Publikationsserver der RWTH Aachen University
Tektono-metamorphe Kontrolle der archaischen Goldmineralisation in der New Consort Gold Mine, Barberton Grünsteingürtel, Südafrika
Abstract
dc:descriptionThe New Consort Gold Mine in the Palaeo- to Mesoarchaean Barberton greenstone belt, South Africa, is one of the oldest recognized orogenic gold deposits on Earth. The gold mineralization is hosted by discrete mylonitic units, the so-called Consort Bar, which occurs at the contact between the c. 3280-3260 Ma Onverwacht Group and the overlying c. 3260-3225 Ma Fig Tree Group. The host rocks comprise metaultramafics and amphibolites of the Onverwacht Group as well as metapelites of the Fig Tree Group. The geochemical characteristics of the host rocks are consistent with a magmatic arc setting for their formation. The precursors of the metamorphic host rocks were komatiites, basaltic to andesitic volcanics and pelites. The metamorphism of the structurally highest Fig Tree Group reached conditions of the upper greenschist facies (510-530°C, 4 kbar). In contrast, the metamorphic conditions in the basal Onverwacht Group reached the upper amphibolite facies (600-700°C, 6-8 kbar). Detailed structural and petrological investigations indicate that the Consort Bar represents a major structural break, which is largely responsible for the telescoping of metamorphic isograds within the structural sequence. This shear zone was responsible for the loss of about 6-9 km crust. Two stages of mineralization can be distinguished. Loellingite, pyrrhotite, and a calc-silicate alteration assemblage characterize an early high-T mineralization event, which is restricted to upper amphibolite facies rocks of the Onverwacht Group. This early mineralization may correlate with the local D1 deformation. The second and main stage of gold mineralization was associated with renewed ductile shearing during D2. The D2 deformation resulted in the formation of a NNW trending, steeply dipping, strike-slip shear zone system, the Shires Shear Zone, which separates two regional, SE plunging D1 synclines. The mineralized shear zones are intruded by abundant syn-kinematic pegmatite dykes. The ore bodies are controlled by the interaction of the D2 strike-slip shear zones with the Consort Bar. Petrological and geothermobarometric data on ore and alteration assemblages indicate that the main stage of gold mineralization, which affected a crustal profile of c. 1.5 km, was characterized by increasing temperatures (c. 510°C to 590°C) with increasing structural depth. Sulfide assemblages in the ore bodies change progressively with metamorphic grade, ranging from arsenopyrite + pyrite + pyrrhotite in the structurally highest, to arsenopyrite + pyrrhotite + chalcopyrite + loellingite in the structurally deepest part of the mine. The main stage of gold mineralization was broadly synpeak metamorphic with respect to the Fig Tree Group, but postdates the peak of metamorphism in upper amphibolite facies rocks of the structurally underlying Onverwacht Group. As the footwall rocks were already on their retrograde path, metamorphic devolatilisation reactions within the greenstone sequence can be ruled out as the source of the mineralizing fluids. Therefore an external source for the ore fluids is more likely. The age of the gold mineralization of the New Consort Gold Mine is c. 3030 Ma and therefore considerably younger than the age of the Fairview Mine. The major and trace element characteristics are consistent with one large hydrothermal system that was operating contemporaneously at different crustal levels. In general the mineralization throughout the mine is characterized by an addition of silica, alkalis, gold, arsenic, sulphur, copper and antimony to the altered host rocks. However, there are differences depending on the fluid-rock ratio.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2008
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Otto, Alexander
- Contributors dc:contributor
-
- Meyer, Franz Michael
Subjects
dc:subject × 11Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- ger