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.