Petrography, ore mineral paragenesis, and gold deportment of the Twin Hills Gold Deposit, central Namibia
Date
2024
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Abstract
The Twin Hills Gold Deposit is an orogenic gold deposit hosted within the metamorphic turbidites of the Kuiseb Formation in the Southern Central Zone of the Damara Belt. This study aimed to understand the petrography of the host rocks, the formation of the ore minerals and asses the deportment of the Twin Hills Gold Deposit. Methods used in this study included drill core logging, transmitted and reflected light microscopy and scanning electron microscopy (SEM) based TESCAN Integrated Mineral Analyzer (TIMA). The host rocks are characterized by graded bedding with gradations ranging from pelitic mica-rich interbeds to psammitic quartz-rich interbeds, together forming interbedded metagreywacke. The interbedded metagreywacke is the main host unit of the gold mineralization and is made up of quartz, biotite, muscovite, plagioclase, orthoclase and cordierite. The ore mineral assemblage at the Twin Hills Gold Deposit is characterized by the prescence of gold and three major sulphide minerals namely, pyrrhotite, arsenopyrite, and pyrite. Gold mineralization at the Twin Hills Gold Deposit is associated with two main hydrothermal alterations, namely, potassic alteration and silicification. This research suggests at least three stages of mineralization within the Twin Hills Gold Deposit. The first stage is the diagenetic stage, which involved the enrichment of diagenetic arsenian pyrite (Py0) in the turbiditic sediments with gold and other trace elements via the exhalation of reduced, deep-seated Au-As enriched H2S-rich basinal brines onto the sea floor. The second stage involved the release of gold initially locked up within the arsenian pyrite (Py0) crystal lattice into the metamorphic fluids and the subsequent precipitation of free gold (Au1) from the gold bearing fluids via phase separation due to a drop in fluid pressure. This resulted in the formation and concentration of disseminated gold to economic grades within the metagreywacke. The third stage is the quartz veining stage, which was characterized by increased permeability and advanced fluid flow due to progressive metamorphism and deformation. This stage was responsible for the remobilization of fine gold grains (Au1) that were disseminated within the groundmass of the metagreywacke and concentrating them into the biotite selvages. This stage is also presumed to be responsible for the exsolution of gold from the arsenopyrite crystal lattice resulting in very fine gold (Au2) grains included within arsenopyrite crystals (Asp2). The third stage led to the further upgrading of the gold grades to higher economic levels. The gold deportment analyses revealed three modes of occurrences of gold within the Twin Hills deposit. This includes gold disseminated within the groundmass, gold grains disseminated within biotite selvages associated with quartz veinlets and gold grains included within arsenopyrite crystals. This study recommends a two stage recovery procedure to ensure optimum gold recovery, this includes gravity separation methods followed by cyanide leach process. The outcomes of this research further reiterate the importance of the Kuiseb Formation meta-turbidites as a significant fertile source for gold and mineralizing fluids in sediment hosted gold deposits in the Damara Belt. A sediment hosted gold deposit model should be adopted in conjunction with the mineral systems approach in identifying targeting vectors in exploration programs. This exploration approach should also be applied to other meta-sediment hosting stratigraphic sequences within the Damara Belt such as the Arandis and the Okonguarri Formations. These stratigraphic horizons have proven gold fertility as they host the only two currently producing gold mines in Namibia, the Navachab gold mine and the Otjikoto gold mine
Description
A research report submitted in partial fulfilment of the requirements for the degree Master of Science to the Faculty of Science, School of Geosciences, University of the Witwatersrand, Johannesburg, 2023
Keywords
Gold deposit, Petrography, Host rocks