AngloGold Ashanti Limited <R&R> 2021
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GEITA CONTINUED
Africa
Geology
GGM is hosted in the Geita Greenstone Belt (GGB), which
is a northern segment of the Sukumaland Greenstone Belt,
located in the northwestern part of the Tanzania Craton and
south of Lake Victoria. This Archaean sequence strikes almost
east west, extending for about 80km and is up to 20km wide.
The GGB sits dominantly within the Nyanzian Supergroup
stratigraphy that is sub-divided into the Lower Nyanzian and the
Upper Nyanzian groups.
The Lower Nyanzian Group is composed of mafic volcanic units
(basalts, pillow basalt, minor gabbro and dolerites). This group of
rocks within the GGB is collectively termed the Kiziba Formation.
The Upper Nyanzian Group consists of black shales, banded iron
formation, clastic sedimentary rock, tuffs, agglomerates and
felsic volcaniclastics. The entire package (Nyanzian stratigraphy)
is intruded by a variety of mafic to felsic rocks. The supra-crustal
package shows variable thickness and is estimated to be more than
500m thick in places, mostly underlain by intrusive complexes.
Deposit type
A simplified stratigraphy of the main igneous rocks in the Geita
area is summarised as: Archaean Gabbro, Basalts, intermediate to
acid volcanoclastic sediments, Diorites, Tonalites-Granodiorites,
Granites and Proterozoic Gabbro dykes.
Across the Archaean-Proterozoic rocks there is a property-wide
paleo-drainage system, which likely flowed towards Lake Victoria.
These late sediments likely represent the remnants of a much
thicker package that might have covered all the hills exposed
today. Both the Archaean-Proterozoic rocks and paleo-alluvials are
covered by ferricrete at different levels of induration and evolution,
up to 15m thick.
The region hosts several world-class shear-hosted Archaean
lode gold deposits and forms the northern portion of the regional
Sukumaland Greenstone Belt, itself one of several belts that
comprise the Lake Victoria goldfields. Other gold mines hosted in
the Lake Victoria Goldfields include Golden Pride (Resolute Mining
Limited), Bulyanhulu, Tulawaka, Buzwagi and North Mara (all
Barrick-owned).
The Geita gold deposits are shear hosted, Archaean orogenic
gold deposits. Within GGML leases the GGB is subdivided into
three major mineralised trends:
•
Geita Central Trend, hosting the Nyankanga, Geita Hill and Lone
Cone deposits
•
Nyamulilima Trend in the west, hosting Star and Comet, Ridge 8
and Nyamulilima deposits
•
Matandani-Kukuluma Trend to the northeast, hosting Matandani
and Kukuluma deposits
The Geita Central Trend contains three major gold deposits
occurring along a NE-SW mineralised trend. These are from
northeast to southwest: Geita Hill, Lone Cone and Nyankanga.
Other prospects occur singly: Chipaka in the centre of the
greenstone belt, and Kalondwa Hill, P30, Fukiri-Jumanne along
an NW-SE trending ironstone ridge. Geita Hill, Lone Cone and
Nyankanga occur along a moderately NW dipping system
of reverse faults that have been multiply reactivated during
subsequent deformation events. The mineralisation is mainly
related to diorite-and BIF contacts exploited by the shear system.
The alteration is restricted within the ore zone and consists of
secondary sulphide (mainly pyrite), silica, carbonate and moderate
potassic alteration.
The Nyamulilima Trend contains three major gold deposits on
an approximately NW-SE mineralised trend. These are from SE
to NW: Ridge 8, Star and Comet and Nyamulilima (historically
named Roberts). Individual deposits occur along a series of N-S
trending, steeply dipping, left stepping en-echelon fault zones that
cut across the ironstone-rich sediments and granite-granodiorite-
tonalite intrusions. Mineralisation is preferentially localised along
fault zones where they cut the ironstone-granitoid contacts. The
mineralisation is associated with secondary pyrite and minor
pyrrhotite, silica, carbonate and actinolite alteration.
The Kukuluma Trend contains five gold deposits distributed along
an approximately E-W mineralised trend. These are from east
to west: Area 3 South, Area 3 Central, Area 3 West, Kukuluma
and Matandani. The mineralisation is steeply dipping along the
contacts of intermediate fine-grained intrusions and magnetite
rich chert and ironstone showing a general en-echelon, left
stepping geometry. The gold is associated with secondary pyrite,
arsenopyrite and minor pyrrhotite. Magnetite, silica, carbonate
and amphibole alteration are variably present within
the mineralised zone.
Mineralisation style
Deformation in the GGB comprises of early stages of ductile
shearing and folding (D1 to D5), with periodic emplacement of
large diorite intrusive complexes, sills, and dykes. Later stages
of deformation (D6 to D8) involved development of brittle-
ductile shear zones, with faults developed in the later stages of
deformation, with late emplacement felsic porphyry dykes within
the greenstone belt, and granitic intrusions located on the margins
of the greenstone belt.
Gold mineralisation occurred late in the tectonic history of the
greenstone belt, synchronous with the development of brittle-
ductile shear zones (D6). Mineralisation is dominantly sulphide
replacement of magnetite-rich layers in ironstone, with local
replacement of ferromagnesian phases and magnetite in the diorite
intrusions. Primary gold mineralisation is associated with the
intersection of the brittle-ductile shear zones and pre-existing fold
hinges, with higher grade concentrations associated with banded
iron formation lithologies and with diorite dyke and sill contacts.
Mineralisation characteristics
The mineralisation in the GGB is preferentially hosted within
deformation zones developed along the contact of banded iron
formation and porphyries of various compositions and associated
with major shear systems. The structures associated with
the mineralised system are well defined, the alteration zone is
restricted to the mineralised zone, quartz veins are rare or missing
although silicification is common.