predictive mineral discovery CRC Conference Barossa Valley 1-3 June 2004 165 Hydrothermal alteration footprints and gold mineralization in the St Ives gold camp Peter Neumayr 1 , John L. Walshe 2 , Leo Horn 3 , Klaus Petersen 1 , Cari Deyell 4 , Kate Moran 5 , Daniel Howe 6 , Karen Connors 6 , Ned Stolz 6 , Robert S. Morrison 6 , Steffen G. Hagemann 1 1 pmd*CRC, Centre for Global Metallogeny, The University of Western Australia 2 pmd*CRC Divison of Exploration and Mining, CSIRO 3 CSIRO, Division of Exploration and Mining (Present address: Goldfields Agnew Gold Mining Company Pty Ltd) 4 CODES, University of Tasmania 5 Lion Ore Pty Ltd 6 St. Ives Gold Mining Company Pty Ltd. Introduction This research project focuses on delineating hydrothermal alteration patterns in three dimensions and through time as well as at different scales from camp- to deposit-, to ore-shoot scale. The analyses of these hydrothermal “footprints” are primarily guided by the geometry and kinematics of fault zones at all scales and geophysical signatures such as magnetics and gravity that delineate porphyry bodies at depth. Detailed field and petrographical analyses are then complemented by whole rock and trace element analyses of alteration zones, mineral analyses of key hydrothermal alteration minerals as well as fluid inclusion and stable isotope analyses. The spatial distribution of hydrothermal alteration types and their mineralogical and chemical signals are then interpreted with respect to fluid sources, ore transport and depositional processes to provide guides to gold mineralization. The key to unravelling hydrothermal alteration footprints and their use for exploration targeting is to understand the relative timing of alteration types as well as their spatial distribution. In addition, spatial information on the variation of stable isotope compositions is vital to determine metal transport and depositional processes, and in combination with multi-element geochemistry and fluid end- member composition data sets, provide critical input parameter into geochemical modeling of metal transport and depositional processes. The aim of this paper is to summarize the key results of current studies (some are still on-going) and also discuss potential models to explain the mineralogical and chemical footprints with respect to ore deposition. Geological setting The St Ives gold camp is located in the southern part of the Norseman-Wiluna greenstone belt within the Eastern Goldfields Province of the Yilgarn Craton of Western Australia. The St Ives gold camp is hosted in predominantly mafic-ultramafic lavas and intrusions that have been metamorphosed to upper greenschist and lower amphibolite facies. The greenstone sequence has been intruded by felsic to intermediate porphyry stocks which predate, are synchronous with and post-date gold mineralization. The St Ives gold camp is bounded by two major NNW-trending regional structures: the Boulder- Lefroy fault to the east and the Merougil fault to the west, and has undergone four major Archaean deformation events. The first (D1) produced regional south-over-north thrusts. The second (D2) produced upright, NNW-trending, gently plunging folds, such as the Kambalda anticline. The third event (D3) generated brittle-ductile, NNW-trending, oblique-sinistral strike- slip fault systems that localized major N-trending, reverse, gold-bearing shear zones. During the fourth event (D4), NE-trending, dextral faults offset stratigraphy and earlier fault systems, and