Malaputese Group at Gwayi River Mine (dumps around the Adder adit).
Submitted by Andrew du Toit on Wed, 15/10/2014 - 13:30 | Last updated: Wed, 29/10/2014 - 10:57
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Gwayi River Mine (dumps around the Adder adit).
The Gwayi River Mine (formerly Gwaai River Mine) was operated by Messina (Transvaal) Development Corporation from 1970 until 1975, when the mine was closed for economic reasons. The mines have been dormant ever since, but since 2010, a Chinese company, Sino-Mining, has been doing exploration in the area.
At Gwayi River mine, the dumps around the Adder adit contain amphibolites, and calc-silicate-bearing marbles, which are the country rocks surrounding the mineralized host rocks (metapsammites with stratabound chalcopyrite and pyrite mineralization, oxidized to malachite and azurite). The mineralized horizons are interpreted as interflow sedimentary rocks intercalated with mafic metavolcanic rocks (now represented by amphibolites).
Although copper staining is widespread in the Malaputese Group, significant copper mineralization is restricted to rocks of the Eastern Domain, mainly in association with the mafic amphibolites (Lockett, 1979a). There was one operating copper mine in the area, the Gwayi River Mine (also spelt Gwai or Gwaai), and many small claims and workings. At the Gwayi River Mine, there are three ore shoots in the form of flattened elongate cigar-shaped pods which are restricted to the noses of fold closures, and which plunge parallel to co-linear F1 and F3 deformation structures (Lockett, 1979a,b; Bahnemann and Lockett, 1979). There are two types of copper occurrences: in shears and fractures within tremolite and cummingtonite-anthophyllite amphibolites which are enclosed in hornblende amphibolites; and in siliceous and calc-silicate-rich interflow metasedimentary intercalations in the mafic sequence. The sulfides consist of chalcopyrite, pyrite and pyrrhotite. At the Adder shoot at Gwayi River Mine, coarse sulfides occur together with vein quartz filling breccias in calc-silicate rocks in the nose of a fold. At the Puff Adder shaft, finely disseminated sulfide mineralization is localized within bands of vitreous grey quartzite grading outwards into cross-cutting veins and blows of quartz (Lockett, 1979a). The orebody varies from 3 to 15m in width, and has a strike length of 400m, and a down-dip extent of at least 470m (Lockett, 1979a).
The ores appear to have been metamorphosed at high grades, and then retrograded. Ore minerals are intergrown with metamorphic minerals like almandine garnet, cordierite, tremolite, cummingtonite, anthophyllite and epidote. The ores predate regional granitic events, as they are cut by undeformed granitic intrusions. Bahnemann and Lockett (1979) attributed the origin of the Gwayi River Mine mineralization to “syngenetic volcanic exhalations next to fumarolic vents” or to a “final degassing of the lava pile”. These volcanogenic deposits were then metamorphosed, stretched and deformed into the shape seen today.
An alternative explanation of the origin of these deposits is that they were generated by metamorphic fluids which leached copper from the volcanic pile during the first deformation and high-grade metamorphic event (Master, 1991). The alteration of mafic volcanics is a good potential source of copper, and may have provided the metal in many copper districts. The metamorphic fluids were channeled along permeable pathways, such as interflow sediments intercalated with the volcanics, as well as along faults, shears, and brecciated fracture zones in the noses of folds. The copper-bearing fluids would have replaced early-formed pyrite to produce chalcopyrite in these permeable zones, giving rise to the elongate shapes of the orebodies. Pyrrhotite may have formed during the metamorphism by desulfidation of pyrite. The same metamorphic fluids would have given rise to the very widespread, but uneconomic, veins and hydrothermal fracture fillings that cut all the Precambrian rocks of the area (Lockett, 1979a). The copper deposits in the Malaputese Group should thus be regarded as metamorphogenic or “metamorphic” rather than “metamorphosed”. The mineralization at Gwayi River Mine shows similarities with mineralization in the Matchless Belt of the Damara Orogenic Belt in Namibia, especially of the Matchless and Otjihase Mines, and Gorob and Hope prospects. It also shows similarities with some zones of stratiform chalcopyrite replacements in interflow pyritic sedimentary intercalations within the Silverside mafic volcanics, in the basal Deweras Group of the Magondi Supergroup in NW Zimbabwe (Master, 1991).
Geological Society of Zimbabwe Annual Summer Symposium, Victoria Falls, 28-30 November 2013, Field Excursion Guidebook "An Introduction to the geology and geochronology of the Dete-Kamativi Inlier" by Dr Sharad Master of the Economic Geology Research Institute, School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa, firstname.lastname@example.org