Geochemistry of supergene manganese oxide deposits, Groote Eylandt, Australia

Abstract

Some 20 different supergene Mn oxide ores and associated secondary rocks from the Groote Eylandt manganese deposit were analyzed for their major, minor, and trace element content. This is the first detailed account of the trace element geochemistry from this deposit and the data presented here should aid in future comparative studies. Due to the complex nature of the secondary ores and rocks at Groote Eylandt, it is difficult to differentiate supergene products purely on the basis of their trace element composition. However, we are able to correlate groups of trace elements to individual minerals using a quantitative normalization technique that determines the mineralogical composition of the samples. Members of these groups tend to be elements with specific ionic radii, contained in the following host phases: pyrolusite ( approximately 0.4 Aa and > or =1 Aa), cryptomelane and todorokite ( approximately 0.7Aa), romanechite (no significant correlation detected), kaolinite ( approximately 0.6, 0.8, and 1.0 Aa), and goethite ( approximately 0.4 Aa). Generally, distinctive supergene horizons, enriched in certain elements (e.g., Zn, Sr) and minerals, develop within the stratigraphic section. Kaolinite, in particular, seems to form laterally continuous layers that crosscut primary sedimentary units. Most other minerals also show a stratigraphy-controlled abundance. Cryptomelane, todorokite, romanechite, and gibbsite usually concentrate in higher parts of the profile, whereas pyrolusite and quartz increase down the section.