The mineralisation, alteration paragenesis and hydrothermal fluids at the Geita Hill gold deposits, NW Tanzania

Van Ryt, Matthew (2019) The mineralisation, alteration paragenesis and hydrothermal fluids at the Geita Hill gold deposits, NW Tanzania. PhD thesis, James Cook University.

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Archean greenstones represent a large percentage of worlds total gold endowment and are actively mined on every continent barring Antarctica. Greenstone-hosted gold deposits often have complex deformation and alteration histories, and a general deposit model remains controversial. The aim of this thesis is to improve our understanding of gold mineralisation in Archean greenstone belts, based on a comprehensive case study of the world-class Geita Hill deposit in Tanzania. Geita Hill is one of the largest gold deposits within the Geita Greenstone Belt in north-western Tanzania and has been mined as an open pit since 2002. The deposit is hosted within a greenschist facies metamorphosed and complexly deformed sedimentary package dominated by ironstone and intruded by diorite dykes. The gold mineralisation is spatially associated with the Geita Hill Shear Zone which, is a NE-trending, moderately west dipping deformation zone of discontinuous shear fractures. Detailed structural studies have defined a deformation history for the deposit, providing an opportunity for an in-depth study of the hydrothermal alteration and fluids associated with gold mineralisation.

The first component of this thesis builds a paragenetic framework for the Geita Hill deposit. The regional metamorphism is characterised by biotite + chlorite + actinolite + K-feldspar + magnetite ± pyrrhotite ± pyrite indicating upper greenschist facies metamorphism. The gold-related alteration overprints the regional metamorphism and is characterised by silicification and sulfidation fronts that end within one meter of the mineralised zone. Locally, the silicification and sulfidation of the wall rock occurs along a series of mineralised quartz veins which have a sub-vertical dip and tend E-W. Paleostress analysis of the mineralised shear fractures of the Geita Hill Shear Zone suggests vertical maximum compressive stress (σ₁) and northerly extensional stress (σ₃) consistent with the orientation of the mineralised quartz veins and indicating N-S extension. The composition of the mineralised quartz veins is characterised by quartz + biotite + K-feldspar + pyrite, which also overprints the metamorphic mineral assemblage. Gold is closely associated with secondary pyrite and occurs as free gold and gold tellurides (sylvanite, calaverite and nagyagite). It occurs mainly as inclusions in pyrite and as invisible gold in pyrite but gold inclusions in biotite and along quartz grain boundaries are also present. The gold-bearing pyrite is associated with secondary biotite and K-feldspar. Two distinct textural styles of auriferous pyrite can be distinguished: inclusion rich subhedral pyrite and inclusion free euhedral pyrite. It is common for the inclusion rich pyrite to have thick rims of inclusion free pyrite. The mineralising alteration is overprinted by barren, multi-phase quartz-carbonate, and carbonate-chlorite veins. This alteration is characterised by the assemblage calcite + siderite + chlorite ± quartz ± pyrite ± barite.

The thesis then builds on the paragenetic framework through silicate and sulfide geochemistry. Biotite was identified as a primary mineral both in the metamorphic assemblage and gold-related hydrothermal alteration assemblage. The study of silicates was conducted through detailed core logging, petrography, SEM mineral identification of alteration assemblages, SWIR measurements and microprobe analyses of biotite in order to identify the nature of the mineralising fluid and its spatial effect across the Geita Hill deposit. Results show that the mineralised assemblage is slightly more oxidized (pyrite + magnetite) compared to the metamorphic background (pyrrhotite + magnetite). The intense sulfidation within the ore zone resulted in the formation of Mg-rich biotite, which grades into more Fe-rich biotite away from the ore zone. This change in biotite composition can be detected using short wavelength infrared spectra, though a shift in the Fe-OH 2250 nm absorption feature to lower wavelengths. This shift is also correlated with an increase in gold grade within the mineralised zone. Halogen chemistry of the biotite implied the presence of multiple hydrothermal fluids during mineralisation, suggesting that within the ore zone a metamorphic fluid in equilibrium with the host rock was overprinted by and mixed with an infiltrating fluid that was enriched in fluorine.

