Geochronology of basin phases in the western Mt Isa Inlier, and correlation with the McArthur Basin∗

Tables 2–11 [indicated by an asterisk (∗) in the text and listed at the end of the paper] are Supplementary Papers lodged with the National Library of Australia (Manuscript Section); copies may be obtained from the Business Manager, Geological Society of Australia

Abstract

Zircon geochronology—using U‐Pb SHRIMP methods—has been applied to investigate the Palaeoproterozoic history of basin development and sedimentation in the western Mt Isa Inlier and eastern McArthur Basin. Ages of the sediment packages have been measured using the abundant felsic volcanics in lower parts of the sequences, and overlying sediments have been dated through the use of zircons in sparse, concordant tuff beds that punctuate these successions.

The earliest volcanic phases north and south of the Murphy Inlier are close to 1725 Ma, and may be part of a much larger extensional tectonic regime now identified in the central and eastern Mt Isa Inlier. A somewhat younger episode of volcanism took place in the central Lawn Hill Platform (Fiery Creek Volcanics and Bigie Formation) at about 1710 Ma, approximately coeval with volcanism throughout much of the McArthur Basin. The most southerly volcanic suite below the Mt Isa Group is 30–40 million years younger, closer to the ca 1650 Ma age of overlying strata and suggesting a rather complex pattern of basin evolution. There is evidence that the present concept of discrete cover sequences over the entire Mt Isa Inlier may no longer be sustainable.

SHRIMP U‐Pb zircon results for the McNamara/Mt Isa Groups which host syngenetic or late diagen‐etic Pb‐Zn‐Ag mineralisation constrain the time(s) of deposition of associated stratabound ores. For the Mt Isa and Hilton orebodies, the depositional ages are 1652 ± 7 Ma and 1654 ± 5 Ma, confirming correlation to the lower McNamara Group which contains tuff interbeds dated at 1653 ± 7 Ma. The turbiditic facies of the upper McNamara Group is at least 20 million years younger than this. The deeper water basin phase represented by the Lawn Hill Formation in the uppermost McNamara Group is younger (1595 ± 6 Ma) and has no analogue in the exposed Mt Isa Group.

The new results provide a good time connection between parts of the Fickling and McArthur Groups that host the Walford Creek and McArthur Pb‐Zn deposits. Tuffs in these host sequences have U‐Pb zircon ages of 1640 ± 7 Ma and 1639 ± 3 Ma, respectively. Thus, deposition of the predominantly shallow‐water sediments was approximately synchronous across both the Mt Isa and McArthur regions in the interval 1655–1640 Ma.

Key words:

• geochronology
• McArthur Basin
• Mt Isa Basin
• Pb‐Zn deposits

Notes

Tables 2–11 [indicated by an asterisk (∗) in the text and listed at the end of the paper] are Supplementary Papers lodged with the National Library of Australia (Manuscript Section); copies may be obtained from the Business Manager, Geological Society of Australia