Pyrite-rich clay sediments from a brackish-water lake on Rennell Island provide new evidence for bauxite genesis. The sediment represents an intermediate product and is considered to have been produced by biogenic activity in a shallow-water environment. Several sources of possible parent material are considered, and it is concluded that the bauxite deposit of northwest Rennell is derived from volcanic ash of hornblende andesite composition erupted from Pliocene to Recent volcanic centers. The diameters of residual ilmenite grains common to both the bauxite and the lake sediment are suggestive of a volcanic source some 200 km distant.

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