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Marramamba Tiger Eye

Marramamba Tiger Eye. (banded iron formation/ microbialite)
Age: Archean (2.6 billion years ago).
Location: Marra Mamba Formation, Hamersley Group, Mount Bruce Supergroup.
Mount Brockman, West Australia.

Microbialites are structures which are mediated by microbial activity. This could be in the form of algae mats, but could also be due to the metabolic byproducts of the organisms. The thin iron bands present in this rock is generally agreed to be due to precipitation of iron oxides, which occurred when iron carried in solution met with oxygenated water. The oxygenation of the water was due to the arrival of cyanobacteria. Cyanobacteria produce oxygen as a byproduct of photosynthesis and this eventually lead to a rise in atmospheric oxygen levels. Banded Iron formations mark the 2 billion year period during which this change occurs (approximately 3.8 billion - 1.2 billion years ago, although there is a short period when BIFs reform around the time of snow ball Earth). The rise in oxygen levels eventually lead to the Great oxygenation event, which is probably also the world's first mass extinction event, which lead to the demise of all organisms not adapted to these new conditions.

The Hamersley Basin, contains an almost continous record of this process, and consists of multiple banded iron formations, each of which contains a record of the local and global conditions of the time. The Marra Mamba Formation are the oldest rocks in the Hamersely group. During this period local volcanism decreased and the majority of the rocks that formed were due to chemical processes within the ocean e.g carbonates and cherts. The formation contains abundant banded iron. The rocks have been folded and metamorphosed. One of the most obvious changes is the abundance of "tiger's eye" or crocidolite. This is a form of micro quartz. It formed when warm chemically rich fluids circulated through the rock.

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