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Metamorphism of the Witwatersrand gold fields: conditions during peak metamorphism |
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| Authors: | GN PHILLIPS |
| Institution: | Department of Geology, University of The Witwatersrand, Johannesburg, South Africa |
| Abstract: | Abstract Pelitic assemblages from all major Witwatersrand gold fields record metamorphic conditions of the greenschist facies, with minimal regional grade changes over at least 200 km strike length. Diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites, the actively-exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons. Bulk rock composition has been a major control on assemblage development. Key metapelitic assemblages include pyrophyllite, chloritoid, chlorite and muscovite in each gold field, with less common metamorphic biotite. Accessory minerals are pyrite, tourmaline, rutile and zircon. The abundance of chloritoid and pyrophyllite in thin shaly units, together with their minor, but widespread, distribution in quartzites and conglomerates, indicate that metamorphic temperatures reached 350°C ± 50°C in all the gold fields. Pressures are less-well constrained, 1–2 kbar being inferred. Outside the gold fields, higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions. The temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to (re)mobilize gold and may explain its secondary textural features. |
| Keywords: | Key-words: chloritoid gold greenschist pyrophyllite Witwatersrand |