LETTER Oxidized and reduced mineral assemblages in greenstone belt rocks of the St. Ives gold camp, Western Australia: vectors to high-grade ore bodies in Archaean gold deposits? Peter Neumayr & John Walshe & Steffen Hagemann & Klaus Petersen & Anthony Roache & Peter Frikken & Leo Horn& Scott Halley Received: 6 July 2006 / Accepted: 1 November 2007 / Published online: 12 December 2007 # Springer-Verlag 2007 Abstract Hydrothermalsulfide–oxide–goldmineral assemblages in gold deposits in the Archaean St. Ives gold camp in Western Australia indicateextremely variable redox conditions during hydrothermal alteration and gold mineralization in spaceand time. Reducedalteration assemblages (pyrrhotite–pyrite) occurin depositsin the southwest of the camp (e.g., Argo,Junction deposits) and moderately to strongly oxidized assemblages (magnetite– pyrite,hematite–pyrite) occurin deposits in the Central Corridorin the northeast (e.g.,North Orchin,Revenge deposits). Reduced mineral assemblages flank the Central Corridorof oxidized deposits and,locally,cutacrossit along E–W trending faults. Oxidized mineral assemblages in the Central Corridor are focused on gravity lows which are interpreted to reflectabundantfelsic porphyritic intrusionsat about1,000 m below presentsurface. Hydrothermal magnetite predates and is synchronous with early phasesof gold-associated albite–carbonate–pyrite– biotite–chlorite hydrothermal alteration. Later-stage, gold- associated pyrite is in equilibrium with hematite. The spatial distribution and temporal sequence of iron sulfides and oxides with gold indicate the presence of at leasttwo spatially restricted but broadly synchronous hydrothermal fluidswith contrasting redox states. Sulfurisotope con- straintssupporttheargument thatthedifferentmineral assemblages reflectdifferences in redox conditions. The δ 34 S valuesfor pyrite forthe St.Ivesgold camp range between −8.4‰ and +5.1‰ withthe negativevalues occurring in oxidized magnetite-rich domains and slightly negative or positive values occurring in reduced, pyrrhotitic domains. Preliminary spatial and paragenetic analysis of t distribution of iron sulfides and oxides in the St. Ives camp suggests that gold grades are highest where the redox sta the hydrothermal alteration assemblages switchesfrom relatively reduced pyrrhotite–pyrite to relatively oxidized magnetite–pyrite and hematite–pyrite both in space and t Gold deposition is inferred to have occurred where fluids o contrasting redox state mixed. Keywords Redox minerals . Orogenic gold . St. Ives gold camp . Australia Introduction Despite the large database on, and the myriad of genetic for, Archaean orogenic gold deposits (see Groves 1993; Hagemann and Cassidy 2001 and references therein) our Miner Deposita (2008) 43:363–371 DOI 10.1007/s00126-007-0170-2 DO00170; No of Pages Editorial handling: Adrian Boyce P. Neumayr (*) : S. Hagemann : K. Petersen Centre for Exploration Targeting, School of Earth and Geographical Sciences, University of Western Australia, Nedlands, WA 6009, Australia e-mail: pneumayr@segs.uwa.edu.au J. Walshe : P. Frikken : L. Horn CSIRO-Exploration and Mining, 26 Dick Perry Ave, Kensington, Perth,WA 6151, Australia S. Halley Mineral Mapping Pty. Ltd., 24 Webb St., Rossmoyne, WA 6148, Australia A. Roache St.Ives Gold Mining Company Pty. Ltd., Kambalda-West, WA 6444, Australia