Genesis of low iron bauxite, northeastern Cape York, Queensland, Australia

Abstract

Several small deposits of bauxite with an iron content of less than 2.0 percent were discovered recently on the remote northeast tip of Cape York Peninsula, Queensland, Australia. The low iron bauxite occurs in the low parts of a gently undulating pre-Pliocene lateritic surface which formed on Mesozoic sediments. The low iron bauxite grades into low-grade high iron (>15.0% Fe 2 O 3 ) bauxite on the ridges of the pre-Pliocene surface through a thin transition zone. In the transition zone the high iron bauxite changes downward into red pisolitic bauxite with white (low iron) cement, through bauxite containing pisolites with red cores and white rims in white cement, to all white low iron bauxite. In places boulder-size blocks of dense, red high iron bauxite occur as scattered remnants in the white low iron bauxite. The low iron bauxite is overlain by brown carbonaceous sandstone, whereas the high iron bauxite is exposed or covered by sand containing no carbonaceous material. The abundance of carbon and plant remains indicates that the Eh and pH of water draining through the brown sandstone since its deposition would have been moderately low. The bauxite is highly permeable except for the dense blocks. The low iron bauxite was formed by leaching of iron from preexisting high iron bauxite. The leaching of iron is thought to have been accompanied by both precipitation of alumina and removal of free quartz. In the transition zone between the high and low iron bauxites leaching is incomplete.