The noble gas systematics of late-orogenic H 2 O–CO 2 fluids, Mt Isa, Australia M.A. Kendrick a,⇑ , M. Honda b , N.H.S. Oliver c , D. Phillips a a The School of Earth Sciences, The University of Melbourne, Vic. 3010, Australia b RSES, Australian National University, Canberra, ACT, Australia c The School of Earth and Environmental Sciences, James Cook University, Qld, Australia Received 17 March 2010; accepted in revised form 29 November 2010; available online 11 December 2010 Abstract The noble gases (He, Ne, Ar, Kr and Xe) are powerful geochemical tracers because they have distinctive isotopic compo- sitions in the atmosphere, crust and mantle. This study illustrates how noble gases can be used to trace fluid origins in high- temperature metamorphic and mineralising environments; and at the same time provides new information on the composition of noble gases in deeper parts of the crust than have been sampled previously. We report data for H 2 O and CO 2 fluid inclusions trapped at greenschist to amphibolite facies metamorphic conditions associated with three different styles of mineralisation and alteration in the Proterozoic Mt Isa Inlier of Australia. Sulphide fluid inclusions are dominated by crustal 4 He. However, co-variations in fluid inclusion 20 Ne/ 22 Ne, 21 Ne/ 22 Ne, 40 Ar/ 36 Ar and 136 Xe/ 130 Xe indicate noble gases were derived from three or more reservoirs. In most cases, the fluid inclusions elemental noble gas ratios (e.g. Ne/Xe) are close to the ranges expected in sedimentary and crystalline rocks. However, the elemental ratios have been modified in some of the samples providing evidence for independent pulses of CO 2, and interaction of CO 2 with high-salinity aqueous fluids. Compositional variation is attributed to mixing of: (i) magmatic fluids (or deeply sourced metamorphic fluids) characterised by basement-derived noble gases with 20 Ne/ 22 Ne  8.4, 21 Ne/ 22 Ne  0.4, 40 Ar/ 36 Ar  40,000 and 136 Xe/ 130 Xe  8; (ii) basinal– metamorphic fluids with a narrow range of compositions including near-atmospheric values and (iii) noble gases derived from the meta-sedimentary host-rocks with 20 Ne/ 22 Ne  8–9.8, 21 Ne/ 22 Ne < 0.1, 40 Ar/ 36 Ar < 2500 and 136 Xe/ 130 Xe  2.2. These data provide the strongest geochemical evidence available for the involvement of fluids from two distinct geochem- ical reservoirs in Mt Isa’s largest ore deposits. In addition the data show how noble gases in fluid inclusions can provide infor- mation on fluid origins, the composition of the crust’s major lithologies, fluid–rock interactions and fluid–fluid mixing or immiscibility processes. Ó 2010 Elsevier Ltd. All rights reserved. 1. INTRODUCTION The noble gases (He, Ne, Ar, Kr and Xe) have been widely used to provide insights into fluid processes and/or the presence of mantle-derived gases in a variety of sedi- mentary environments (Ballentine et al., 2002). This has been possible because the noble gases are transported by major volatile phases (H 2 O and CO 2 ) and have isotopic compositions that vary by orders of magnitude between dif- ferent geochemical reservoirs (Ozima and Podosek, 2002). Noble gas isotopic variations result from the non-radio- genic isotopes ( 20 Ne, 36 Ar, 84 Kr and 130 Xe) being over- whelmingly concentrated in the exosphere, and radiogenic isotopes (e.g. 4 He, 21 Ne * , 40 Ar * and 136 Xe * ) 1 having differ- 0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.12.005 ⇑ Corresponding author. E-mail address: mark.kendrick@unimelb.edu.au (M.A. Ken- drick). 1 The asterisk indicates that the radiogenic/nucleogenic/fissio- genic isotope has been corrected for an atmospheric contribution, e.g. 40 Ar * = 40 Ar total  40 Ar Atm = 40 Ar total  ( 36 Ar  296). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 75 (2011) 1428–1450