Molango: Giant Sedimentary Manganese Deposit in Mexico
Patrick M. Okita, J. B. Maynard, A. Martinez Vera
The Molango deposit is the only large manganese deposit known in North America and contains perhaps a billion tons of manganese. Mineralization is entirely manganese carbonate in a finely laminated bed about 20 m thick with a strike length of over 50 km. This bed forms the basal unit of the Chipoco (or Taman) Formation of Kimmeridgian age, and is underlain by the Oxfordian Santiago Formation, a laminated black shale with a few fossil-rich beds. Manganese values are highest at the base of the Chipoco and decrease steadily upward. The dominant mineralogy follows this trend, going from rhodochrosite to kutnahorite to manganoan calcite.
In thin section, the ore bed is seen to consist of very fine grained carbonate with dispersed organic matter and pyrite. The lamination is wavy and sometimes disturbed by what appear to be slump structures, but bioturbation is absent. In some localities, clotted textures predominate. The ore bed is overlain by about 100 m of similar appearing limestone with lower manganese content, but this unit is succeeded by a calcarenite bed composed of micritized ooliths and scattered large quartz grains.
Descriptions of Upper Jurassic rocks to the north of the area suggest a facies model for Molango. Very deep water lay to the east in the spreading Gulf of Mexico. This spreading center may have provided manganese in solution. The Gulf was bordered by a series of islands associated with the Tamaulipas arch. West of this archipelago was a deeper water (below storm wave base) basin that passed westward into continental deposits farther west. Molango seems to have lain on the west side of the island chain, at the boundary between reducing basinal waters to the west and shallow oxidizing water to the east.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.