The Diamond-Bearing Ultrapotassic (Lamproitic) Rocks of the West Kimberley Region, Western Australia

摘要: The diamond-bearing ultrapotassic (lamproitic) rocks of the West Kimberley region Western Australia are Miocene age (20 m.y.) and comprise more than 100 separate pipes, plugs, sills, rare dykes. recently-discovered ‘kimberlitic’ - olivine lamproites grade petrographically through leucite-bearing olivine-diopside lamproite to better known leucite with phlogopite, diopside potassic richterite as dominant ferromagnesian phase. Mineral compositions in overlap those lamproites: lies range Mg93-77, phlogopite is Ti-rich commonly strongly zoned Ti-rich, Al-poor types, rich Ti poor Al richterite. heavy mineral suite dominated by chrome spinel subordinate pyrope; picroilmenite absent. Rare mantle xenoliths mostly refractory lherzolite or harzburgite depleted lithophile elements. The from ultrabasic (20-29% MgO) basic (5% MgO less) compositions: all have very high K2O contents (4-12%), K2O/Al2O3 (average 1.2) K2O/Na2O (typically greater 10) ratios. All saturated oversaturated silica, SiO2 Al2O3 increase decreasing content Mg/(Mg+Fe) ratio. characterised F, Ba, Rb, Sr, Pb, Th, U, Ti, Zr, Nb light earth elements (LREE), low concentrations CaO, CO2 Sc. REE patterns highly fractionated enriched LREE at 500-2000x chondritic abundances HREE (4-6x chondritic). Rb/Sr 87Sr/86Sr ratios (0.3-0.4, 0.711-0.720) 143Nd/144Nd (eNd = -7 -15), indicate derivation an ancient, large-ion-lithophile-element (LILE)-enriched source. The inferred been derived partial melting phlogopite-rich, metasomatised garnet diopside-poor lherzolite/harzburgite under H2O- F-rich (CO2-poor) conditions. component this peridotite source had previously experienced long term enrichment LILE. comparative rarity ‘kimberlite indicator’ minerals thought be due existence (garnet-clinopyroxene-poor) beneath region. While broadly kimberlitic show significant geological, petrological chemical differences kimberlite. These include association leucite, presence amphibole, absence primary carbonate, ‘indicator’ suite. Diamond grades ‘trace’ certain ‘subeconomic’ (>5 cts/100 tonnes) indicating that represents a potentially important diamond.