Gold mineralization in the Amani area, southwestern Tanzania

Thesis (PhD)--Stellenbosch University, 2021.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.Doctora