LEAD ISOTOPE ZONING IN GALENA: AN ION MICROPROBE STUDY OF A GELENA CRYSTAL FROM THE BUICK MINE, SOUTHEAST MISSOURI.

Ion microprobe techniques have been used to determine the variations of Pb isotope ratios within a single octahedral crystal of galena from the Buick mine, southeast Missouri. The The crystal shows uniform concentric zonation of Pb isotope ratios, with a total variation of approximately 3 percent in **2**0**8Pb/**2**0**6Pb and approximately 4 percent in **2**0**7Pb/**2**0**6Pb and with the crystal margin being less radiogenic (lower 206/204) than the core. The range in the observed **2**0**8Pb/**2**0**6Pb ratio is larger than that observed previously for bulk galena analyses from throughout the mine and is almost as large as the total variation reported for the whole southeast Missouri ore district. The ion probe results are demonstrated to be accurate to 0. 1-0. 2 percent by comparison with conventional mass spectrometric analyses performed on microsamples of the same crystal. The ion probe technique holds promise for establishing a Pb isotope chronostratigraphy for the ore-bearing solutions, which will be useful in delineating temporal and spatial aspects of the fluid circulation patterns.