On the ore-deposition and geochemistry of manganese

Abstract

The distribution of manganese in eruptive rocks has been elaborated together with A. Berger (1965). There are systematically higher MnO-values in intrusive rocks than in the corresponding extrusives with the same SiO₂-content. Thus, negative values of “potential ore metal” are characteristic for manganese. Therefore, Mn is strongly concentrated in magmatic residual solutions, contrary to chromium and titanium which have positive values of potential ore metal, and which are concentrated in the very early products of fractional crystallization of basaltic magmas. Under fresh water conditions with low ionconcentrations, small asbolan deposits may be formed on peridotite-serpentinites (New Caledonia Type). Rich ore concentrations may be formed in connection with lateritic weathering, above all on the southern continents (old Gondwana Shield) on the basis of primarily poor protores. Special reaction processes of two groundwater masses may lead to the Lindener Mark Type in the southern part of the Rheinisches Schiefergebirge as well as to the big ore bodies of Postmasburg in South Africa. Under marine conditions with the development of a “carbonic-acid-zone”, manganese may be dissolved in solutions with much higher pH- and more positive Eh-values. Thus, the separation of Mn and Fe and of both from the main mass of Si-Al-components can take place. Pure manganese ore deposits of the Tschiaturi/Nikopol Type in an “expanded Black Sea” could thus be built up. On the other side, the “volcanic-sedimentary” type with shale-chert-spilite-manganese ore formation is strictly bound to residual solutions of basaltic provenance in an eugeosynclinal realm. The different genetic types are schematically demonstrated in Fig. 2. The physicochemical conditions for the separation of Mn and Fe are elucidated in Fig. 9.

Zusammenfassung

Auf der Grundlage einer umfassenden Untersuchung von A. Berger (1965) über die Lagerstättenkunde und Geochemie des Mangans werden die natürlichen Prozesse, welche von der geochemischen Verdünnung zu Konzentrationen und Lagerstättenbildungen führen können, kritisch untersucht. Der Vergleich der MnO-Gehalte der verschiedensten Typen von Eruptivgesteinen (von den ultrabasischen bis zu den sauersten) zeigt, daß die Ergußgesteine stets höhere Mangangehalte aufweisen, verglichen mit den Tiefengesteinen gleichen SiO₂-Gehalts. Daher ergeben die Differenzen stets positive Werte des „potentiellen Lagerstättenmetalls“, was gleichzeitig für die starke Anreicherung des Mangans in hydrothermalen Restlösungen charakteristisch ist. Im Mittelpunkt der Gesamtuntersuchung steht das lagerstättenkundlich besonders wichtige Verhalten des Mangans — verglichen mit dem Fe — in kontinentalen Verwitterungslösungen und im marinen Bereich.