Queensland Geological Record 2008/05
Joint GSQ-GA NGA geochronology project
Mount Isa region, 2007-2008
As part of remapping the Mount Isa Inlier by the Geological Survey of Queensland, a programme of geochronology has been undertaken in order to establish a time framework in the inlier. Particularly in the Eastern Fold Belt where stratigraphic relationships are difficult to establish, geochronology is necessary in order to establish correlations with the Western Fold Belt.
In the Eastern Fold Belt, a correlation between the Toole Creek Volcanics and the Answer slate is suggested by the similar detrital zircon populations between the two units. Both have recorded maximum depositional ages of 1658±5Ma and 1654±11Ma respectively. We interpret this to mean the two units are correlative.
A low grade psammite from the host sequence of the Dugald River Pb-Zn deposit has yielded a maximum detrital age of 1686±7Ma. This result indicates a possible correlation with the Gun Supersequence in the Western Fold Belt.
The Knapdale Quartzite is part of the Mount Albert Group, the age of which is uncertain. The Knapdale Quartzite has yielded a maximum depositional age of 1728±5Ma.
The Quamby Conglomerate is an apparently younger unit infilling a graben in the Quamby 1:100 000 Sheet area. A maximum depositional age from detrital zircons is 1754±10Ma. This age seems to reflect source region rather than the age of the unit.
An unnamed tuff from the Boomarra Horst has yielded a magmatic age of 1776±8Ma. This is interpreted as a magmatic age which dates part of the sequence in the horst as being equivalent to the Argylla Formation.
An unnamed gneissic granite in the southern ARDMORE 100K Sheet area has yielded a magmatic age of 1782±5Ma. As this rock intrudes sandstone equated to the Eastern Creek Volcanics, this correlation is highly unlikely.
A sample of Argylla Formation from the MARY KATHLEEN 100K Sheet area has yielded a magmatic age of 1780±4Ma, confirming that this unit is equated to the Argylla Formation in the surrounding areas. The Leichhardt Volcanics have yielded a magmatic age of 1864±3Ma, which also confirms this outcrop as part of the Kalkadoon Leichhardt block.
Two samples from the St Ronans Metamorphics have been dated to try to determine the age of this unit. One yielded a magmatic age of 1787±4Ma and is interpreted as an intrusive into the metamorphics. A felsic volcaniclastic which overlies the St Ronans Metamorphics has yielded a maximum depositional age of 1823±8Ma.
These results have significantly improved our knowledge of time frames and correlations within the Mount Isa Inlier.
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