Precambrian Research 137 (2005) 93–114 A trace element study of siderite–jasper banded iron formation in the 3.45 Ga Warrawoona Group, Pilbara Craton—Formation from hydrothermal fluids and shallow seawater Robert Bolhar a,∗ , Martin J. Van Kranendonk b , Balz S. Kamber a a Advanced Centre for Queensland University Isotope Research Excellence (ACQUIRE), University of Queensland, St. Lucia, Qld 4072, Australia b Geological Survey of Western Australia, 100 Plain St., East Perth, WA 6004, Australia Received 25 June 2004; received in revised form 3 December 2004; accepted 1 February 2005 Abstract Shale-normalised rare earth element and yttrium (REE + Y) patterns for siderite–jasper couples in a banded iron formation of the 3.45 Ga Panorama Formation, Warrawoona Group, eastern Pilbara Craton, display distinct positive Y and Eu anomalies and weak positive La and Gd anomalies, combined with depleted light REE relative to middle and heavy REE. Ambient seawater and hydrothermal fluids are identified as major sources of REE + Y for the BIF. In the case of siderites, strong correlations between incompatible trace elements and trace element ratios diagnostic of seawater indicate variable input from a terrigenous source (e.g. volcanic ash). We propose a volcanic caldera setting as a likely depositional environment where jasper and siderite precipitated as alternating bands in response to episodic changes in ambient water chemistry. The episodicity was either driven by fluctuations in the intensity of hydrothermal activity or changes in magma chamber activity, which in turn controlled relative sea level. In this context, precipitation of jasper probably reflects background conditions during which seawater was saturated in silica due to evaporative conditions, while siderites were deposited most likely during intermittent periods of enhanced volcanic activity when seawater was more acidic due to the release of exhalative phases (e.g. CO 2 ). © 2005 Elsevier B.V. All rights reserved. Keywords: Archaean; Banded iron formation; Caldera; Hydrothermal fluids; Panorama Formation; Pilbara Granite-Greenstone Terrain; Rare earth elements ∗ Corresponding author. Tel.: +61 7 3346 9751; fax: +61 7 3365 8530. E-mail address: r.bolhar@acquirelab.uq.edu.au (R. Bolhar). 1. Introduction There is now widespread agreement that the trace element inventory in banded iron formation is strongly dominated by a hydrothermal component, derived from alteration of basalt at mid oceanic ridges and hot-spots 0301-9268/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2005.02.001