Final tabulation of sulfur--isotope analyses

Uranium host rocks commonly contain two or more types of sulfides differing in mineral species, crystal size or habit, firmness of attachment to detrital grains, or other parameters. It may be assumed that different types of sulfides may have formed at different times and under different conditions and may thus differ in isotopic composition. Since no method of concentration is 100 percent effective, it is likely that different methods (flotation, heavy liquids, sieving, etc.) should preferentially concentrate one type or another in relation to one or more of these parameters and thus to isotopic composition of sulfur. It has been found, however, that variations in isotopic composition of splits of concentrates carefully prepared to be as nearly identical as possible tend to obscure any differences due to possible selectivity of preparation methods. Pairs of sulfide-concentrate samples prepared from the same field sample are listed in the tabulation, along with brief statements of preparation methods. These results suggest that differences (here considered to be a combination of analytical and sampling error) between supposedly identical samples are large enough to obscure all but the greatest differences due to sample preparation and that the isotopic composition of sulfur in disseminated sulfides of no rock sample represented by this tabulation is known within about two permil. Fractions prepared from the same field sample by highly selective methods, such as hand picking, showed considerable differences. (U.S.)