[H-RE13] 資源地質学

[HRE13-P09] Characterisation of Archean greenstone belt hosted gold mineralisation at the Mupane gold mine, Botswana

*Onameditse Lulu Seaba¹、Akira Imai¹、Kotaro Yonezu¹、Thomas Tindell¹、Kemmonye Baliki² (1.Kyushu University、2.Mupane Gold Mine)

Archean Banded Iron Formation (BIF) hosted gold deposits are widely distributed in greenstone belts in Archean cratons. Greenstone belts have contributed a significant amount of gold worldwide, mainly from Canada, Australia, Southern Africa, Brazil, and India. The origin of these deposits has been debated since the 1970s because of post-depositional deformation, metamorphism, and lack of modern referents for comparative studies. The present research investigates the origin of the Mupane gold deposits, situated in the Archean Tati greenstone belt succession of Botswana based on ore petrography, geochemistry of the host rocks and mineral chemistry of sulphides. The Mupane gold deposits consist of three main strata-bound orebodies, namely the Tau (Tawana), Kwena, and Tholo deposits. Mineralogical characteristics of the host rocks indicate that the Tau deposit is hosted in an oxide-silicate-carbonate facies BIF based on the dominant iron minerals. The results show a close textural relationship between native gold mineralisation and quartz, grunerite, sulphides, carbonates, biotite, and chlorite. Arsenopyrite-rich zones are associated with biotite-chlorite veins indicating that arsenopyrite precipitation might have been concomitant with potassic alteration. Pyrrhotite replaced magnetite in some samples suggesting that sulphidation might have been the dominant gold precipitation mechanism because it destabilises thiocomplexes in the ore fluids. The average gold fineness of native gold in fractures, as inclusions in sulphides and as free gold associated with silicates from the Tau gold deposit as determined by scanning electron microscope (SEM) is 914, which is typical of Archean BIF-hosted gold deposits. Based on textural relationships and chemical composition, arsenopyrite is interpreted to reflect two generations being arsenopyrite 1 (Apy1), which is possibly early in origin, sieve textured with abundant inclusions, overgrown by late arsenopyrite 2 (Apy2), which is late and commonly rims Apy1 with no porous texture and rare inclusions. Sulphidation reactions proceeded via coupled dissolution reprecipitation reactions as evidenced from porous inclusion-rich sulphides. Gold mineralisation was introduced because of focused fluid flow and sulphidation of the oxide facies BIF, leading to an epigenetic gold mineralisation. The mineralogical assemblages, textures, mineral chemistry and geothermometric data at the Tau gold deposit revealed two-stage gold mineralisation commencing with the deposition of invisible gold in arsenopyrite followed by the later formation of native gold during hydrothermal alteration and post-depositional recrystallisation of arsenopyrite. LA-ICP-MS data of arsenopyrite from the Tau deposit revealed that the hydrothermal event responsible for the formation of late gold also affected the distribution of other trace elements within the grains. The gold-bearing assemblages from the Tau deposit have a restricted range of δ³⁴S compositions of 1.6‰ to + 3.9‰, which is typical of Archean orogenic gold deposits and indicates that overall reduced hydrothermal conditions prevailed during the gold mineralisation process at the Tau deposit. The results from this study suggest that gold mineralisation involved multi-processes such as sulphidation, metamorphism, deformation, hydrothermal alteration, and gold remobilisation.