The Marra Mamba banded iron formation of Karijini’s Hamersley Range in northwestern Australia is an archaic natural artwork, created by the first microorganisms that inhabited the early Earth. About 2500 million years ago, the shallow seas off the coast of the primordial Pilbara continent underwent powerful tectonic changes that opened up a new ocean basin and, at the same time, created calm and stable environmental conditions on the continental shelf.
The only living organisms that inhabited Earth at that time were bacteria that fed on iron and sulphur in a world without oxygen. The iron and sulphur were dissolved in the murky brown seawater.
These mineral assemblages were abundantly released from hydrothermal vents along the newly formed mid-ocean volcanic ridge. The bacteria utilized this abundance. As a by-product of their anoxygenic photosynthesis, compounds containing oxygen combined with the dissolved iron, which was precipitated from the water as iron oxides and accumulated as gel-like sedimentary mud on the seafloor offshore from the Pilbara continent. A newly established group of cyanobacteria, which increased in numbers at this time, began to produce free oxygen as a waste product of oxygenic photosynthesis. This biologically produced oxygen also immediately combined with the dissolved iron and added to the sediments on the seafloor.
The fascinating colors of the Marra Mamba iron ores were created by several complex geological processes. These include both primary mineral assemblages formed during the sedimentation of the gel-like mud 2500 million years ago and later secondary mineral reorganizations. The Marra Mamba iron ores fractured into small veins in which hot hydrothermal fluids leached the rocks and altered them chemically. The associated growth of microscopic crystals created a colorful spectacle of rearranged iron oxides and silica. This richness of colors and shapes affected every scale of the rock, down to the tiniest fractures, a characteristic that makes the Marra Mamba banded iron ores unique.
Yellow shades were formed by microscopically small crystals of the iron oxide mineral goethite. Larger crystals of goethite colored the rock brownish. The blood-red colors were formed by tiny crystals of the iron mineral hematite. Larger hematite crystals change the color to metallic gray. The green colors were created by a mineral mixture of blue riebeckite and yellow goethite.
White and sometimes even transparent inclusions of pure quartz complete the color palette of the iron ores. Quartz itself, when crystallized in microcrystalline form, can take on a wide range of colors through the addition of iron oxides. Chalcedony is microcrystalline quartz, called jasper when it is red. Black colors are caused by the magnetic iron mineral magnetite. A spectacular and unique feature of the Marra Mamba iron ores is the chatoyant mineral tiger’s eye with its silky luster. It consists of quartz with impurities of the red and yellow iron ores hematite and goethite and additionally contains crocidolite. Crocidolite is a blue, lustrous, fine-needled to furry-fibrous natural asbestos that originated from the mineral riebeckite. During crystallization, the riebeckite was replaced by quartz, more precisely by silica, similar to the process of petrifying wood, in which the organic wood material is also replaced by silica. The tiger’s eye fills veins in the quartz rock through secondary mineralization, whereby the asbestos fibers always extend perpendicular to the vein.
The bacteria that played a decisive role in the deposition of these sediments in a world almost devoid of oxygen created a natural painting of unprecedented beauty. It is not only the incredible richness of colors and shapes that makes these banded iron ores so unique, but above all their inconceivable age. Knowing that our planet is restlessly changing its appearance, it is almost incomprehensible that these sediments have survived all this time since their formation essentially unaltered. Here, we are truly looking at a snapshot of the early Earth and the beginning of life.
