A collision-related pressure-temperature-time path for Prieska copper mine, namaqua-natal tectonic province, South Africa
Prieska Copper Mine lies in highly metamorphosed gneisses of the 1100 Ma Namaqua-Natal tectonic Province of South Africa, close to its boundary with the Kaapvaal Craton. The complex stratigraphy of the mining district is thought to have formed in a 1300 Ma arc environment, while the ore is regarded as a volcanogenic massive sulphide deposit. Petrographic study of key peraluminous and metabasic rock types led to the recognition of four principal metamorphic events M1-4 which, together with three deformation phases, constitute an orogenic cycle. Staurolite-garnet-kyanite M1 parageneses reflect medium pressure metamorphism, but are largely obscured by the dominant M2 muscovite-free, cordierite-garnetsillimanite assemblages. The latter reflect the metamorphic peak at about 730°C and 4.1 kbar, with partial melting suppressed by dry conditions. M2 peak temperatures at different localities within the 30-km-long mining district vary between 550 and 800°C, suggesting a localized heat source. M3 sillimanite-phlogopite grain boundary reactions represent a brief hydration event in P-T conditions close to those of M2. M4 parageneses which feature retrograde kyanite, staurolite, garnet, muscovite and chlorite are developed only in F3 shear zones, and represent a near-isobaric return to a geotherm close to that which prevailed during M1. P- T data for the metamorphic cycle is integrated with comprehensive geochronology to develop a model PTt path for the Prieska Mine region. The model envisages an orogenic cycle at the end of a 1300-900 Ma Wilson Cycle, characterised by collision, crustal thickening followed by a long period of uplift and erosion. The peak metamorphic conditions are ascribed to two short-lived, essentially contact metamorphic events, due to the intrusion of the Plat Sjambok Anorthosite Suite. The 400 Ma Wilson Cycle, 300 Ma orogeny and 0.05 mm/yr uplift rate envisaged for Prieska Mine are long and slow compared with Phanerozoic examples, however, similar parameters seem to characterize more than one Proterozoic event. © 1992.