Abstract

Potassium-rich tuff beds occur interbedded with major stratiform Pb-Zn mineralization in shales and dolomitic shales of the Mount Isa Group, northwest Queensland, and the McArthur Group, Northern Territory. These deposits are some 600 km apart, yet they exhibit many similarities, which, together with regional correlations, suggest that the deposits formed by similar processes and that the successions developed more or less contemporaneously.Zircon is sparsely distributed as tiny crystal fragments or phenocrysts in the tuffs, and its morphology and U-Pb systematics affirm that it is primarily of igneous origin. U-Pb zircon data from a tuff in the mineralized H.Y.C. Pyritic Shale Member of the McArthur Group give an age of the tuff of 1,690 (super +29) (sub -25) m.y., and from a tuff in the mineralized Urquhart Shale of the Mount Isa Group, an age of 1,670 (super +20) (sub -17) m.y. These results are consistent with all other geologic controls and are regarded as the best estimates of the depositional ages of the Groups. The ages of the Mount Isa and McArthur Groups are thus the same to within the present experimental uncertainties, essentially confirming geologic synchrony of these important sedimentary successions.Pooled whole-rock data from several of the Mount Isa tuff marker beds give an updated Rb-Sr age of about 1,480 m.y., interpreted as indicating that isotopic exchange and regional metamorphism had essentially ceased by this time.

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