A Landscape of Aluminum Pebbles
The vast expanse of the Weipa bauxite deposit on the Cape York Peninsula is geologically unique. It is composed almost entirely of pisolites—small, spherical concretions of aluminum ore, typically just 2 to 8 millimeters in diameter. This deposit is one of the world's largest, covering an area of about 11,000 square kilometers with an average thickness of 2.5 meters. The bauxite layer, which can be up to 12 meters thick in some places, consists of these loose, reddish-brown pebbles overlying a classic lateritic profile of deeply weathered rock.
The formation of these pisolites results of intense and prolonged chemical weathering, a process known as lateritization. This occurs in hot, humid climates with distinct wet and dry seasons, like that of northern Queensland. Heavy rainfall leaches soluble elements such as silica, sodium, and calcium from the parent rock, leaving behind less mobile elements like aluminum and iron. The parent rocks at Weipa are primarily Cretaceous marine sediments of the Rolling Downs Group and the younger, Paleogene Bulimba Formation, which consists of terrestrial sands and muds. Over millions of years, this weathering process concentrated aluminum hydroxides—mainly the mineral gibbsite, with some boehmite—into the small spheres seen today.
Layers of Time
Each pisolite tells a story of its formation. They have a core, which can be a fragment of a broken pisolite, an accumulation of smaller ooliths (grains under 2 mm), or other earthy material. Around this core, concentric layers, or cortices, of aluminum and iron oxides have precipitated from groundwater. A single pisolite can have up to seven distinct cortices, each representing a period of accretion. This multi-stage process involves repeated episodes of formation, erosion, fragmentation, and renewed growth, showing a dynamic geological environment over long timescales.
The composition of the pisolites varies across the deposit. At East Weipa, where the bauxite overlies the quartz-rich Bulimba Formation, the ore contains up to 30% quartz and around 5% iron oxide (Fe2O3). In contrast, at the Andoom deposit to the north, which sits on the Rolling Downs Group, the bauxite has very little quartz but can contain up to 25% iron oxide. On an anhydrous basis, the ore across both deposits contains approximately 75% aluminum oxide (Al2O3), 10% silicon dioxide (SiO2), and 10-20% iron oxide. The process is ancient; evidence suggests that ongoing weathering and bioturbation continue to modify the deposit today.