Australian Journal of Earth Sciences (2003) 50 , 19–38 Constraints on the composition of ore fluids and implications for mineralising events at the Cleo gold deposit, Eastern Goldfields Province, Western Australia* S. M. BROWN, 1† C. A. JOHNSON, 2 R. J. WATLING 3 AND W. R. PREMO 2 1 Centre for Global Metallogeny, Department of Geology and Geophysics, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. 2 US Geological Survey, Box 25046, MS 963, Denver Federal Center, Denver, CO 80225, USA. 3 Department of Applied Chemistry, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia. The Cleo gold deposit, 55 km south of Laverton in the Eastern Goldfields Province of Western Australia, is characterised by banded iron-formation (BIF)-hosted ore zones in the gently dipping Sunrise Shear Zone and high-grade vein-hosted ore in the Western Lodes. There is evidence that gold mineralisation in the Western Lodes (which occurred at ca 2655 Ma) post-dates the majority of displacement along the Sunrise Shear Zone, but it remains uncertain if the ore in both structures formed simultaneously or separately. Overall, the Pb, Nd, Sr, C, O and S isotopic compositions of ore-related minerals from both the Western Lodes and ore zones in the Sunrise Shear Zone are similar. Early low-salinity aqueous– carbonic fluids and late high-salinity fluids with similar characteristics are trapped in inclusions in quartz veins from both the Sunrise Shear Zone and the Western Lodes. The early CO 2, CO 2–H2O, and H 2O- dominant inclusions are interpreted as being related to ore formation, and to have formed from a single low-salinity aqueous–carbonic fluid as a result of intermittent fluid immiscibility. Homogenisation temperatures indicate that these inclusions were trapped at approximately 280 C and at approxi- mately 4 km depth, in the deeper epizonal range. Differences between the ore zones are detected in the trace-element composition of gold samples, with gold from the Sunrise Shear Zone enriched in Ni, Pb, Sn, Te and Zn, and depleted in As, Bi, Cd, Cu and Sb, relative to gold from the Western Lodes. Although there are differences in gold composition between the Sunrise Shear Zone and Western Lodes, and hence the metal content of ore fluids may have varied slightly between the different ore zones, no other systematic fluid or solute differences are detected between the ore zones. Given the fact that the ore fluids in each zone have very similar bulk properties, the considerable differences in gold grade, sulfide mineral abundance, and ore textures between the two ore zones most likely result from different gold-deposition mechanisms. The association of ore zones in the Sunrise Shear Zone with pyrite-replaced BIF suggests that wall-rock sulfidation was the most significant mechanism of gold precipitation, through the destabilisation of gold–bisulfide complexes. The Western Lodes, however, do not exhibit any host-rock preference and multistage veins commonly contain coarse-grained gold. Fluid-inclusion characteristics and breccia textures in veins in the Western Lodes suggest that rapid pressure changes, brought about by intermittent release of overpressured fluids and concomitant phase separation, are likely to have caused the destabilisation of gold–thiocomplexes, leading to formation of higher-grade gold ore zones. KEY WORDS: Cleo, Eastern Goldfields Province, fluid inclusions, gold, isotopes, ore fluids, Yilgarn Craton. INTRODUCTION Cleo, an Archaean orogenic lode-gold deposit (after the criteria of Groves et al. 1998), is located 55 km south of Laverton in the Eastern Goldfields Province of the Yilgarn Craton in Western Australia (Figure 1). A north–south tenement boundary (Figure 2) separates Cleo, operated by AngloGold Australia, from Sunrise, operated by the Granny Smith Joint Venture. The Cleo deposit together with the contiguous but independently mined Sunrise deposit contain over 248 t of gold in resources ( ~ 168 t) and past production (~ 81 t), derived from primary ore and palaeochannel placer deposits. This paper relates specific- ally to the Cleo deposit, although the conclusions should apply equally well to the adjacent Sunrise deposit. The Sunrise Shear Zone divides the mine sequence into a hangingwall and footwall, and divides the Western Lodes into the hangingwall Western Steep Lode and the footwall *Table 8 (indicated by an asterisk [*] in the text and listed at the end of the paper) is a Supplementary Paper; copies may be obtained from the Geological Society of Australia’s website (www.gsa.org.au) or from the National Library of Australia’s Pandora archive (http://nla.gov.au/nla.arc-25194). † Corresponding author: susieb@geol.uwa.edu.au