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Geological Society, London, Special Publications; 1997; v. 119; p. 265-280;
DOI: 10.1144/GSL.SP.1997.119.01.17
© 1997 Geological Society of London

Cenozoic Deposits

An EPMA and SEM study of the Mn-oxide mineralization of Kato Nevrokopi, Macedonia, northern Greece: Controls on formation of the Mn4+ oxides

K. M. Michailidis1, K. Nicholson2, M. K. Nimfopoulos3 & R. A. D. Pattrick4

1 Department of Mineralogy, Petrology & Economic Geology, Aristotle University, 540 06 Thessaloniki, Macedonia, Greece
2 Environment Division, School of Applied Sciences, The Robert Gordon University, Aberdeen AB1 1HG, UK
3 Institute of Geology & Mineral Exploration, Thessaloniki Branch, 1 Fragon Street, 546 26 Thessaloniki, Macedonia, Greece
4 Department of Geology, The University, Manchester M13 9PL, UK

EPMA and SEM studies of tunnel-structured todorokite and nsutite and layer-structured chalcophanite and birnessite from the Kato Nevrokopi battery grade Mn-mineralization were performed. The chemistry of todorokite, formed at hypogene and supergene environments in an increasing order of weathering, reflects different pH and oxidation conditions. The data also point to a clear dependence of the development of late stage hypogene and early supergene Mn4+-oxide paragenesis on the host rock and protore composition and the mobilities of base metals (mainly Zn), alkalis and alkaline earths (mainly Ca).

The chemistry and development of (large cation-bearing) nsutite as in situ oxidations and banded poorly crystalline (with structural defects) aggregates in veins and associated karstic cavities, together with birnessite, indicates the influence of climate, oxidation and water table fluctuations. The data on chalcophanite revealed a deposition, under low fO2 conditions, from acid meteoric fluids produced by weathering of vein sulphides. Birnessite formation reflects a prevalence of higher fO2 in weathered veins and karstic cavities.

The evolution of well developed layer structures through poorly crystalline to amorphous tunnel-structured nsutite is probably controlled by a Zn-saturation of the ore fluids.