KARST OCCURRENCE, CHARACTER, AND PHOSPHATE MINERALOGY OF JAMAICA BAUXITES ORES

The bauxite deposits of Jamaica formerly were the world’s largest producers and currently are the largest suppliers of bauxite ore to the United States. The bauxite ores are closely controlled by karst structures in the surfaces of Tertiary high-purity limestones. In the northern and western portions of the island, the ores occur at the bottoms of 300-foot, steep-sided karst sink structures. In the southern portions of the island, the bauxite ores occur in blanket deposits on the surfaces of highly karstified limestone.

The origin of the bauxite involved deep lateritic weathering and leaching of aluminum from Quaternary air-fall volcanic ash derived from volcanoes in the Lesser Antilles, together with additions of Sub-Saharan dust blown from Africa, and probably sediments from a more local alluvial source, all deposited on the karst host limestones. The presence of altered bentonitic Miocene tuffs on the nearby ocean floor, together with abundances and ratios of certain elements in the bauxite, have provided support for these interpretations.

Jamaican bauxites have the world’s highest phosphate contents, reaching a maximum of 32% P₂O₅. The phosphorus is a deleterious constituent. Bauxites with the highest phosphate contents are left behind unmined. Mined bauxite ores must have their phosphate contents eliminated during metallurgical treatment of the ores. The eliminated phosphorus is partitioned into the red mud waste. The mineralogy of Jamaica bauxite phosphorus has previously been inadequately studied. The current study has employed petrographiy, ore microscopy, SEM-EDS, XRD, and cathodoluminescence (CL) microscopy to examine the mineralogical character of the phosphorus-bearing grains. Apatite and collophane, thought by some to constitute the major phosphorus phases, were not observed in low-phosphorus Jamaican bauxite. SEM-EDS analyses instead indicate that the small amounts of phosphorus in those bauxites occur as rare earth (Ce, Nd, and Dy) phosphates.

By contrast, in high-phosphorus bauxites, 200-500 µm crandallite [CaAl₃(PO₄)₂(OH)₅^.H₂O] fecal pellets constitute the dominant phosphorus mineral. The crandalite fecal pellets are interpreted to have been deposited by invertebrate organisms rummaging through the organic-rich materials beneath rookeries of sea birds such as cormorants.