Ironstone-Gossan discrimination: Pitfalls of a simple geochemical approach — A case study from northeast Scotland

Abstract

Located in northeast Scotland, the Lecht manganiferous ironstone occurs as several minor and one principal outcrop within deeply weathered Dalradian meta-sediments. The distribution of these shows is controlled primarily by an underlying porous breccia pipe and not by Dalradian stratigraphy or faulting, as previously suggested. The deposit is composed principally of goethite and cryptomelane, with minor hematite, ramsdellite, pyrolusite, lithiophorite, chalcophanite and woodruffite. The ironstone is enriched in several target and pathfinder elements, particularly Zn and Ba which are primarily concentrated in the manganese oxides. Detailed examination of the geochemistry demonstrates that the enrichments are actually more typical of non-economic ironstones (particularly bog-ore) than gossans (a conclusion supported by field, textural and mineralogical evidence), illustrating the danger of relying upon simple geochemical surveys alone for ironstone-gossan discrimination. No relict sulphides, secondary ore minerals, native metals, gangue minerals or “boxwork” textures were observed in either hand specimen or polished section. The morphology and textures of the Lecht ironstone are typical of those observed in bog-iron ores and in weathered profiles. The Lecht ironstone is considered to have been derived from prolonged weathering of the local Dalradian meta-sediments. These are enriched in target and pathfinder elements and are regarded as a prospective sequence. Cementation of the subsequent regolith by solutions rich in iron, manganese and other elements, combined with bog-ore formation and penetration of the breccia pipe by these solutions, produced the complex and varied morphology and geochemistry seen in the deposit today. The Lecht deposit may represent the distal manganiferous expression of a goldrich zinc-lead exhalative deposit hosted by the Dalradian meta-sediments of the region.