Proton-induced X-ray and gamma-ray emission analysis of unopened fluid inclusions

Abstract

An external beam (avg diam 20 mu m) of protons from a 4.0-MV Van de Graaff accelerator has been used to excite, simultaneously, characteristic X-rays and gamma rays from a wide range of elements contained in individual, unopened, fluid inclusions in doubly polished mineral plates. We report qualitative and semiquantitative compositional data obtained using the proton-induced X-ray (PIXE) and gamma-ray (PIGE) emission techniques for selected, near-surface (10-40 mu m deep) inclusions from three major, well-studied ore deposits.Proton-induced X-ray emission analysis of numerous low-salinity, CO 2 -bearing inclusions in quartz from the Panasqueira wolframite-quartz vein system, Portugal, did not detect Na, Cl, or K, presumably because of X-ray attenuation, whereas proton-induced gamma-ray emission analysis revealed Na in all inclusions. The gamma-ray spectrum yielded by an inclusion containing a daughter or accidentally trapped crystal resembling muscovite confirmed the presence of F, Na, and Al; and several X-ray spectra recorded K, Ti, Mn, Fe, Cu, Zn, Rb, and Sn.Proton-induced X-ray emission analyses of multiphase fluid inclusions in quartz from a molybdenite-rich veinlet in the core of the Bingham, Utah, porphyry Cu(-Mo) deposit confirmed the presence of the S, Ca, Cl, K, and Fe, expected from the occurrence of anhydrite, sylvite, and hematite daughter crystals. Quantification of the abundances of the lighter elements was subject to considerable error, but the data permit reasonable estimates of several heavier elements: Mn, 1,700; Fe, 28,000; Cu, 700; Zn, 1,800; Pb, 700; and Br, 200 (all in ppm).Two fluid inclusions were examined in secondary spodumene from the Tanco, Manitoba, rare element pegmatite: both contained a highly birefringent daughter crystal initially assumed to be the new mineral diomignite (Li 2 B 4 O 7 ). The proton-induced gamma-ray emission spectra were compared to those obtained from Li 2 B 4 O 7 sealed within SiO 2 glass capillaries. Boron was clearly detected in the capillaries but not in two large daughter crystals, one of which contains major amounts of Ca (tens of wt %) and is tentatively identified as calcite.Although these data are preliminary and mainly qualitative, they demonstrate that the combined proton-induced X-ray and gamma-ray emission method has great potential for fluid inclusion studies. Considerable improvements in instrumentation and techniques for fluid inclusion analysis are anticipated, as are more accurate determinations through precise standardization employing synthetic inclusions of known composition.