PHOSPHORUS MINERALOGY OF THE JAMAICA BAUXITE ORES

The bauxite deposits of Jamaica formerly were the world’s largest producers, and they currently are the largest source of bauxite imported into the United States. They occur within karst sink structures or on karst surfaces of unusually pure Tertiary limestones. Hypotheses on the origin of the bauxite have involved deep lateritic weathering to produce aluminum hydroxide from: 1) residual clays from the host limestones, 2) older uplifted volcanic rocks, and 3) younger air-fall volcanic ash from Central America together with dust blown from Africa both deposited on the karst host limestones. The presence of altered bentonitic Miocene tuffs on the nearby ocean floor has supported the third hypothesis.

The Jamaican bauxite ores contain the highest phosphorus amounts of any bauxite ores in the world, and samples examined for this study contained up to 32% P₂O₅. Because apatite was thought by some to contain much of the phosphorus in Jamaican bauxites, a cathodoluminescence (CL) microscopy was initiated to examine the character of the phosphorus-bearing grains. Apatite and collophane commonly exhibit distinct CL in other ores and rocks including sedimentary iron ores. Those Jamaican bauxite ores with the highest P₂O₅ were found to contain phosphorus largely in 200-500µm crandallite fecal pellets. Additional phosphorus occurs in phosphatic collophane bones. These areas of bauxite probably served as roosting areas for sea birds like cormorants that had feasted nearby on anchovy and other fish and subsequently contributed the fish bones to the underlying bauxite. Bauxite ores with lower phosphorus contain only bone fragments. In bauxite ores with the least phosphorus, SEM-EDS analyses indicate that phosphorous is entirely present as rare earth (Ce, Nd, and Dy) phosphates.

Reflected light and SEM-EDS study of opaque grains in a black sand concentrate at Blue Hole spring, speculated to be derived from bauxite protore by underground drainage, identified hematite, martite, titaniferous magnetite, titaniferous hematite with abundant especially well-developed exsolution ilmenite, and ilmenite. The recognition that titanium iron oxides also occur in the bauxite ores suggests that the black sands at Blue Hole were derived from the bauxite protores and that they provided an alluvial addition to those protores.