartz vein with zones of intensive shearing and silicification. At
Mega, gold occurs in BIF, which displays abundant boxworks after pyrite.
Bulyanhulu Mine.
The Bulyanhulu Mine is located 45 Km south of Lake Victoria or 65 km
north of Kahama in northern Tanzania. The stratigraphy of Bulyanhulu is
characterized by a Lower Nyanzian volcano � sedimentary sequence largely
comprising bimodal mafic and felsic extrusive and intrusive rocks. All
litholotgical units have undergone greenschist facies metamorphism.
The Bulyanhulu greenstone succession comprises a sequence of mafic
basaltic volcanics overlain by an argillitic sequence (Mustones and shales)
of variable thickness along strike. Argillites are in turn overlain by
felsic pyroclastic rocks (Fig.27).
Lithological units generally strike at 315 degree ,320 degree and dip at
80 degree to 85 degree in a northeasterly direction.
The greenstones are hosts to gold, silver and copper mineralisations
that occur in shear zones comprised of a complex of graphite and quartz sulphide veins. Reef horizons are subparallel to the lithological units and
are typically steeply dipping (80 degree to 85 degree) , (Fig.28).
Underground mine development started in May 1997, and construction of the
$ 280 million underground mine began in the third quarter of 1999.
Bulyanhulu Mine started production in early 2000 with the process plant
commissioned in 2001, and during the year gold production was 241,000 oz.
The average annual production ranges from 350,000 to 365,000 oz of gold.
Proven reserves are 20 Moz of gold at an average grade 15 g/t.
The ore zones are characterized by the presence of sub continuous
lenses and veins of distinctive black quartz concentrated in narrow shear
zones (0.5 m - 6m), developed within both argillaceous and volcanic units.
The main zone of mineralisation, Reef 1, is developed at this boundary
between the felsic tuffs and basalts. Reef horizons are sub parallel to
the lithological units are and are typically steeply dipping (80 degree - 85
degree). Reef 0 is a narrow quartz veins shear zone, which occurs in the
western portion of the mine and can contain minor amounts of arsenopyrite
and galena.
The main auriferous structure (Reef 1 and Reef 0) is
graphitic quartz - sulphide sheared sedimentary rock at the contact between hanging wall felsic
volcanic rock and footwall intermediate volcanic rock. The sulphide
mineralisation within the main reef consists of abundant pyrite and lesser
chalcopyrite and pyrrhotite. Numerous sub parallel mineralized zones exist
in close proximity to the main reef. Reef 2 is located 500 m north of the
main Reef 1 as a series of narrow high grade ore shoots. It consists of a
series of quartz sulphide veins within intermediate volcanic rocks. The
main gold bearing body (Reef 1 & 0) is tabular shaped, averages 4.4 m in
true thickness, and dips steeply to the northeast at 80 & (Fig.28). The reef
structure is very regular, undeformed, and continuous in both dip and strike
directions. The structure pinches and swells, but generally shows increased
thickness and lower grade to the east, and less thickness and higher grade
to the west.
Itetemia.
The itetemia property is contiguous to the east with the Bulyanhulu
Mine, which is operated by Barrick Gold. It is being explored by Tan
Range and Barrick Gold as a joint venture. A diamond drill programme has
been conducted to test the down dip extension of this Golden Horseshoe Reef
deposit. Gold distributions within the high grade core from 35m
to 200m level in one of the sections are as follows: