Mineralogy and petrology of the Aries diamondiferous Kimberlite Pipe, Central Kimberley Block, Western Australia

Aries is a deeply weathered micaceous kimberlite pipe (approximately 820 Ma) consisting of four lobes: South, Central, North, and North Extension. It is the largest (approximately 18 ha) and most diamondiferous of the few kimberlites currently known on the Australian continent, and is rich in country-rock (dolerite and quartzite) xenoliths. Three textural varieties of Aries kimberlites can be recognized, together with autoclastic breccias: (1) macrocrystal medium-grained; (2) aphanitic (less-than-or-equal-to 5 vol.% olivine macrocrysts); and (3) macrocrystal segregated. The kimberlites contain two generations of olivine pseudomorphs (30-40 vol.%), and two of phlogopite (up to 60 vol.%), in a groundmass of apatite, calcite, diopside, sphene, spinels, serpentine, talc, and accessory groundmass minerals including aeschynite [(Ce, Ca) (Ti, Nb)2O6], barite, ilmenite, monazite, rutile, siderite, and unidentified Nb-Fe-titanates. Phlogopite zoning is complex and differs from lobe to lobe, but general compositions and trends resemble phlogopites from kimberlites (TiO2 0.5-4 wt.%, Al2O3 9-16%); tetraferriphlogopite substitution is indicated by low Al in some grains. Diopside is low in Cr, Al, Na, and Ti, with high mg-number [molecular Mg/(Mg+Fe2+) approximately 93]. Apatite contains up to 17.5% SrO, calcite up to 1.7% SrO but little MgO or FeO, sphene up to 1.5% Nb2O5, and ilmenite 2-6% Nb2O5 and 16% MnO but no detectable MgO.Extremely complex morphological, textural, and compositional variations are present in spinels. They can be divided into five textural-genetic types: cognate Groundmass chromian spinels (Type G); Inclusions of chromian spinels in olivine macrocrysts (Type 1), probably representing either early phenocrysts or mantle xenocrysts: Macrocryst chromian spinels (Type M), probably representing xenocrysts; late-stage groundmass Ferrian spinels (Type F), derived from serpentinization of olivine; Alteration ferrian spinels (Type A), found as inclusions associated with siliceous melt inclusions, in Types I and M, and probably representing interaction of these earlier types with late-stage melts. Some of these, particularly Types M and F, show further textural sub-types with no obvious genetic significance.The pipe formed from several magma-pulses. All four lobes may contain at least one pulse in common, but Central and South Lobes include additional pulse(s) which yielded distinctive phlogopite zoning, whereas North Lobe and North Extension include pulse(s) which may have originated at higher mantle levels and yielded more evolved phlogopites.Aries most resembles South African Group II kimberlites mineralogically, certain West African micaceous kimberlites geochemically, and Group I kimberlites isotopically. A distinctive mantle source-region is implied by high Nb/U, Ce/Sr, Ce/P, Rb/Ba, and especially Nb/Zr ratios. Similar anomalous geochemical signatures are shared with two other contemporaneous (approximately 800 Ma) lamprophyric intrusions in the east Kimberley (at Maude Creek and Bow Hill), suggesting that a scattered alkaline province exists in the Kimberley Block, generated from a regionally anomalous mantle source.