DETERMINATION OF TRACE-METAL DISTRIBUTIONS IN THE IRON-OXIDE PHASES OF RED-BED SANDSTONES BY CHEMOMETRIC ANALYSIS OF WHOLE-ROCK AND SELECTIVE LEACHATE DATA

Hematite is known to be a sink for trace metals and study of the trace metal distributions within hematite-rich formations can provide evide nce of past groundwater activity. The aim of this study is to develop a method specifically for determining the trace metal content of natur ally occurring hematite in a sandstone formation. The elemental compos itions of twelve samples of permo-triassic Red Bed sandstones from the St. Bees formation, Sellafield, Cumbria, mere determined. An acid dig estion procedure was used to produce solutions suitable for ICP-AES an alysis for the major and trace elements (Al, Fe, Mg, Ca, Na, K, Ti, Mn , Ba, Sr). The same solution was used for ICP-MS analysis for lower ab undance trace elements (Cr, Ni, Pb, Sn, Th, U, V, Zr). Fusion with LiB O2, followed by an ion exchange preconcentration-separation procedure, provided solutions for rare earth elements, La, and Y determination b y ICP-AES. Solutions for ICP-AES analysis of Si in the whole rock were prepared by a modified fusion procedure using 4 + 1 mixture of LiBO2 and Li2B4O7 A leaching procedure was developed to selectively extract the hematite phase of the rock samples. This procedure consisted of a preliminary extraction with 1 AI acetic acid for 20 h at 20 degrees C to remove carbonates, followed by an extraction with 0.1 M oxalic acid -0.175 M ammonium oxalate (Tamm's reagent) for 20 h at 70 degrees C to dissolve the iron oxide. The Tamm's reagent leachate was analysed by ICP-AES and ICP-MS for the same suite of elements as the whole rock. P rincipal component analysis was applied to the whole rock and leachate data matrices. Qualitative assessment of the resulting principal comp onents revealed elemental groupings which can be assigned to minerals known to exist in this rock type and showed the leaching procedure was not fully selective for the iron oxide phase. Chemometric mixture dec omposition applied to the leachate data sets identified the extraction of four different mineral sources tentatively assigned to hematite, d olomite, chlorite and iron oxy-hydroxides. Quantitative estimates of t he composition and proportion each of these components in each rock sa mple were calculated.