The kinetics of fluoride uptake by synthetic hydroxyapatite

The kinetics of fluoride uptake by synthetic hydroxyapatite from aqueous solution has been studied. Experiments involved exposing 0.1 g of synthetic hydroxyapatite to 5 cm3 of sodium fluoride solution in the concentration range 100-1000 ppm fluoride and determining fluoride concentration at regular time intervals with a fluoride ion-selective electrode. In all cases, uptake was found to follow pseudo-second order kinetics with correlation coefficients of at least 0.998; all systems equilibrated by 24 hours with equilibrium uptake values that varied with the initial fluoride concentration. The kinetic results differ from those previously reported for much lower concentrations of fluoride, but in the present case, the concentrations were of clinical relevance, as they are those used in fluoride-containing dental products. Further work is necessary to determine how well these findings model uptake by natural hydroxyapatite and hence the extent to which they might apply in vivo

The Uitkomst intrusion and Nkomati Ni-Cu-Cr-PGE deposit, South Africa: trace element geochemistry, Nd isotopes and high-precision geochronology

The Uitkomst intrusion is a tubular mafic-ultramafic layered body that hosts one of South Africa’s largest Ni-Cu-Cr-PGE deposits, Nkomati. The sulphide ore occurs in the form of massive lenses in the immediate quartzitic footwall and as disseminations within peridotite. The chromite ore forms an up to ∼10-m-thick layer in the lower portion of the intrusion. Uitkomst has generally been interpreted as a magma conduit, possibly related to the Bushveld event. Here, we present a new high-precision U-Pb zircon date of 2057.64 ± 0.69 Ma that overlaps with the age of the Merensky Reef of the Bushveld Complex and thus demonstrates a coeval relationship between the intrusions. Based on incompatible trace elements as well as O- and Nd isotope data (εNd −4.5 to −6.2), we show that the Uitkomst parent magmas were contaminated with up to 20% Archean upper crust prior to emplacement, and with up to 15% dolomitic country rock during emplacement. Ore formation at Nkomati was critically aided by substantial devolatisation and removal of dolomitic floor rocks leading to hydrodynamic concentration of sulphide and chromite during slumping of crystal mushes into the trough-like centre of the subsiding intrusion and its footwall

The Uitkomst intrusion and Nkomati Ni-Cu-Cr-PGE deposit, South Africa: trace element geochemistry, Nd isotopes and high-precision geochronology

The Uitkomst intrusion is a tubular mafic-ultramafic layered body that hosts one of South Africa’s largest Ni-Cu-Cr-PGE deposits, Nkomati. The sulphide ore occurs in the form of massive lenses in the immediate quartzitic footwall and as disseminations within peridotite. The chromite ore forms an up to ∼10-m-thick layer in the lower portion of the intrusion. Uitkomst has generally been interpreted as a magma conduit, possibly related to the Bushveld event. Here, we present a new high-precision U-Pb zircon date of 2057.64 ± 0.69 Ma that overlaps with the age of the Merensky Reef of the Bushveld Complex and thus demonstrates a coeval relationship between the intrusions. Based on incompatible trace elements as well as O- and Nd isotope data (εNd −4.5 to −6.2), we show that the Uitkomst parent magmas were contaminated with up to 20% Archean upper crust prior to emplacement, and with up to 15% dolomitic country rock during emplacement. Ore formation at Nkomati was critically aided by substantial devolatisation and removal of dolomitic floor rocks leading to hydrodynamic concentration of sulphide and chromite during slumping of crystal mushes into the trough-like centre of the subsiding intrusion and its footwall

Olivine and sulfur isotopic compositions of the Uitkomst Ni-Cu sulfide ore-bearing complex, South Africa: evidence for sulfur contamination and multiple magma emplacements

The Uitkomst Complex in northern South Africa is host to a Ni–Cu deposit containing a minerable reserve of 2.9 Mt massive ore with grades of 2% Ni, 1% Cu and 6 ppm Pt+Pd and an indicated plus inferred resource of 98 Mt disseminated ore with grades of 0.6% Ni, 0.2% Cu and 1 ppm Pt+Pd. Based on similar age and composition to that of the Bushveld Complex, the Uitkomst Complex is believed by some investigators to be a satellite body of the Bushveld Complex. It has a tubular shape with an exposed surface area of about 0.8×8 km and a thickness of up to 1000 m, apparently exploiting the bedding planes of gently dipping footwall sediments that include shale, quartzite and dolomite. The Uikomst Complex comprised of stratiform mafic and ultramafic rock units that from base to top include gabbro, sulfide-mineralized harzburgite, unmineralized harzburgite, pyroxenite, gabbronorite and gabbro. The compositions of olivine from different rock units are significantly different. Olivines from the gabbronorite unit are highly fractionated, containing <30 mol% Fo (forsterite, Mg2SiO4) and <300 ppm Ni. Olivines from the underlying pyroxenite and harzburgite units are much more primitive, containing 86–91 mol% Fo. Abrupt changes in the contents of Ni in olivine occur within the pyroxenite unit, and between the mineralized and unmineralized harzburgite units. The contents of Ni in olivine from the unmineralized harzburgite unit are between 2900 and 3600 ppm. The contents of Ni in olivine from the underlying mineralized harzburgite units are ∼1500 ppm lower. Olivines from the lower part of the pyroxenite unit are similar to olivines from the underlying harzburgite unit, whereas olivines from the upper part of the pyroxenite unit contain much lower Ni (<600 ppm). Numerical modelling suggests that the strikingly different compositions of olivine are related to different parental magmas with different MgO/FeO ratios and/or Ni contents. The δ34S values of the basal gabbro and unmineralized rock units range from −0.9‰ to 2.6‰, which are similar to typical mantle values (∼0±3‰). In contrast, the δ34S values of the sulfide ores in the harzburgite units are significantly lower, varying between −2.6‰ and −7.1‰. The low δ34S values of the sulfide ores indicate addition of 34S-depleted crustal sulfur. The obvious sources of such crustal sulfur are the Malmani dolomite and the Timebal Hill shale that contain pyrite with negative δ34S values up to −11‰ and −18‰, respectively. Sulfide saturation in the magma of the mineralized harzburgite units is thought to have resulted from addition of the crustal sulfur. The resulting immiscible sulfide liquid droplets were then concentrated in the base of the magma channel. Some of the sulfide liquid was expelled into the basal gabbro that separated the active magma channel from the footwall quartzite. New magma then entered the channel, displacing most of the resident magma to form the unmineralized harzburgite. The overlying pyroxenite, gabbronorite and gabbro units formed either by in situ differentiation of the same magma giving rise to the underlying unmineralized harzburgite or by subsequent emplacement of a more evolved magma