ABSTRACT: Anoxic Marine Lakes--an Analogue Environment for Insular Phosphorite Formation

William C. Burnett

Hundreds of islands in the tropical Pacific Ocean contain phosphate deposits ranging from inconsequential to economically significant in size. Although many of these deposits clearly have formed by the interaction of avian guano with underlying limestone, some display evidence of having developed within an aqueous environment. Several of the emergent carbonate islands in the southern part of Palau contain phosphate deposits that we speculate formed in anoxic marine lakes, similar to those which still occur on a few of these islands.

Lake water, sediments, and sediment pore waters from Jellyfish Lake, on the island of Eil Malk in Palau, were analyzed during an expedition in 1987. The results of this investigation supported, but did not provide, conclusive evidence of our hypothesis. Pore water profiles of phosphate and fluoride confirmed precipitation of carbonate fluorapatite. However, the extremely high bulk sediment accumulation rate, driven by the high biological productivity of the surface waters of the lake, dilutes authigenic phosphate to low levels. We have refined our original proposal to suggest that phosphate deposits may form either by: (1) subaerial weathering and concentration of phosphatic sediments after these lakes disappear; or (2) interaction of phosphate-enriched sediment pore solutions with li estone at the underlying contact. Another expedition to test these concepts is being planned.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990