I think soliceous carbonate rocks have been formed in ocean floor must be answere of this question.

Dear All: yes, oceanic floor sediments are rich in Mn oxides, but also some basáltic flows. Usually these rocks are metamorphosed, so that the Mn-rich siliceous sediments become a Quartz-Mn-silicate rock called gondite in India. The basalt becomes Mn-rich amphibolite, but some other proceses should occur to these primary rocks in order to form deposits, such as tropical lateritization, which oxidises Mn-silicates with a low valence Mn-oxides (like braunite) to high valence oxides and hydroxides, like psilomelane and pyrolusite; this happened in the large Brazillian, Ghanan and Indian deposits. Instéad, in the huge Kalahari Manganese Fields, sedimentary Mn deposits suffered low grade metamorphism and hydrothermal alteration, and formed mixed valence oxides, like hausmannite, and hidrated silicates, like inesite. With higher metamorphism pyroxenoids like rodonite and pyroxmangite are formed, together with Mn-spinel (galaxite). In Venezuela some Archean gondites and amphibolites have formed small pyrolusite deposits, and some Proterozoic metabasalts too, Regards, Sebastián

See the table and the comment on the physical chemical regime

Magmatic manganese deposits

(1)Sedimentary/volcanic-exhalativemarine Mn deposits(SEDEX toVOLCEX)

(2)Volcano-sedimentary lacustrine

(3)Epithermal Mn in places with gem quality rodochrosite

(4)Rhodonite skarn

(2)Structure-related manganese deposits

(1)Hydrothermal unconformity-related vein-type deposits

(2)Mixed-type unconformity-related vein-type deposits

(3)Rodochrosite veins

(3)Sedimentary manganese deposits

(1)Shallowmarine Mn deposits with /without remobilization in subaerial regimes

(1)Clastic-hosted deposits

(2)Carbonate-hosted

(2)Mn deposits in black shales

(3)Mn nodules in deep-marine depositional environments (in parts endogeneous)

(4)Supergene manganiferous deposits

(1)Bog iron ores on siliciclastic bedrocks

(2)Manganiferous replacement deposits hosted by calcareousrocks

(4)Metamorphic manganese deposits (prevalently sedimentaryprotore)

(1)Manganiferous Banded Iron Formation

(2)Rhodonite gemstone deposits

The main process is pH- and Eh- driven. It is the process were the stability field of Mn oxy-hydrates get enlarged very much relative to the sister element bivalent Fe which is the case in alkaline conditions, e.g, on carbonate and basic rocks. Calcareous rock are poor in Mn but nevertheless they foster the separation of Fe from Mn and thereby favor the built-up of Mn depoists. In all cases Fe is too much predominating the chemical system and has a diluting effect Mn-bearing solutions.

HGD