Manganese deposits in the global lithogenetic system: Quantitative approach

Abstract

A computerized data base, including 330 localities and believed to represent at least 95% of the presently recorded spot accumulations of ore-grade Mn on land, is the basis for a quantitative analysis of terrestrial Mn resources. This file is reprinted in full (Appendix). The present subaerially exposed global ore Mn resources are calculated as 17.9 × 10 9 t Mn 1 1 All tonnages in this paper are in metric tonnes (t), except where directly quoted from the literature. In such case, they are in short tons (tons), i.e., about 0.9 tonne. . In this figure are included the actually mineable straight Mn deposits (8.7 × 10 9 t Mn); potentially mineable land-based Mn accumulations (9.2 × 10 9 t Mn), and actual or potential Mn that could be extracted as a byproduct of mining other metals (0.6 × 10 9 t Mn). This distribution is strongly influenced by giant accumulations, where the single, exceptional Kalahari Mn field contains over 50% of the presently economic Mn ore reserves, or 23.42% of the global land Mn resources. A set of attributes has been selected to treat the global ore Mn population in terms of genesis, geotectonic and environmental setting, and lithologic associations. In terms of genesis, precipitation from aqueous solutions was responsible for at least 99% of the contemporaneous, and probably also the past Mn accumulations now exposed on land. Weathering of Mn orebodies has left its mark on 93% of the Mn localities, and 24% are now represented entirely by supergene assemblages. Less than 0.01% of the ore Mn resources, however, are formed by weathering-related accumulation over silicate rocks (ultramafics). In terms of geotectonic environments, the bulk of the land-based Mn deposits (97%) formed in intraplate and stable continental margin settings; 3.1% formed along Pacific-type and rift-type continental margins; and only 0.00045% of the deposits formed in an oceanic setting. This is in contrast with the outstanding Mn-accumulating capacity of the present ocean and is a consequence of the low preservation potential of the oceanic domain. In terms of lithologic associations, 96% of the Mn in land-based deposits is present in marine-sedimentary associations (70% of Mn is in banded iron formations, 14.4% is in detrital and 11.1% is in carbonate-dominated associations). Chert and jasper, limestone, sandstone, shale, and banded iron formation are statistically the most common immediate hosts to Mn ores with recorded hosting frequencies of 79, 50, 45, 37 and 35, respectively. In terms of geological history, the lower Proterozoic accounts for 58.9% of the prreserved ore Mn on land, followed by Oligocene (17.2%), Jurassic (6.2%) and middle Proterozoic (4.5%). In terms of the intensity of Mn accumulation per one million years of geological time, Oligocene (110 × 10 6 t Mn/ma) is two orders of magnitude greater than the nearest time periods: Jurassic (8.9 × 10 6 t Mn/ma) and lower Proterozoic (6.5 × 10 6 t Mn/ma). The historical distribution pattern of the land-based Mn deposits seems to indicate that accumulation of the bulk of the present ore-grade Mn is the result of repeated recycling with a land → ocean trend, abruptly initiated at the time of early cratonization (about 2.5 Ga). This has been supplemented by a substantially less significant, but remarkably steady reverse trend of addition of juvenile Mn released from the mantle into the crust. Mafics and particularly basalts are the most important intermediaries in the cumulative secular increase of liberated and accumulated Mn in the crust. Direct to indirect, proven to hypothetical spatial coincidence of “basalts” and Mn ores can be demonstrated on at least 169 localities out of 330 (= 51%) evaluated.