Mineralogical and sulfur isotopic characteristics of Archean greenstone belt‐hosted gold mineralization at the Tau deposit of the Mupane gold mine, Botswana

Abstract

The Mupane gold deposit, which is one of the numerous gold occurrences in the Tati Greenstone Belt in the northeastern part of Botswana, consists of four orebodies, namely Tau, Tawana, Kwena, and Tholo deposits. The present research, which focuses on the genesis of the Tau deposit, was based on ore petrography, mineral chemistry of sulfides, and sulfur isotope data. Mineralogical characteristics of the host rocks indicate that banded iron formation at the Tau deposit includes iron oxides (magnetite), carbonates (siderite and ankerite), silicates (chlorite and amphibole), and sulfides (arsenopyrite and pyrrhotite). The deposit features arsenopyrite‐rich zones associated with biotite‐chlorite veins, which are indicative of the precipitation of arsenopyrite concomitant with potassic alteration. The replacement of magnetite by pyrrhotite in some samples suggests that sulfidation was likely the dominant gold precipitation mechanism because it is considered to have destabilized gold‐thiocomplexes in the ore‐forming fluids. Based on textural relationships and chemical composition, arsenopyrite is interpreted to reflect two generations. Arsenopyrite 1 is possibly early in origin, sieve textured with abundant inclusions of pyrrhotite. Arsenopyrite 1 was then overgrown by late arsenopyrite 2 with no porous textures and rare inclusions of pyrrhotite. Gold mineralization was initiated by focused fluid flow and sulfidation of the oxide facies banded iron formation, leading to an epigenetic gold mineralization. The mineralogical assemblages, textures, and mineral chemistry data at the Tau gold deposit revealed two‐stage gold mineralizations commencing with the deposition of invisible gold in arsenopyrite 1 followed by the later formation of native gold during hydrothermal alteration and post‐depositional recrystallization of arsenopyrite 1. Laser ablation inductively coupled plasma mass spectrometric analysis of arsenopyrite from the Tau deposit revealed that the hydrothermal event responsible for the formation of late native gold also affected the distribution of other trace elements within the grains as evidenced by varying trace elements contents in arsenopyrite 1 and arsenopyrite 2. The range of δ34S of gold‐bearing assemblages from the Tau deposit is restricted from +1.6 to +3.9‰, which is typical of Archean orogenic gold deposits and indicates that overall reduced hydrothermal conditions prevailed during the gold mineralization process at the Tau deposit. The results from this study suggest that gold mineralization involved multi‐processes such as sulfidation, metamorphism, deformation, hydrothermal alteration, and gold remobilization.

Nature and conditions of gold mineralization at the Tau deposit, Mupane mine, Tati Greenstone Belt, Botswana: Evidence from fluid inclusion and arsenopyrite geothermometer

“…The pre‐ore stage consists of primary minerals such as quartz, magnetite, and carbonates, which were precipitated during the late diagenesis of BIF. Minerals from the pre‐ore stage were recrystallized during metamorphism marked by the triple granoblastic texture of quartz and euhedral magnetite grains during the main stage (Figure 3a, f; Seaba et al, 2023). The main stage comprises the gold mineralization event was accompanied by metamorphism, deformation, and remobilization of earlier gold into fractures of the host minerals (Seaba et al, 2023).…”

“…Minerals from the pre‐ore stage were recrystallized during metamorphism marked by the triple granoblastic texture of quartz and euhedral magnetite grains during the main stage (Figure 3a, f; Seaba et al, 2023). The main stage comprises the gold mineralization event was accompanied by metamorphism, deformation, and remobilization of earlier gold into fractures of the host minerals (Seaba et al, 2023). Metamorphism of the host rocks to middle amphibolite facies at the Tau deposit was indicated by the observed mineralogical assemblages and comprising quartz, grunerite, almandine, chlorite, biotite, pyrrhotite which were formed from either replacement of earlier minerals or recrystallization indicated by textures such the triple granoblastic texture of quartz and euhedral magnetite grains (Figure 3; Seaba et al, 2023).…”

“…The main stage comprises the gold mineralization event was accompanied by metamorphism, deformation, and remobilization of earlier gold into fractures of the host minerals (Seaba et al, 2023). Metamorphism of the host rocks to middle amphibolite facies at the Tau deposit was indicated by the observed mineralogical assemblages and comprising quartz, grunerite, almandine, chlorite, biotite, pyrrhotite which were formed from either replacement of earlier minerals or recrystallization indicated by textures such the triple granoblastic texture of quartz and euhedral magnetite grains (Figure 3; Seaba et al, 2023). Sulfidation was marked by the replacement of magnetite by pyrrhotite suggesting its role in the precipitation of gold because it destabilized gold‐bisulfide complexes in the ore‐forming fluid (Figure 3f; Seaba et al, 2023).…”

“…Sulfidation was marked by the replacement of magnetite by pyrrhotite suggesting its role in the precipitation of gold because it destabilized gold‐bisulfide complexes in the ore‐forming fluid (Figure 3f; Seaba et al, 2023). In addition, mineral textures indicated that gold mineralization at the Tau deposit occurred after the deposition of BIF implying its epigenetic nature (Seaba et al, 2023). Arsenopyrite at the Tau deposit reflected two generations (Figure 3h, i).…”

“…The first stage of gold mineralization consisted of the deposition of invisible gold in the crystallographic structure of Apy1 and the second stage consisted of the formation of native gold as inclusions in Apy2 during hydrothermal alteration and post‐depositional recrystallisation of Apy1 to Apy2 (Figure 3h). The ore textures at the Tau deposit revealed that gold mineralization occurred as a result of multiple processes including sulfidation, metamorphism, deformation, hydrothermal alteration, and gold remobilization (Figure 3; Seaba et al, 2023).…”