The Petrology and Geochemistry of Alteration Assemblages in the Eastern Creek Volcanics, as a Guide to Copper and Uranium Mobility Associated with Regional Metamorphism and Deformation, Mount Isa, Queensland

Abstract

Summary At least four alteration types are identified in the basaltic Eastern Creek Volcanics of the Mount Isa Inlier. The earliest alteration type, found in predominantly massive rocks, contains albite + actinolite with lesser chlorite + epidote + sulphides + magnetite. These basalts are enriched in K 2 O and S, and Cu has been locally redistributed within the lava flows. The remaining three alteration types are only found in deformed rocks. Two are closely related and are dominated by either epidote + sphene, or calcite + magnetite, with the mineralogy of the alteration type controlled by the concentration of CO 2 in the fluid phase. Both assemblages are depleted in S, K 2 O, Cu, Rb, Ba, and Sr, whilst Zr, V, Y and Cr are enriched, and these samples also have the highest ratio of Fe 2 O 3 /FeO. These two alteration types are associated with major regional metamorphism, and some uranium deposits in the western Mount Isa Inlier and possibly some small Cu deposits are related to them. The final alteration was the most profound, producing rocks dominated by albite + chlorite + rutile, and strongly depleted in CaO, MnO, Fe 2 O 3 , Cu, Sr, Ba, and Ni, and enriched in MgO and SiO 2 . This last alteration is believed to be linked to the formation of the Mount Isa copper ore body.