Résumé
Oxygen isotopic and geochemical analyses on whole rock and quartz veins are combined with structural observations in order to constrain the fluid circulation history within the Mesoarchaean Warrawoona syncline of the North Pilbara Craton, Western Australia.
The plumbing system which is the focus of this study is localized in the so called Fielding's Find shear zone (FFSZ), a km-scale shear zone formed during the burial of greenstones and coeval exhumation of granitic complexes. This shear zone runs parallel or close to the axial plane of the syncline. It involves a prominent quartz vein network and is lined with strongly hydrothermally-altered mafic, felsic and sedimentary rocks. Towards the FFSZ, felsic and mafic volcanic rocks become intensely silicified with an increase in bulk rock 8180 values from + 10.8%. to +25.1 parts per thousand% for altered felsic volcanics and from +7.1 parts per thousand to 18.3 parts per thousand for altered mafic volcanics. Geochemical modelling ascribes the silicification to a dissolution/precipitation process. REE and most other trace elements are strongly depleted in the silicified units, with the exceptions of elements such as V. Cr, Ni and Co, which are enriched. Throughout the Warrawoona syncline, vein quartz 8180 data are within a small range of + 13.2 +/- 2%., significantly lower than their silicified host rocks. These data are interpreted as the result of two main paleo-fluid circulation stages. Intense silicification and (18)O enrichment represent alteration driven by low-temperature hydrothermal convection probably involving Archaean seawater. In contrast, the quartz veins network is related to the infiltration of metamorphic and/or magmatic fluids during a later deformation episode. These quartz veins represent the event responsible for the bulk of economic lode-gold formation in the area.
The protracted fluid-rock interaction history in the Warrawoona syncline may have played a major role in setting the stage for the late mineralizing event. The early hydrothermal circulation could have formed an efficient Plumbing system characterized by high permeability, low reactivity and possibly Au-enrichment. upgrading the Au-endowment to the late hydrothermal fluids. Crown Copyright (c) 2008 Published by Elsevier B.V. All rights reserved.