The world-class gold deposits in the Geita greenstone belt, Northwestern Tanzania

Dirks, P.H.G.M., Sanislav, I.V., van Ryt, M.R., Huizenga, J.-M., Blenkinsop, T.G., Kolling, S.L., Kwelwa, S.D., and Mwazembe, G. (2020) The world-class gold deposits in the Geita greenstone belt, Northwestern Tanzania. In: Sillitoe, Richard H., Goldfarb, Richard J., Robert, Francois, and Simmons, Stuart F., (eds.) Geology of the World’s Major Gold Deposits and Provinces. Special Publications of the Society of Economic Geologists, 23 . Society of Economic Geologists, Littleton, CO, USA, pp. 163-185.

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Abstract

The Geita mine is operated by AngloGold Ashanti and currently comprises four gold deposits mined as open pits and underground operations in the Geita greenstone belt, Tanzania. The mine produces ~0.5 Moz of gold a year and has produced ~8.3 Moz since 2000, with current resources estimated at ~6.5 Moz, using a lower cut-off of 0.5 g/t.The geologic history of the Geita greenstone belt involved three tectonic stages: (I) early (2820–2700 Ma) extension (D1) and formation of the greenstone sequence in an oceanic plateau environment; (II) shortening of the greenstone sequence (2700–2660 Ma) involving ductile folding (D2–5) and brittle-ductile shearing (D6), coincident with long-lived igneous activity concentrated in five intrusive centers; and (III) renewed extension (2660–2620 Ma) involving strike-slip and normal faulting (D7–8), basin formation, and potassic magmatism. Major gold deposits in the Geita greenstone belt formed late in the history of the greenstone belt, during D8normal faulting at ~2640 Ma, and the structural framework, mineral paragenesis, and timing of gold precipita-tion is essentially the same in all major deposits. Gold is hosted in iron-rich lithologies along contacts between folded metaironstone beds and tonalite-trondhjemite-granodiorite (TTG) intrusions, particularly where the contacts were sheared and fractured during D6–7 faulting. The faults, together with damage zones created along D3 fold hinges and D2–3 hydrothermal breccia zones near intrusions, formed microfracture networks that were reactivated during D8. The fracture networks served as conduits for gold-bearing fluids; i.e., lithologies and structures that trap gold formed early, but gold was introduced late.Fluids carried gold as Au bisulfide complexes and interacted with Fe-rich wall rocks to precipitate gold. Fluid-rock interaction and mineralization were enhanced as a result of D8 extension, and localized hydro-fracturing formed high-grade breccia ores. Gold is contained in electrum and gold-bearing tellurides that occur in the matrix and as inclusions in pyrrhotite and pyrite. The gold mineralization is spatially linked to long-lived, near-stationary intrusive centers. Critical factors in forming the deposits include the (syn-D2–6) formation of damage zones in lithologies that enhance gold precipitation (Fe-rich lithologies); late tectonic reactivation of the damage zones during extensional (D8) faulting with the introduction of an S-rich, gold-bearing fluid; and efficient fluid-rock interaction in zones that were structurally well prepared.

Item ID: 65951
Item Type: Book Chapter (Research - B1)
ISBN: 978-1-629493-12-1
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Copyright Information: © 2020 Society of Economic Geologists, Inc.
Date Deposited: 18 Feb 2021 02:01
FoR Codes: 37 EARTH SCIENCES > 3705 Geology > 370508 Resource geoscience @ 50%
37 EARTH SCIENCES > 3705 Geology > 370511 Structural geology and tectonics @ 30%
37 EARTH SCIENCES > 3703 Geochemistry > 370301 Exploration geochemistry @ 20%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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