Abstract
The Greenhills Complex is a composite ultramafic to mafic layered igneous intrusion located on the Bluff Peninsula, 30 km south of Invercargill. Two principal lobes range from dunite at their base through olivine clinopyroxenite to gabbro. On the basis of crystallisation sequence, mineral composition and melt inclusion compositions (Spandler et al., 2000), the intrusion appears to have formed from primitive hydrous basaltic magmas in feeder chambers beneath an island arc volcanic complex as part of the Permian Brook Street Terrane.
Dunite of the south lobe is well exposed in the Greenhills Quarry. The dominant primary mineral is cumulus olivine with minor cumulus chromian spinel, intercumulus clinopyroxene and rare orthopyroxene and plagioclase. A series of dykes of varying composition cross cut the dunite, forming small zones of plagioclase-rich rocks.
A number of zircon grains extracted from a plagioclase-rich felsic dike intruding the dunite of the Greenhills complex have been dated using LA-ICP-MS analysis of U/Pb isotope ratios to an age of 261.5 ± 2.3 Ma. This is consistent with the minimum age of 246 ± 10 Ma obtained from K/Ar dating of hornblende in gabbroic rock by Aronson (1968) and the age reported by Spandler et al. (2003) of 265 Ma obtained by Kimbrough et al. (1992) using U/Pb isotope ratios.
Spandler et al. (2000) report the occurrence of platinum-group minerals (PGM) in chromian spinel at one location just south of the Greenhills dunite quarry. LA-ICP-MS analysis of chromian spinel in this study has revealed ppb concentrations of PGE and ppm concentrations of gold in a small number of chromian spinel grains.
Olivine in the dunite is fractured and partially replaced by serpentine and carbonate. XRD analysis of nine alteration rims of the dunite and one white vein has identified calcite, chlorite, chrysotile and other rarer phyllosilicates. L.O.I. analysis reveals the average volatile component of the dunite to be 7.74 wt% while bulk rock volatile element (CNS) analysis reveals the average concentration of naturally occurring carbonates in the dunite to be 0.6 wt% per sample with some samples having as much as 4.5 wt% carbonate and up to 11.5 wt% H2O.
Recent studies have shown that natural rates of carbonation of olivine-rich peridotite (Keleman & Matter, 2008) and serpentinite mine tailings (Wilson et al., 2009) are much faster than previously thought. These rates should be further enhanced in the subsurface because the carbonation reactions are exothermic and, in the case of serpentine-bearing rock, lead to increased porosity. If the dunite has sufficient fracture permeability at depth, it is conceivable that CO2 could be injected at a flow rate sufficient for heating due to carbonation to balance cooling due to advection and diffusion in order to maintain an optimal temperature for rapid reaction.
Alternatively the natural rates of carbonation of olivine at the surface can be enhanced by simply crushing and spreading on the Earth’s surface (Schuiling and Krijgsman, 2006). When mixed with a slow release fertiliser, crushed dunite could potentially act as a cheaper substitute for lime.