Synchrotron X-ray fluorescence analysis of zoned carbonate gangue in Mississippi Valley-type deposits (U.S.A.)

Abstract

Synchrotron X-ray fluorescence analysis (SXRFA) was used to analyze the concentrations of Fe and Mn in breccias and gangue carbonate minerals of the Right Fork area in the Central Tennessee zinc district. The technique permits the detection of elements with atomic numbers > 20 at the ppm level. The detection limits for Fe and Mn are 2–3 ppm and the analytical precision is estimated to be within ±10–20% (relative), based on a comparison with standards previously analyzed using an electron microprobe (EMP). Individual zones in gangue dolomite, recognized using cathodoluminescence (CL) microscopy, were found to contain different concentration levels of Fe (ranging from 20 to 36,000 ppm) and Mn (ranging from 30 to 900 ppm) depending on the nature of the host mineral and the spatial distribution of the sample within the Right Fork area. Small differences in trace-element composition may be responsible for visually observable differences in the intensity of the luminescence associated with the zones. Bright-red luminescence of dolomite was not suppressed in zones having Fe/Mn ratios as high as 12, at least when the Fe concentration was <3000 ppm. The concentrations of Fe and Mn in zoned dolomite gangue were from 4 to 40 times higher than their concentrations in the adjacent breccia fragments; hence, it is unlikely that these elements were derived locally by the mineralizing fluid. The presence of fluid inclusions in the samples did not seem to affect the elemental analyses, but areas which contain solid inclusions should be avoided because they can have a significant influence on trace-element concentrations. Combined with information obtained using CL microscopy, isotope analysis and other techniques, SXRFA offers the possibility of tracing zoned carbonate gangue over long distances and, perhaps, even determining the direction of fluid flow.