Manganese ores are well developed in Japan and in Australia. The Japanese deposits are mostly volcanic hosted and are Permian or younger. All are small, less than 1 million tonnes. Younger volcanic hosted deposits like those of Japan are typically enriched in silica, as shown below for world-wide deposits. The increase in silica after 150 Ma slightly precedes the radiation of diatoms.
Australia has deposits of Archean and Cretaceous ages ranging in size from a few million tonnes up to the giant deposit on Groote Eylandt with 179 million tonnes of Mn.
Mn ore bed at Groote, from Ferenczi (2001)