Younger ca 1500 Ma granites of the Williams and Naraku Batholiths, Cloncurry district, eastern Mt Isa Inlier: Geochemistry, origin, metallogenic significance and exploration indicators∗

Tables 1 and 2 [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

The last major felsic intrusive event in the Mt Isa Inlier occurred between 1520 and 1490 Ma with the emplacement of the oxidised, high‐uranium l‐(granodiorite) type plutons which form the bulk of the Williams and Naraku Batholiths in the Eastern Fold Belt. These young plutons were emplaced shortly after a major period of deformation and metamorphism (D₂), but are themselves in many places affected by major north‐northwest or north‐northeast structures (D₄ and D₅). They crop out over at least 2400 km² and are inferred from geophysical data to extend to the north and south of the Mt Isa Inlier beneath the Carpentaria, Eromanga and Georgina Basins. These post‐D₂ intrusions are mineralogically and chemically distinct from any other felsic intrusives in the Mt Isa Inlier and consist of a series of coeval but compositionally distinct plutons. Rock types present are very heterogeneous ranging from mafic magnetite‐hornblende‐biotite diorites and monzonites, through coarse porphyritic granodiorites and monzogranites to high‐SiO₂ leucogranites enriched In U, Th and F. It is believed that their source region was underplated or intruded into the lower crust up to 1000 million years earlier. Metasomatic alteration of both the country rocks and the intrusives is widespread, whilst brecciation is more common in the calc‐silicate country rocks. The alteration, which is more common around the more felsic plutons, is complex with end‐members varying from albitites to late cross‐cutting K‐feldspar + hematite veins (with up to 15 wt% K₂O). Deposits of Cu and Au and minor occurrences/deposits of Ag, U and Co occur both within and around the margins of these post‐D₂ plutons, although the most significant deposits are hosted by iron‐rich facies some distance from known intrusions. For exploration purposes, the presence of late high‐K alteration facies combined with appropriate structures and suitable host rocks are considered more important than the primary compositions of the individual intrusions. These ca 1500 Ma intrusions of the eastern Mt Isa Inlier closely resemble, in their chemical composition and association with regional‐scale breccia systems, the ca 1590 Ma granites of the Stuart Shelf Region, including those hosting the major Cu‐Au‐U deposit at Olympic Dam.

Key words:

• breccias
• copper
• gold
• l‐type granite
• mineralisation.

Notes

Tables 1 and 2 [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.