Syn- and epigenetic sulphides in Archaen BIFs of NW-Tanzania and their significance to gold mineralisation

摘要

The Sukumaland Greenstone Belt of NW-Tanzania hosts gold-free and gold-bearing sulphide mineralisation of various styles. Laminated stratiform pyrrhotite-pyrite occurs interlayered with quartz and magnetite in Algoma-type BIF with stratabound spheroidal pyrites representing a facies variation of this type. These sulphides are of synsedimentary exhalative origin and are void of any gold mineralisation or anomalous gold contents. Epigenetic sulphides occur in and as replacements adjacent to magmatically and/or tectonically induced features such as andesitic porphyries, breccias, quartz-veins, and shear zones in a wide variety of host rocks. A gold-porphyry system has possibly been active at Nyamulilima where large volumes of sub-economic, finely disseminated gold-pyrite-pyrrhotite occur in an andesitic porphyry. The wall rocks to this porphyry are syngenetic sulphide and oxide BIF and these host economic gold ores in contact-breccias, shear zones and as replacements. Gold-porphyries, formed in an Archean magmatic arc environment are a probable source of the porphyry- and of the BIF-hosted gold and could be more common in this region than previously anticipated.