Gold mineralization in the Amani area, southwestern Tanzania
Date
2021-03
Authors
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Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Amani area is located in the Njombe Region of southwestern Tanzania,
approximately 40 km east of Lake Malawi, and is geologically situated within the
Upangwa terrane of the poly-orogenic Ubendian Belt. This subtropical area is richly
endowed in gold mineralization and local artisanal miners have successfully recovered
gold from unaltered quartz-carbonate veins occurring in the mountainous terrane,
gossan caps that directly overlie these veins and gravel horizons occurring at lower
elevations in the Amani River. Since the discovery of alluvial gold in the Amani River
during the early 1990’s, no detailed research has been done to determine the local
geology, the controls and timing of primary gold mineralization and the genesis of the
alluvial deposits. The unique nature of gold mineralization within the study area, also
allows for further investigation into the geochemical behaviour and transformation of
gold when introduced into low-temperature and subtropical environments. This
dissertation aims to resolve these knowledge gaps by utilizing several analytical
techniques (U-Pb and 40Ar/39Ar geochronology, oxygen isotope analysis, fluid
inclusion microthermometry, Raman spectroscopy, X-ray computed
microtomography, scanning electron microscopy and major and trace element
geochemistry) on geological samples obtained from fieldwork conducted in the Amani
area.
Four main lithological groups were identified in the Amani area (Kabelege Group,
Ulembo Suite, Rafiki Group and Amani Group) and field relationships combined with
U-Pb zircon-rutile and 40Ar/39Ar muscovite geochronology, suggest that the Amani
area experienced at least two temporally distinct deformational events at ~1.8 Ga and
~0.55 – 0.6 Ga. These regional metamorphic events are related to the Ubendian
orogeny and Kuunga orogeny, respectively. The first deformational event (D1) is
characterized by high-grade metamorphism of the Kabelege Group ortho- and
paragneisses during the Paleoproterozoic. D2 occurred during the Neoproterozoic and
is characterized by NW-trending fabrics, large scale folding and thrusting and
greenschist facies metamorphism of the Amani and Rafiki Groups. Hydrothermal gold
mineralization in the Amani area is temporally associated to this ~550 – 600 Ma D2
event and is controlled by kilometre-scale, NW-SE to E-W trending D2 brittle-ductile
reverse shear zones. These structures contain a network of en echelon, steeply
dipping shear- and sub-horizontal extensional quartz-carbonate veins with pyrite + chalcopyrite ± pyrrhotite sulphide assemblages, with free-milling gold occurring
predominantly in quartz and along sulphide grain boundaries. The mineralized veins
are hosted predominantly in the Rafiki Group (impure micaceous marbles and banded
iron formations) and Amani Group (quartz-muscovite schists). Fluid inclusion
microthermometry revealed that these veins record a bimodal occurrence of early lowtemperature
(140 – 320 °C) and low-salinity (≤23.2 wt.% NaCl eq.) fluids that are
overprinted by later, high temperature (210 – 560 °C) and hypersaline fluids (32.8 –
54 wt.% NaCl eq.). These ore-forming fluids contain H2O – NaCl ± CO2 ± CH4
compositions and were trapped at near-lithostatic to supralithostatic conditions of 330
– 400 °C and 1.7 – 6.9 kbar. These fluids were likely variably sourced (i.e.
metamorphic and magmatic origin) and were focussed along kilometre-scale brittleductile
reverse shear zones in the Amani area, where fluid-rock interactions led to gold
deposition.
Detailed examination of both alluvial and gossan-hosted gold mineralization indicates
that low-temperature surficial processing of hypogene vein gold can lead to the
formation of supergene gold with unique geochemical and textural features. Our
results highlight that both settings can: (1) concentrate dispersed and finer-grained
gold particles into larger masses; (2) facilitate gold remobilization via an array of
potential Au-complexing ligands that promote the dissolution and re-precipitation of
gold in the weathering environment; (3) form high-purity supergene gold via Ag-Au
decoupling due to differences in the solubility of their respective complexing ligands;
and (4) develop unique supergene gold morphologies that are indicative of their setting
and that cannot be produced in high-temperature hydrothermal systems. These
findings suggest that oxidative weathering of sulphides and subsequent lowtemperature
gold dissolution re-precipitation reactions in both gossan and fluvial
settings, are crucial in the formation of large alluvial gold nuggets in the Amani area.
Collectively, these insights represent a novel contribution towards our understanding
of hydrothermal and alluvial gold mineralization in the Amani area and the mobility of
gold throughout its geochemical life-cycle in subtropical environments. This
dissertation further highlights the potential for large-scale exploration in this
understudied and overlooked area of southwestern Tanzania.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (PhD)--Stellenbosch University, 2021.
Keywords
Gold mines and mining -- Environmental aspects -- Tanzania -- Amani area, Gold -- Milling, Mineralogy, Determinative, Gold mineralisation, Alluvial mining, Orogeny, Hydrothermal deposits, UCTD