Continental drift secret to mineral deposits
20 December 2001
Geologists at Geoscience Australia are using the movement of the continents over millions of years to better understand the origin of major mineral deposits in northern Australia and use this information to predict where additional resources may be located.
The recently published results of a multidisciplinary study in Geoscience Australia have shown that the age of major Pb-Zn-Ag mineral deposits from northern Australia (Mount Isa, Century, and McArthur River) coincide with bends on the Apparent Polar Wander Path (APWP) curve. These rocks are 1700-1575 million years old. The rocks accumulated in marine environments that once covered most of northern Australia.
Most rocks on earth contain small quantities of the mineral haematite and magnetite. When the sediments are deposited these magnetic minerals form tiny 'grains' that align with the earth's magnetic field like a compass. As the sediments are buried and gradually turn to rock the 'grains' retain their magnetic alignment. With the use of specialised laboratory techniques it is possible to decode the palaeomagnetic signal recorded in the rocks.
Using these techniques it is possible to determine where the north and south poles were relative to the rocks over time. Because the rocks on the earth's crust have been moving due to plate tectonics, it appears from the palaeomagnetic signal that the poles have moved. However, in reality the poles remain stationary and it is the overlying crust that wanders due to plate tectonics. Hence the term Apparent Polar Wander.
By plotting the palaeomagnetic data over time you end up with a curve, the APWP, which shows how the continents have drifted over millions of years. The curve has straight line segments and sharp bends or cusps. Straight-line segments indicate continuity of plate motion. Bends or cusps form when a change takes place in the direction of plate motion. Changes in plate direction take place when continents collide, causing inter- and intraplate stresses in the crust.
Data from rock samples dating back to the late Palaeoproterozoic and Mesoproterozoic ages is being used to reconstruct basin shape and sediment architecture at the time of fluid migration and sulphide precipitation to develop new predictive exploration models. The models will provide a template to assess the mineral potential of rocks of similar age elsewhere in Australia and around the world.
Rocks of late Palaeoproterozoic and Mesoproterozoic age outcrop in the Arnhem Land McArthur River, Mount Isa, Cloncurry and Georgetown regions of northern Australia, the Gawler Craton of SA and in the Broken Hill region of NSW.
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