Manganese precipitates in the karst terrain of Grand Cayman, British West Indies

Abstract

Ten morphologically distinct forms of Mn precipitates (probably birnessite) occur in a wide variety of paleokarst features that characterize dolostones of the Bluff Formation on Grand Cayman. These Mn-rich precipitates, which commonly contain variable amounts and selections of Al, Si, Fe, K, Ti, Ni, Na. Mg, and Ca, are not evenly distributed throughout the Bluff Formation of Grand Cayman. Thus, some areas have abundant Mn precipitates, whereas other areas are devoid of such precipitates. Mn precipitates, of variable morphology, have teen found in stalactites, karst breccia, caymanite, terrestrial oncoids, and root calcretes and along fractures and cavities in the dolostone. There does not appear to be any correlation between the morphology and composition of the Mn precipitate and its host substrate.The Mn and its associated elements may have been derived from the terra rossa and (or) swamps that formed on the surface of the Bluff Formation at various times. The scattered distribution of the soils and swamps, with respect to time and space, accounts for the patchy occurrence of the Mn precipitates presently found in the Bluff Formation. Although some of the Mn precipitates are abiogenic in origin, others appear to have formed through the direct or indirect intervention of various microbes.Paragenetic analysis of the Mn precipitates and their associated cements shows that Mn precipitation has not been a constant, ongoing process. Indeed, Mn precipitation does not appear to be occurring at the present day. Available evidence suggests that Mn precipitation occurred at various times when climatic conditions were suitable for Mn mobilization.