Study of the sulfides in the deposit was conducted through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of pyrite and magnetite grains. The gold distribution correlates well with Te, Ag, Bi and Pb consistent with its occurrence as micro- and nano-inclusions of gold bearing telluride minerals. As, Co and Ni distribution in pyrite grains hosted in quartz veins is patchy, whereas in pyrite grains from ironstone and diorite these elements show zoning characteristic of growth pattern. Elements including As, Ni, Co, Cu and Zn appear to be dominantly locally derived, and remobilised into the pyrite during sulfidation. The concentrations of these elements are highly lithologically controlled, and they are not consistently incorporated into the pyrite after initial stages of growth. Au, Ag, Te, Sn, Bi and Pb appear to be dominantly externally derived, and closely correlate in all varieties of pyrite. The Se content is typical of pyrite from Archean gold deposits (~30ppm) and reflects to an average temperature of ~340°C for the mineralising fluid on the basis of temperature dependent incorporation into pyrite.

Lastly, the gold-bearing hydrothermal fluids were studied directly through detailed microthermometry and raman microspectroscopy, and three principle fluids were identified: (1) A low salinity, carbonic-rich (XCO₂ > 0.8) fluid with minor N₂ (XN₂/(XCO₂ + XN₂) <0.15) that was interpreted to be metamorphic in origin; (2) A high salinity (NaCl+CaCl₂ > 20 wt.%; NaCl/(NaCl+CaCl₂) mass ratio > 0.45), aqueous brine that was interpreted to be magmatic in origin; and (3) A low salinity (NaCl < 5 wt.%) aqueous fluid that was interpreted to be meteoric in origin. Preserved fluid assemblages imply mineralisation occurred at pressures of less than 2 kbar, likely from 1.4 to 1.7 kbar, at temperatures of approximately 350 °C. C-O-H fluid modelling of the carbonicrich fluid has constrained ƒO₂ fluid to 1.5-1.8 log10 units above ƒO₂ FMQ corresponding to absolute values of 10- 30.5 bar. The gold was likely transported in the high salinity brine as Au-bisulfide complexes with tellurium, potentially introduced as a vapour. Deposition of Au was triggered via interaction of gold-bearing fluids with the relatively reduced Fe-rich host rocks and the low salinity CO₂-rich fluid.

Item ID: 63541
Item Type: Thesis (PhD)
Keywords: alteration paragenesis, Anglogold Ashanti, Archean gold, biotite, Geita Gold Mine, greenstone belt, quartz veins, short wavelength infrared spectroscopy, Tanzania, wavelength dispersive spectroscopy
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Copyright Information: Copyright © 2019 Matthew Van Ryt.
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Three publications arising from this thesis are stored in ResearchOnline@JCU, at the time of processing. Please see the Related URLs field. The publications are:

Chapter 2: van Ryt, Matthew R., Sanislav, Ioan V., Dirks, Paul H.G.M., Huizenga, Jan M., Mturi, Marwa I., and Kolling, Sergio L. (2017) Alteration paragenesis and the timing of mineralised quartz veins at the world-class Geita Hill gold deposit, Geita Greenstone Belt, Tanzania. Ore Geology Reviews, 91. pp. 765-779.

Chapter 3: Van Ryt, Matthew, Sanislav, Ioan, Dirks, Paul, Huizenga, Jan, Mturi, Marwa, and Kolling, Sergio (2019) Biotite chemistry and the role of halogens in Archaean greenstone hosted gold deposits: a case study from Geita gold mine, Tanzania. Ore Geology Reviews, 111. 102982.

Chapter 4: Van Ryt, M.R., Sanislav, I.V., Dirks, P.H.G.M., and Huizenga, J. (2020) Trace element associations in magnetite and hydrothermal pyrite from the Geita Hill gold deposit, Tanzania. Journal of Geochemical Exploration, 209. 106418.

Date Deposited: 23 Jun 2020 00:25
FoR Codes: 04 EARTH SCIENCES > 0402 Geochemistry > 040201 Exploration Geochemistry @ 35%
04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 35%
04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 30%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840105 Precious (Noble) Metal Ore Exploration @ 50%
97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 50%
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