An ancient manganese-iron deposit of freshwater origin, Islay, Argyllshire

Abstract

Synopsis Veins composed of manganese and iron oxides occur within the Dalradian Jura Quartzite (Argyll Group) on the SW coast of Islay. The manganese oxides, which are composed principally of hollandite with minor pyrolusite and rare groutite, frequently display a colloform texture centred upon silicate grains. In a single section an oolitic texture was displayed by hollandite in a hematite groundmass. Hematite and goethite, in variable amounts, constitute the iron oxides, and usually enclose fragments of the host quartzite. The manganese and iron oxides frequently occur as separate, discrete patches within the vein material. The geochemistry of the oxides shows that, of the target and pathfinder elements, only Ba attains elevated levels (up to 4.9%). The absence of enrichments in the diagnostic hydrothermal assemblage (As–Ba–Cu–Mo–Pb–Sb–Sr–V–Zn) and in the marine assemblage (Na–K–Ca–Mg–Sr; Co–Cu–Ni) imply that this is a freshwater deposit. This conclusion was confirmed by discriminatory scatter plots of Na vs Mg and (Co + Ni) vs (As + Cu + Mo + Pb +V + Zn) which indicate a fresh-water, non-hydrothermal origin for the veins. The oxides were deposited from groundwater which penetrated the Jura Quartzite via a local fault system probably in Permo-Triassic time. The veins have no economic potential as manganese ore and the prospect of related base or precious metal mineralisation is remote.