Petrochemistry of the metasomatized Neoarchean Lower Transvaal Supergroup carbonate from the Kanye Basin (south Botswana)

The Taupone Dolomite Group (Lower Transvaal Supergroup) is a carbonate platform sequence that rimmed the north western Kaapvaal Craton during the Neoarchean-Paleoproterozoic (ca. 2.6–2.5 Ga). The dolostones of the Taupone Dolomite Group in Botswana are known as the Ramonnedi Formation. These carbonates have been intruded by the granitoids and dolerites of the Moshaneng Igneous Complex (ca. 2.1 Ga) and the Moshaneng Dolerites (ca. 1.9 Ga), which have induced alteration of the Ramonnedi Formation dolostones, thus resulting in calc-silicate metasomatism. Metasomatism of the dolostone host rock induced mobilization of Mg with formation of Mg-bearing silicates and calcite, which has replaced dolomite as dominant carbonate phase. The mineral assemblage of the metasomatised dolostones of the Ramonnedi Formation consists of calcite, chrysotile, talc, fibrous and equant crystals of amphibole, phlogopite, diopside and small amounts of epidote and forsterite. The electron probe-micro-analyzer (EPMA) data generally indicate the presence of Fe-poor amphiboles (tremolite) and minor amounts of Mn-rich fibrous tremolite. The overall Cl-chondrite-normalized REE patterns vary consistently across the studied samples with some samples showing light REE (LREE) and medium REE (MREE) enrichment. The samples show variable Ce and Eu anomalies and near chondritic Y/Ho ratio. The distribution of Eu anomaly suggests that the emplacement of the granitoids of the Moshaneng Igneous Complex has induced prograde metasomatism and fluids characterized by K enrichment and Eu depletion. On the other hand, the emplacement of plagioclase-rich Moshaneng Dolerites has induced formation of fluids characterized by high positive Eu-anomaly due to the breakdown of plagioclase in the dolerites. The presence of diopside, tremolite and phlogopite in the altered samples can be attributed to prograde contact metamorphism and/or prograde metasomatism reflecting an increasing hydrous condition away from the intrusion. The alteration minerals show a coarse-grained texture suggesting that these minerals most likely derive from prograde metasomatism, whereas evidences for contact metamorphism have not been observed.