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

Abstract

The diamond-bearing ultrapotassic (lamproitic) rocks of the West Kimberley region of Western Australia are of Miocene age (20 m.y.) and comprise more than 100 separate pipes, plugs, sills, and rare dykes. The recently-discovered ‘kimberlitic’ rocks - olivine lamproites - grade petrographically through leucite-bearing olivine-diopside lamproite to the better known leucite lamproites with phlogopite, diopside and potassic richterite as the dominant ferromagnesian phase. Mineral compositions in the olivine lamproites overlap those in the leucite lamproites: olivine lies in the range Mg 93-77 , phlogopite is Ti-rich and commonly strongly zoned to Ti-rich, Al-poor types, and diopside is rich in Ti and poor in Al as is potassic richterite. The heavy mineral suite is dominated by chrome spinel with subordinate chrome diopside and rare pyrope; picroilmenite is absent. Rare mantle xenoliths are mostly refractory lherzolite or harzburgite depleted in lithophile elements. The West Kimberley lamproites range from ultrabasic (20-29% MgO) to basic (5% MgO or less) compositions: all have very high K 2 O contents (4-12%), and high K 2 O/Al 2 O 3 (average 1.2) and K 2 O/Na 2 O (typically greater than 10) ratios. All are saturated to oversaturated in silica, and SiO 2 and Al 2 O 3 contents increase with decreasing MgO content and Mg/(Mg+Fe) ratio. The suite is characterised by very high contents of F, Ba, Rb, Sr, Pb, Th, U, Ti, Zr, Nb and light rare earth elements (LREE), and very low concentrations of CaO, CO 2 and Sc. REE patterns are highly fractionated and enriched in LREE at 500-2000x chondritic abundances with very low abundances of HREE (4-6x chondritic). Rb/Sr and 87 Sr/ 86 Sr ratios are high (0.3-0.4, 0.711-0.720) and 143 Nd/ 144 Nd ratios are very low (ε Nd = -7 to -15), and indicate derivation from an ancient, large-ion-lithophile-element (LILE)-enriched mantle source. The West Kimberley rocks are inferred to have been derived by partial melting of phlogopite-rich, metasomatised garnet and diopside-poor lherzolite/harzburgite under H 2 O- and F-rich (CO 2 -poor) conditions. The depleted component of this peridotite source had previously experienced long term enrichment in LILE. The comparative rarity of ‘kimberlite indicator’ minerals is thought to be due to the existence of depleted (garnet-clinopyroxene-poor) peridotite beneath the region. While broadly kimberlitic the West Kimberley lamproites show significant geological, petrological and chemical differences to kimberlite. These include the association with leucite, presence of amphibole, absence of primary carbonate, and the rarity of garnet and picroilmenite from the ‘indicator’ suite. Diamond grades range from ‘trace’ in certain leucite lamproites to ‘subeconomic’ (>5 cts/100 tonnes) in the olivine lamproites indicating that the lamproite suite represents a potentially important primary source of diamond.