SPINELS FROM THE ELLENDALE OLIVINE LAMPROITES (WESTERN-AUSTRALIA) SIGNIFICANCE FOR DIAMOND DISTRIBUTION AND EMPLACEMENT HISTORY

Several of the olivine lamproite pipes in the Miocene Ellendale Volcan ic Field (Western Australia) carry diamonds in subeconomic concentrati ons. The observed systematic decrease in diamond grades from the early produced tuffs towards the later produced lava lakes suggests combust ion of diamonds in the late magmatic phase as a result of increased ox ygen fugacity. In order to examine variations in oxygen fugacity withi n and between the pyroclastic and magmatic phases a detailed study of primary spinels was carried out. Compositional variation and zoning of primary spinels follow the magmatic Trend-2 (titanomagnetite trend) O f MITCHELL (1986). It is characterized by increasing FFM-ratios (Fe2+/ [Fe2+ + Mg]) of spinel, probably mainly controlled by olivine crystall ization. In spinels from the slowly cooled lava lake centers a sharp d epletion in Cr on the tetrahedral site is observed. It is replaced by Fe3+ and Ti, together with minor amounts of V and Mn on the tetrahedra l and octahedral sites respectively. Estimates of oxygen fugacity and temperatures based on olivine - spinel equilibria indicate a late stag e oxidation event at near solidus conditions which affected only the s lowly cooled central parts of the lava lakes whereas the tuffs and the marginal parts of the lava lakes were emplaced at temperatures about 1050 - 1250-degrees-C under more reducing conditions at or slightly be low the FMQ-buffer. Thus, the diamond distribution in the Ellendale la mproites is not controlled by variations in oxygen fugacity.