The Phalabowra (Palabora) carbonatite-hosted magnetite-copper sulfide deposit, South Africa: an end-member of the iron-oxide copper-gold-rare earth element deposit group?

摘要

Olympic Dam-type iron-oxide copper-gold deposits are widely recognised in terrains with significant Palaeoproterozoic to Mesoproterozoic granitic magmatism. Most researchers favour a magmatic association for these deposits, but none of the 100-2000 Mt recognised copper-bearing deposits has a spatial and/or temporal relationship to an intrusive body of sufficient dimensions to produce the hosting giant breccia and/or hydrothermal systems. In other words, if the recognised ore-bodies are magmatic-hydrothermal, they must be classified as distal deposits. The magnetite-copper-phosphate-rare earth element pipe-like carbonatite-hosted orebody at Loolekop, within the larger Phalabowra Carbonatite Complex, has many features to suggest that it represents an end member of the Olympic Dam-type deposit class hosted within its magmatic source rock. It (1) falls broadly within the appropriate age range, (2) has a similar giant size and low copper grade, (3) is dominated by magnetite, (4) has sulfur-poor copper-sulfide minerals and lacks iron sulfides, (5) is silica-poor, and (6) is enriched in REE, particularly LREE, as well as P, F, U and Th. As and end member of the Olympic Dam-type deposits, the Loolekop orebody can help explain the common siting of the deposits at craton edges or other lithospheric boundaries, where decompression melting of metasomatised mantle can produce volatile-rich alkaline melts (rich in REE, P, F, etc.). Such melts are capable of generating sulfur-deficient volatiles enriched in copper and gold, among other elements, as well as phreatic breccia pipes and associated intense metasomatism, the most common features of the Olympic Dam-type deposits.