Abstract

The 40 Ar/ 39 Ar dating of hydrothermal micas is used to determine the late metamorphic history of metacarbonate-hosted Cu ore deposition, regional metabasalt alteration, and relative uplift near the Proterozoic base metal deposit at Mount Isa, Queensland, Australia. Biotite that is broadly contemporaneous with the formation of the alteration envelope to the breccia-hosted Cu orebodies (silica-dolomite alteration) in dolomite-rich metasediments at the Mount Isa mine yields ages of ca. 1523 to 1505 Ma; ca. 1523 Ma is presently interpreted as the most likely approximation to the time of Cu mineralization. District-scale fracture zones in metabasalts to the east of Mount Isa, which are associated with characteristic metabasalt alteration and local U mineralization, show the same relative timing to regional metamorphic foliation as the Cu-mineralized breccias, and have identical fluid inclusion and isotopic characteristics to the Cu-related silica-dolomite alteration at the Mount Isa mine. The 40 Ar/ 39 Ar dating of hydrothermal biotite from a uranium prospect associated with a zone of intense carbonate-Fe oxide + or - biotite alteration, approximately 15 km northeast of Mount Isa, constrains the transition from peak regional metamorphism to late metamorphic hydrothermal activity at ca. 1534 Ma, and suggests that the regional metabasalt alteration could have overlapped the silica-dolomite alteration and Cu mineralization at the Mount Isa mine. More extensive dating of the regional alteration is needed to fully determine the timing of hydrothermal activity but it appears that carbonate-Fe oxide-altered fracture zones in metabasalt may be district-scale exploration indicators of the mildly oxidizing hydrothermal systems capable of forming Mount Isa-style, late metamorphic, Cu ores. 40 Ar/ 39 Ar dating of hornblende, which is in broad accord with a published zircon date, suggests that the highgrade metamorphic rocks on the western side of the Mount Isa fault zone were not juxtaposed with the low-grade metamorphic rocks to the eastern side of the zone at the time of Cu ore formation. The earliest the high-grade rocks could have been uplifted into their current position would be after ca. 1460 Ma. Reverse movement on the Mount Isa fault zone thus probably postdated, at least in part, the major hydrothermal system that lead to Cu ore formation at Mount Isa. Micas from faults in and near the Mount Isa mine yield younger ages of ca. 1400 to 1325 Ma, which may reflect localized resetting by minor fluid flow rather than the time of Cu mineralization or the last major movement on the structures.

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