Evolution of continental crust in southern Africa

Nd isotopic data from the Zimbabwe and Kaapvaal cratons and the Limpopo, Kalahari, Namaqualand and Damara mobile belts imply that over 50% of present-day continental crust in this region had separated from the mantle by the end of the Archaean and that< 10% of continental crust of southern Africa has formed in the last 1.0 Ga. Such a growth rate implies that average erosion rates through geological time were high and that evolution of continental crust has been dominated by crustal growth prior to 1.4 Ga, and crustal reworking since that time. The evolution of average crust is not represented directly by clastic sediment samples but may be determined from sediment analyses if both the time of orogeneses and the average erosion rate are known. Both trace element data from southern Africa granitoids and the high erosion rates implied by the isotopic study suggest that growth of continental crust in the Archaean was by underplating rather than lateral accretion, but arc accretion was the dominant mechanism after 2.0 Ga.     

Direction changes in thrusting of the Schistes Lustrés in Alpine Corsica

The direction of thrusting contemporaneous with high pressure-low temperature (HP/LT) metamorphism of the ophiolite Schistes Lustrés nappes in Cap Corse, Alpine Corsica has changed from being towards the northwest to towards the southwest during Upper Cretaceous obduction.Similar anticlockwise changes in thrusting have been observed in other regions of Alpine Corsica, Calabria and Southern Betic Cordilleras. A model is proposed for the Alpine evolution of this part of the Western Alps involving a sinistral component of transcurrent movement added to the northwest thrusting. These events have been followed by Eocene backthrusting of nappe of southern-Alpine origin in northwest Cap Corse towards the southeast with associated backfolding of the underlying Schistes Lustrés.     

Associated chemical and carbon isotopic composition variations in diamonds from Finsch and Premier kimberlite,South Africa

The carbon isotopic composition of 66 inclusion-containing diamonds from the Premier kimberlite, South Africa, 93 inclusion-containing diamonds and four diamonds of two diamond-bearing peridotite xenoliths from the Finsch kimberlite, South Africa was measured. The data suggest a relationship between the carbon isotopic composition of the diamonds and the chemical composition of the associated silicates. For both kimberlites similar trends are noted for diamonds containing peridotite-suite inclusions (P-type) and for diamonds containing eclogite-suite inclusions (E-type): Higher δ¹³C P-type diamonds tend to have inclusions lower in SiO₂ (ol), Al₂O₃ (opx, gt), Cr₂O₃, MgO, $\text{Mg}\text{(Mg + Fe)}$ (ol, opx, gt) and higher in FeO (ol, opx, gt) and CaO (gt). Higher δ¹³C E-type diamonds tend to have inclusions lower in SiO₂, Al₂O₃ (gt, cpx), MgO, $\text{Mg}\text{(Mg + Fe)}$ (gt), Na₂O, K₂O, TiO₂ (cpx) and higher in CaO, $\text{Ca}\text{(Ca + Mg)}$ (gt, cpx).Consideration of a number of different models that have been proposed for the genesis of kimberlites, their xenoliths and diamonds shows that they are all consistent with the conclusion that in the mantle, regions exist that are characterized by different mean carbon isotopic compositions.     

Heavy metals distribution in Jamaica Bay sediments

Ninety-two sediment samples collected at 53 stations in Jamaica Bay from November 1970 to October 1973 were analyzed for Pb, Cr, V, Cu, Cd, Zn, Ni, Co, and percent organic carbon (ORC). Selected samples were analyzed for Hg. The concentrations of all metals correlate with each other and with organic carbon. Vanadium, cobalt, and nickel stand out as a separate component and are enriched relative to other metals in areas affected by petroliferous pollution. The metal concentrations are strongly influenced by the strength of tidal currents and proximity to pollution sources. Amount of rainfall and/or season affects metal concentrations in Grassy Bay (a deep restricted body of water within Jamaica Bay).     

Impeller speed,air, and power requirements in flotation machine scale-up

Experimental and operational data, flotation criteria, machine design and scale-up principles are reviewed and combined in a chart encompassing scale-up detail for various machine types. Topics requiring further study are mentioned.     

Rare-earth abundances in indian kimberlite

Abundances of some rare-earth elements (REE), together with Hf, Ta, Th, and Ba have been determined by neutron activation analyses in twelve kimberlite samples from Peninsular India. The kimberlites have fractionated REE patterns with enrichment of light rare earths; La/Yb ratio varies from 39 to 166. A systematic relationship exists between the total rare-earth abundances and the petrochemistry of the rocks. The rare-earth abundances observed in the kimberlites are consistent with their derivation by partial melting of a hydrous garnet peridotite mantle and subsequent fractional crystallization of the melt.     

The application of spinel-bearing metapelites to P/T determinations: An example from South India

Metasedimentary migmatites from the Archean charnockitic terrain of South India contain the five phase equilibrium assemblage spinel-cordierite-garnet-corundum-sillimanite. The assemblages is a result of anatexis which has generated a silica-deficient anhydrous restite. Peak metamorphic conditions are defined by the intersection of two divariant reactions in the A1₂O₃-SiO₂-FeO-MgO system at which the five phases coexist. These reactions are univariant and their intersection invariant if the Fe/Mg ratio of at least one femic phase is fixed.The location of the invariant point in P/T space is derived from extracting standard stage thermodynamic data from published equilibria experiments in the system Al₂O₃-SiO₂-FeO. Microprobe analyses of coexisting spinel, almandine and cordierite specify the Fe/Mg distributions between phases and allow the computation of the five phase invariant point for =P _total (770° C, 5.9 kb) and =O (740° C, 4.8 kb). A low , implied by evidence of extreme anatexis, indicates a P/T field of T=740±20° C and P_total=4.8±0.5 kb which is consistent with the field of equilibration of interlayered charnockites computed from garnet-hypersthene and garnet-plagioclase pairs.     

Quantitative analysis of anomalous sources and geochemical signatures in the Walker Lake quadrangle of Nevada and California

A new quantitative transportation model for geochemical elements in stream sediment samples is proposed and used to estimate mineral sources of anomalous concentrations and to adjust both location and concentration for the transportation effect. Subsequent to transportation analysis, data on 14 geochemical elements in the Walker Lake quadrangle are analyzed for geochemical signatures for three types of mineralization environments: epithermal gold-silver deposits, porphyry-skarn copper deposits, and a mixed occurrence. Geochemical signatures are analyzed using a multidimensional scaling method referred to as the p_ijk model, which is a factor-like model generalized to include nonlinear associations, as well as linear, among triple variables (elements). A result of this analysis is that Pb is the best indicator for epithermal gold-silver mineralization, Mo and Zn for mixed mineralization, and Cu for porphyry-skarn mineralization.