ARTICLE Geology, geochemistry and mineralogy of the lignite-hosted Ambassador palaeochannel uranium and multi-element deposit, Gunbarrel Basin, Western Australia Grant B. Douglas & Charles R. M. Butt & David J. Gray Received: 5 July 2010 / Accepted: 22 March 2011 / Published online: 27 April 2011 # Springer-Verlag 2011 Abstract The Ambassador U and multi-element deposit occurs on the SW margin of the Gunbarrel Basin, Western Australia. Low-grade, flat-lying U mineraliza- tion averaging about 2 m thick at 0.03% U occurs in lignites at the redox front at the base of the weathering profile within a laterally extensive palaeochannel net- work. Uranium is principally associated with organic matter within the lignitic matrix, although rare discrete U minerals, such as coffinite and uraninite, are also present. The lignite is also enriched in a suite of other elements, principally base metals and sulphur, with concentrations of 0.3 ≥ 1% Cu, Pb, Ni, Co, Zn and total rare earth elements (REE) in some samples. Other element enrichments include: Cr, Cs, Sc, Se, Ta, Ti, Th, V and Zr as detrital heavy minerals of Zr, Ti and REE (oxides and silicates) or authigenic minerals of Cu, Bi, Pb, Zn, Ni, Se, Hg, Ti, Cr, Tl, V, U and REE (sulphides, vanadates, selenides, oxides, chlorides and native metals) and diffuse lignite impregnations. The Ambassador deposit probably formed from the conver- gence of redox-active weathering processes to unique source/host rocks, constrained within the palaeochannel. A proximal source of U and trace elements of lamp- roite/carbonatite origin is probable, as constrained by U– Pb isotope and U–Th disequilibria studies. Uranium and other metals were precipitated syngenetically with organic matter as it was deposited during a humid phase in the Late Eocene. Remobilization subsequently concentrated the metals in the upper 2 m of the lignite. This may have occurred during one or more periods of weathering and associated diagenesis, with the latest episode in the last 300,000 years. Keywords Ambassador deposit . Gunbarrel basin . Western Australia . Uranium . Lignite . Redox . Palaeochannel Introduction Mid-Eocene lignite in several palaeochannels at Mulga Rock in the south west of the Gunbarrel Basin, Western Australia (Fig. 1), is strongly enriched in U, at potential ore grades (Douglas et al. 2005). The principal U deposits are Shogun, Emperor and Ambassador. The enrichment is generally in a 1- to 2-m interval immediately below a sharp redox front with overlying clay. Uranium mainly occurs within the carbonaceous matrix, with some discrete U minerals, such as coffinite, uraninite and unidentified sulphides. The lignite is also enriched in many other elements, principally base metals and S, with concentra- tions of 0.3 ≥ 1% Cu, Pb, Ni, Co, Zn and total rare earth elements (REE) in some samples; Cr, Cs, Sc, Se, Ta, Ti, Th, V and Zr are also enriched. There is a remarkable diversity of different phases, including detrital heavy minerals (Zr, Ti and REE oxides and silicates) and/or authigenic minerals of Cu, Bi, Pb, Zn, Ni, Se, Hg, Ti, Cr, Tl, V, U and REE (sulphides, vanadates, selenides, oxides, chlorides and native metals), as well as diffuse impregnations in organic matter. A detailed study of the Ambassador deposit was undertaken to provide insight into the mechanisms of Editorial handling: M. Cuney G. B. Douglas (*) CSIRO Land and Water, Private Bag No 5, Wembley, WA 6913, Australia e-mail: Grant.Douglas@csiro.au C. R. M. Butt : D. J. Gray CSIRO Earth Science and Resource Engineering, PO Box 1130, Bentley, WA 6102, Australia Miner Deposita (2011) 46:761–787 DOI 10.1007/s00126-011-0349-4