Late quartz veining along the Cloncurry Fault, Eastern Fold Belt, NW Queensland, Australia: stable isotope evidence for the incursion of low-latitude meteoric fluids

The Mount Isa Block (MIB) preserves evidence of significant hydrothermal alteration produced during multiple periods of veining, brecciation and alteration during the Paleo- and Mesoproterozoic. Representing the last phase, are large, quartz-rich veins that form long N-S vein systems associated with brittle movement along major N-S Paleoproterozoic faults. The Cloncurry Fault represents one such system where late quartz veins formed during dilation associated with left-lateral movement. Stable isotope data for these veins show that the fluids had a calculated (at 250degreesC) delta(18)O and deltaD of -4 to +3 parts per thousand, and - 22 to -4 parts per thousand, respectively, which suggests they were of mixed low-latitude meteoric fluid and formation water origin, and are markedly different to the fluids associated with earlier epigenetic Cu-Au mineralization and Na-Ca alteration. The low-latitude character of the hydrothermal system suggests it was formed near to the equator, and correlation to the paleomagnetic data for the MIB, together with structural and geological considerations, and geochronological arguments suggest they formed c. 1.10 Ga or later, some 400 m.y. after Na-Ca alteration and mineralization. These results suggest that the late veining was unrelated to earlier hydrothermal alteration, and as such places into doubt previous models of the evolution of mineralisation and alteration in the Eastern Fold Belt (EFB).