Alteration paragenesis and the timing of mineralised quartz veins at the world-class Geita Hill gold deposit, Geita Greenstone Belt, Tanzania

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.

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The world-class Geita Hill deposit is one of the largest gold deposits located within the Geita Greenstone Belt in NW Tanzania. The deposit is hosted within a 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 NW dipping deformation zone consisting of a network of discontinuous shear fractures that record early thrusting overprinted by later strike-slip and normal events. The regional metamorphic assemblage in the meta-sediments is characterised by biotite +chlorite + actinolite +K-feldspar + magnetite ± pyrrhotite ± pyrite indicating upper greenschist facies conditions. The gold-related alteration overprints the regional metamorphic assemblage, and is characterised by silicification and sulfidation fronts that generally extend out from the mineralised zone by no more than one meter. The alteration assemblage includes sub-vertical, mineralised quartz veins that trend approximately E-W. The mineralised quartz veins are accompanied by alteration halos of quartz +biotite+ K-feldspar +pyrite which overprints the peak metamorphic 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 also as gold inclusions in biotite and along quartz grain boundaries. Two distinct textural styles of auriferous pyrite can be distinguished: inclusion rich subhedral pyrite, hosting invisible gold, and inclusion free euhedral pyrite, hosting visible gold grains. It is common for the inclusions rich pyrite to have thick rims of inclusion free pyrite. The mineralising alteration is overprinted by barren, multiphase quartz-carbonate, and carbonate-chlorite veins. This alteration is characterised by the assemblage calcite +siderite +chlorite ± quartz ± pyrite ± barite. Palaeostress analysis of mineralised shear fractures along the Geita Hill Shear Zone are indicative of sigma 1 being vertical and sigma 3 trending N-S, indicating N-S extension, which is consistent with the orientation of the mineralised quartz veins.

Item ID: 50981
Item Type: Article (Research - C1)
ISSN: 1872-7360
Keywords: Archean gold, alteration paragenesis, greenstone belt, quartz veins, Tanzania
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A version of this publication was included as Chapter 2 of the following PhD thesis: Van Ryt, Matthew (2019) The mineralisation, alteration paragenesis and hydrothermal fluids at the Geita Hill gold deposits, NW Tanzania. PhD thesis, James Cook University, which is available Open Access in ResearchOnline@JCU. Please see the Related URLs for access.

Date Deposited: 26 Oct 2017 01:16
FoR Codes: 37 EARTH SCIENCES > 3703 Geochemistry > 370301 Exploration geochemistry @ 10%
37 EARTH SCIENCES > 3705 Geology > 370508 Resource geoscience @ 60%
37 EARTH SCIENCES > 3705 Geology > 370511 Structural geology and tectonics @ 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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