Nature and development of supergene manganese deposits, Groote Eylandt, Northern Territory, Australia

Supergene manganese ores occur on Groote Eylandt in association with other secondary rocks. Overprinting primary sedimentation features (e.g. oolitic and pisolitic units), secondary Mn-oxides and gangue minerals nowadays belong to the most important components in the deposits. The different primary and supergene rock types are described and discussed with the help of typical geological profiles. In an idealized vertical cross section we present a combination of all observed supergene Mn-ores ("mangcretes"). Many parts of this section can be encountered in various localities of the Groote Eylandt deposits. The most common minerals found on the island are pyrolusite, cryptomelane, romanechite (psilomelane), todorokite, quartz, kaolinite, goethite and hematite. Probably with the exception of todorokite, all other mentioned phases can occur as primary and/or supergene minerals. The dominant process for the dissolution of Mn-oxides is a redox reaction between divalent iron and tetravalent manganese. Other leaching processes are of subordinate importance. We discuss transport and precipitation which lead to the development of a "Mangcrete Model" for supergene Mn-deposits. Finally the supergene products are compared with secondary ores of other manganese deposits and it can be seen that in spite of different protores the supergene materials can be very similar in their physical appearance and mineralogy.