Secondary fluorescence in electron-microprobe analysis near grain boundaries, with emphasis on sphalerite and Au-Ag-Hg alloy

Simplified modeling of electron interactions, X-ray absorption, and secondary fluorescence effects together with a series of test analyses and modeling using CalcZAF/FANAL confirm that electron-microprobe compositions of sphalerite and Au-Ag-Hg alloy are susceptible to analytical contamination from adjacent sulfide grains lying within extensive volumes of secondary fluorescence (i.e., radii of approximately 50 and approximately 20 mu m, respectively). Compositions of sphalerite encapsulated in pyrite for geobarometric analysis, and from a series of points approaching a boundary with pyrite (i.e., concentration-gradients of Fe) for equilibration studies, may give spuriously high Fe contents due to secondary fluorescence of Fe in the adjacent pyrite, and thereby lead to erroneous interpretations. Similarly, anomalously high observed Cu contents of free sphalerite occurring in the Trout Lake Cu-concentrate can be attributed to secondary fluorescence of Cu in chalcopyrite disease in the sphalerite grains or of Cu in adjacent chalcopyrite, and not the preferential recovery of sphalerite with Cu-rich compositions during chalcopyrite or Cu flotation. Potentially false-element concentrations due to secondary fluorescence near grain boundaries is an inherent pitfall in the analysis of very small mineral grains, near grain boundaries, and in all diffusion studies. The factors determining the potential magnitude of secondary fluorescence for the specific mineral pair under investigation should be routinely considered, and probably modeled, when operating near grain boundaries (i.e., within approximately 100 mu m), either by the simplified method used here or by a computer modeling program such as CalcZAF/FANAL.