Browsing by Author "Harris, Christopher"

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    O- And H-Isotope Study of the Carbon Leader Reef at the Tau Tona and Savuka Mines (Western Deep Levels) South Africa: Implications for the origin and evolution of Witwatersrand Basin Fluids
    (2010) Grové, D; Harris, Christopher
    The Carbon Leader Reef is a ~1 m thick conglomeratic unit with a thin bituminous base, and is one of the major gold-bearing conglomerate horizons in the Central Rand Group of the Witwatersrand Basin. It consists of alternating conglomerate and quartzite layers and was metamorphosed under greenschist facies conditions. Bulk rock δ18O and δD values of the Carbon Leader Reef range from 7.2‰ to 10.8‰ (mean = 10.6‰) and −27 to −65 (mean = −41‰), respectively. The narrow range in δ18O values, together with the lack of correlation between the δ18O value and the modal % matrix minerals, suggests that the original detrital minerals and the authigenic matrix minerals have similar oxygen isotope composition. Calculated δD values of the fluid, assuming that it was in isotope equilibrium with the bulk rock at the estimated peak metamorphic temperature of 350°C, range from −1‰ to −40‰. This suggests that the ultimate origin of the fluid was a mixture of meteoric and metamorphic water. These data are not consistent with the ingress of large quantities of externally derived fluid post burial metamorphism. Gold-rich sections of the Carbon Leader Reef do not have significantly different δ18O values than the adjacent gold-poor section, which suggests that gold mineralization is not related to interaction with significant amounts of externally derived hydrothermal fluids.
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    Oxygen isotope composition of garnet in the peninsula granite Cape granite suite South Africa:constraints on melting and emplacement mechanisms
    (2010) Harris, Christopher; Vogeli, Jacques
    Garnet is an accessory mineral in the Cape Granite Suite, and garnet δ18O values in the Peninsula Granite range in from 10.0 to 11.4‰ (mean 10.6 ± 0.6‰, n = 15). These values are consistent with the garnet being produced during incongruent melting of a metapelitic source that has a similar O-isotope composition to the Malmesbury Group. Peninsula Granite quartz δ18O values range from 13.2 to 14.0‰ (mean 13.6 ± 0.3‰, n = 17), at the high end of the range previously observed for the Cape Granite Suite. These high δ18O values are consistent with the source of the Peninsula Granite magma having a greater component of clay minerals, which have inherently high δ18O values. Garnet has a high closure temperature (>800 oC) to oxygen diffusion and its δ18O value should, therefore, correlate closely with that of the source. Quartz has a significantly lower closure temperature (~550 oC) than garnet, and sub-solidus oxygen isotope re-equilibration between quartz and feldspar during slow cooling ought to result in a greater variation in quartz δ18O values compared to that of garnet. That the reverse is the case suggests that granite magmas were derived from a moderately heterogeneous source, as expected for metasedimentary rocks. This source underwent melting to produce different batches of granitic magma containing entrained garnets of slightly different δ18O value. Magma batches were subsequently mixed and homogenized before and/or during the emplacement process, resulting in a narrower spread of quartz δ18O values.