OCCURRENCE, ORIGIN AND MINING OF MESOZOIC MANGANESE NODULES IN COSTA RICA, CENTRAL AMERICA

The geologic basement of southern Central America is made up of oceanic complexes, partly of ophiolitic affinity, with Jurassic to Early Tertiary ages (Denyer & Gazel, 2009). Most of these rocks are deep-sea tholeiitic basalts, gabbros and serpentinite which are regionally associated with units of volcaniclastic breccias as well as radiolarian cherts (Gursky, 1989). In the Nicoya Peninsula (NW Costa Rica) manganese mineralizations are present within some of these oceanic rocks. They occur mostly as clearly hydrothermal ores: in form of massive stratiform and cross-cutting mineralizations, generally associated with recrystallized bedded cherts or bodies. However, radiolarian cherts of probable Middle Jurassic to Cretaceous ages contain in places sedimentary manganese nodules, which either are set in masses within dark-grey manganiferous siliceous mudstone intercalated in radiolarian chert sequences or occur isolated within red relatively pure radiolarian chert (Gursky, 1989). The manganese ores are of minor dimensions and thus were called ‘pockets’ by the early miners. Massive hydrothermal deposits were preferentially mined, as shown by the material of relictic dump ores found locally. Documented mining in small open pits and trenches was active mainly from OCCURRENCE, ORIGIN AND MINING OF MESOZOIC MANGANESE NODULES