Metamorphosed manganese ore. Gwynedd, Wales, Britain

The ore is a banded chloritic pelite from the Hafotty Formation within the Harlech Dome of Wales. Diagenetic calcian rhodochrosite is the primary mineral and relicts are found in light-coloured bands comprising concretionary Mn-Ca-Mg-Fe carbonate, quartz, minor chlorite and spessartine garnet that are interbanded with darker bands with a higher garnet content. Beneath the ore is an iron-rich horizon that now comprises magnetite, pyrite, quartz and chlorite, together with manganese-bearing carbonates, garnet and albite. Submarine exhalative hydrothermal solutions produced the original syngenetic to syndiagenetic mineral assemblage of carbonates and oxides, but this has suffered greenschist facies metamorphism. Later supergene oxidation formed limonite and manganese oxides and hydroxides.

Major Minerals:

Calcian rhodochrosite, spessartine garnet

Minor Minerals:

Magnetite, haematite,- pyrite, rhodonite, rhodochrosite, limonite, psilomelane, siderite

Trace Minerals:

Include chalcopyrite, ferropyrophanite


Radiating and zoned microconcretions of manganese-bearing carbonates have been replaced by limonite and psilomelane. Euhedral magnetite shows incipient martitization.


Mohr, 1964; Bennett, 1987