MINERALOGICAL CHARACTER OF HIGH-PHOSPHORUS JAMAICA BAUXITES ORES

The bauxite deposits of Jamaica formerly were the world’s largest producers. They occur within karst sink structures and on karst surfaces of Tertiary limestones. The origin of the bauxite probably involved deep lateritic weathering and leaching of aluminum from Quaternary air-fall volcanic ash from volcanoes in the Lesser Antilles, together with additions of Sub-Saharan dust blown from Africa, and perhaps sediments from an alluvial source, all deposited on the karst host limestones. The presence of altered bentonitic Miocene tuffs on the nearby ocean floor, together with abundances 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 that must be eliminated during metallurgical treatment of the bauxite ores. The eliminated phosphorus is not recovered but rather is lost in the red mud waste product. Bauxites with the highest phosphate contents are left behind unmined. The mineralogy of Jamaica bauxite phosphorus has 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 species. The crandalite fecal pellets appear to have been deposited by invertebrate organisms rummaging through the organic-rich materials beneath rookeries of sea birds such as cormorants. Small amounts of phosphatic bone fragments, that occur in some Jamaican bauxites, are comprised of weakly CL apatite and non-CL collophane. Importantly, the crandallite fecal pellets are sufficiently large that they probably could be eliminated from high-phosphorus bauxite ores by a preliminary stage of flotation, and thereby could render those high-phosphorus bauxite ores economic.