Precambrian time period is the longest time period on the geological time scale. It covers 90% of the Earth's history - the period from 4600 to 545 million years ago, but we know very little about it compared to later eras.
There were three major changes in the Earth's history. Two of them - the appearance of the oldest rocks (that we know of) and the oldest primitive life, happened during Precambrian. The third one - the appearance of complex life forms started the era of Cambrian (the "Cambrian Explosion")
Archaean Precambrian Rocks So the oldest rocks that we know of formed more than 2500 million years ago, in a time period called Archaean. There are three areas with Archaean rocks in Australia: western Eyre Peninsula in South Australia; and Pilbara region and Yilgarn craton (an area south of Pilbara starting from about Shark Bay in north and extending to south-western WA - that also covers the WA goldfields). Pilbara rocks are the oldest of the three, estimated to have been formed about 3500 years ago. A zircon (mineral) found in Pilbara was dated as old as 4276 million years.
Some of Archaean rocks are sedimentary, but mostly they are igneous, of two types: greenstones and granites. The Yilgarin craton contains mainly granite and gneiss; the Pilbara craton contains sediments and basalts overlying granites and greenstones; and the Gawler craton in South Australia contains mainly granites and gneiss with basaltic volcanic rocks such as komatiites which are exceptionally rich in Magnesium, and known for the so-called Spinifex texture. In the WA Goldfields, komatiites host gold mineralisation. The sedimentary rocks from Archaean time period don't contain any fossils. It was a strange world with exposed continents with shorelines, and waves washing sediments onto beaches, but no plants let alone animals - just rock that streched away to the horizon.
Proterozoic Precambrian Rocks The Proterozoic covers the time between 2500 and 545 million years ago. From about 2500 years ago there is evidence of the first signs of life, the first major continents and the first glaciations. The Acheaean rocks were deformed, eroded, and then covered by Proteozoic sediments. The sedimentary rocks that formed from these sediments carry evidence of life. There are Proterozoic sedimentary rocks in Pilbara and Arnherm Land; and widespread Proterozoic basaltic igneous rocks in central Australia around Mount Isa and Tennant Creek; and also in the Grawler cration in South Australia. Some examples of Proterozoic metamorphics have been found in the Kimberley and Mount Isa regions; and areas around Broken Hill, Georgetown in the Gulf Savannah region, and Albany in Western Australia.
The Large Orebodies in Precambrian Rocks Many of our major sources of gold, silver, copper, zinc, lead and uranium occur in Precambrian rocks. The rocks that surround the orebodies have typically been altered by the circulation of hot, hydrothermal fluids in the Earth's crust.
The Archaean orebodies include the WA Goldfields in the Kalgoorlie-Norseman area; and the Whim Creek gold deposits in the Pilbara region.
The Proterozoic orebodies host many of the most important mines in Australia. They include the copper-lead-zinc orebodies in north Queensland, including Mount Isa; gold-copper orebodies around Tennant Creek and the Tanami Desert; and Broken Hill.
Banded Iron Formations (BIF) in Precambrian Rocks Pilbara's famous banded iron formations - the basis of the Australian iron ore industry, also formed during Proterozoic. They are spectacular reddish layered sedimentary rocks with an iron content about 30%. The Hamersley area in the Pilbara region in Western Australia is the major place where BIFs are found and here they are 500m thick and cover 400-500km. They have all been formed about the same time, which indicates that there was a relatively short period of time when large quantities of iron were deposited over vaste distances.
Glaciations in Precambrian Rocks Proterozoic rocks also show the earliest signs of glacial activity in Australia. There were two phases - the Sturtian phase (780-700Ma) and the Marinoan Phase (610-575Ma). The tickest deposits of Proterozoic ice-affected rock are in the Flinders Ranges in South Australia. Others are on King Island, around Broken Hill, Mount Painter, and several deposits in the Kimberleys and the areas around Alice Springs.
Ancient Stromatolites And finally, the amazing stromatolites of Western Australia are also formed during the Precambrian time. Stromatolites are ancient sedimentary rocks that contain layered structures formed by cyanobacteria - the earliest form of life on the Earth that we know of, and they still live in Shark Bay! Hamelin Pool in the southern Shark Bay area is partly separated from the ocean. The water is about twice as salty here than in the open ocean, thanks to the heat (the water evaporates) and the lack of freshwater influx. Stromatolites grow here because other species that would threaten them, like crabs, fish, and shellfish cannot grow in such salty conditions. During the Precambrian, when there was no competition from any other animal species, stromatolites grew in many other environments.
NOTE: This website is written in British English, which is the English we use in Australia. You will find words like "traveller", "harbour" and "realise", and they are all correct in the language used in Australia.
