Exhalative-sedimentary manganif erous iron ores from the Gasborn area, W. Bergslagen, Central Sweden

Abstract

The Gåsborn metamorphosed manganiferous iron ore horizon consists of Mn-poor (<I wt% Mn) iron ore-bearing marbles and metavolcanics and manganiferous (5-25 wt% Mn) iron ore-bearing metavolcanics and metacherts, displaying lateral facies transitions. The ore horizon is concordantly intercalated in a pile of felsic metavolcanics. It shows a concentric zoning around two pipe-like structures which consist of extremely Mn-poor (<0.1 wt% Mn) iron ore-bearing Mg-enriched metavolcanics, located directly below the ore horizon. Major- and trace-element analyses suggest that (1) the ore horizon is of exhalative-sedimentary origin and (2) the Mg-enriched metavolcanics mark the top of a (now metamorphosed) conduit zone for hydrothermal fluids from which the ores were deposited