Carbonatite veins in lamprophyre dykes, Mt Tapuaenuku, New Zealand.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Carbonatites are rare igneous rocks that are predominantly composed of carbonate minerals. At Mount Tapuaenuku, Inland Kaikoura Ranges, New Zealand, a series of highly alkaline lamprophyre dykes with subordinate calcite and dolomite carbonatite veins intrudes the Tapuaenuku Igneous Complex (TIC). Crystallisation of the Tapuaenuku Igneous Complex produced volatile- and alkali-enriched residual melts present as a series of lamprophyres exposed in the TIC. Cross cutting these dykes are planar laminate carbonatite veins and chaotic non-directional carbonatite veinlets. Previous studies of the TIC field classified these veins as carbonatites; however, indepth investigation of the petrography, geochemistry and petrogenesis of the carbonatites was not completed prior to this thesis. This thesis presents a detailed mineralogical, chemical and stable isotopic investigation of the TIC carbonatite veins that outcrop in the Gut Stream area. The veins exhibit comb and crustiform textures and are primarily composed of rhombohedral limpid calcite and turbid calcite of igneous origin. X-ray diffraction results indicate that the carbonate present in the veins is a mixture of predominantly calcite and subordinate dolomite. Petrographical investigation further indicates accessory quartz, epidote and aegirine are common in the calcite dominated carbonatite veins. The Tapuaenuku carbonatites have a wide range of δ13CPDB (-1.88 to -4.32‰) and δ18OSMOW (+6.09 to +16.75‰) values. Vein type and mineralogy appears to be correlated with oxygen isotope compositions; limpid calcite laminates range from 12.00 to 13.99 ‰, with turbid laminate calcite veins ranging from 7.35 to 8.53 ‰, both limpid and turbid have similar δ13C values and the chaotic spider veinlets exhibiting a larger variation in δ18O ca. +11.41 to +15.15‰ and lower δ13C values of -4.00 to -4.32‰ than laminated veins. Carbon (δ13C) and oxygen (δ18O) isotopic composition of the carbonatite veins and veinlets are compatible with a magmatic carbonate melt/carbohydrothermal fluid origin with δ18O variation attributed to limpid and turbid carbonate kinetic fractionation and isotopic exchange with δ18O magmatic or hydrothermal fluids. The presence of carbonatite veins in the TIC provide insights into the magmatic and processes that generated the TIC in the mid-Cretaceous. The TIC carbonatites represent the second carbonatite occurrence in New Zealand after the Haast Alpine Schist carbonatites and a new site to the list of the 527 globally distributed carbonatite occurrences.