PARTITIONING OF TRACE ELEMENTS IN THE HEAVY MINERAL FRACTION OF OUACHITA MOUNTAIN SHALES

The whole-rock geochemistry of shales has been considered an average of continental crustal geochemistry since the time of Goldschmidt. It has largely been assumed that trace elements reside in the clay minerals that comprise the major component of shales. However, certain heavy minerals concentrate trace-elements to the degree that the overall whole-rock chemistry is influenced by minor variations in their accessory fraction. Samples were collected from nine formations within the Ouachita fold belt. These shales are of deep-water marine clastic origin, and range in age from late Cambrian through late Devonian. Aliquot-pairs consisting of a whole-rock split and a whole-rock split with heavy minerals removed (Li-metatungstate density separation process) were analyzed by XRF. Trace element analyses of Paleozoic deep-water shales from these nine formations indicate significant partitioning of Ti, Zr, Y, Ce, La and Th in the heavy mineral fraction. The aliquots with the heavy mineral fraction removed contain significantly reduced concentrations of these trace elements as compared to the whole-rock sample. A diverse suite of heavy minerals was identified, including zircon, monazite, rutile, tourmaline, and Fe-oxides, and are the most likely mineralogical sites for these elements. The partitioning of these elements and other lithophile trace elements in resistant heavy minerals has implications for interpreting provenance based upon whole-rock geochemistry of shales.