TRACE-ELEMENT ANALYSIS OF SCHEELITE BY EXCIMER-LASER ABLATION INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY (ELA-ICP-MS) USING A SYNTHETIC SILICATE GLASS STANDARD

Concentrations of W and trace elements in scheelites (CaWO4) associate d with Archean lode-gold deposits in Western Australia were determined by ArF (193 nm) excimer laser ablation-ICP-MS with external calibrati on against a silicate glass standard reference material, NIST 610. The excimer laser beam drilled well-defined pits in both scheelite and si licate glass. With internal standardization against Ca alone, W measur ements for the scheelites fall within 5% of the concentrations expecte d from electron microprobe measurements. A matrix effect between schee lite and silicate glass is apparent in the behaviour of W during laser ablation: W is progressively fractionated from Ca in the glass but no t in the scheelite. Proper data reduction, therefore, requires use of the early maximum count rates for W and Ca rather than the mean count rates during ablation. W isotopic ratios measured in the scheelites an d an in-house generated silicate grass, ANU 252, are both within simil ar to 2.5% of the accepted values for the natural ratios and are repro ducible to similar to 1%, if data for the first similar to 11 s of abl ation are excluded. Using the same data reduction techniques employed for W, concentrations of Sr, Y, Mo, REE and Pb, present at ppm levels in the scheelites, were measured with a precision of 4% or less. Measu rements on Th and U, present at the 5-10 ppb level, and P, Mn, Nb and Ta are less precise (similar to 5-40%) and concentrations of Rb, Zr, B a, Sn, Hf, Tl and Bi are, for the most part, below detection limits. F or Sr, Sm and Nd, replicate ELA-ICP-MS measurements made on 80-mu m wi de spots in scheelite largely encompass the concentrations determined by ID-TIMS on bulk samples. REE patterns determined by ELA-ICP-MS for the scheelites vary smoothly as a function of atomic number. Most of t he patterns are hump-shaped but others are rather flat except for posi tive Eu anomalies. This suggests that the hydrothermal fluids that for med the scheelites did not have a common composition and source. (C) 1 997 Elsevier Science B.V.