Strata-bound copper-iron sulfide mineralization in a Proterozoic front arc setting at Boksputs,Northwest Cape,South Africa — a possible Besshi-type deposit

A low-grade, copper-iron sulfide deposit is present at Boksputs in the Northwest Cape, South Africa. It occurs in the Proterozoic amphibolite belt, the Areachap Group, along the eastern margin of the Namaqua mobile belt.The mineralization, which is dominated by pyrite, chalcocopyrite, sphalerite, and magnetite, occurs as disseminated ore and thin layers of massive-type ore in the lower massive amphibiolite and associated amphibole gneisses of the Kraalkop antiform and synform.Chemically, the host-rock amphibolite resembles low-K arc tholeiite and calc-alkaline volcanics, whereas the upper amphibolite is calc-alkaline.The sulfide mineralization and the host-rock composition is explained in terms of modern plate tectonic principles. It is concluded that the Boksputs mineralization is a syngenetic, strata-bound, copper-iron deposit which to some extend resembles Besshi-type deposits. It is associated with low-K arc tholeiite which is tectonically related to a front arc environment or a rifted arc system along the eastern margin of the Namaqua mobile belt.     

Source mechanisms and microfracturing during uniaxial cycling of rock

Focal mechanisms for 116 acoustic emission events associated with uniaxial cyclic loading of Westerly granite have been investigated using P-wave first-motions. Polarities were observed at eight transducers positioned on the sample to provide good azimuthal coverage. The P-wave radiation patterns can be fit by a quadrupole source but not by a simple pure tensional source. The predominant orientations of the two nodal planes for the quadrupole fit were subparallel to and perpendicular to the ₁ direction. Events with close hypocenters, i.e., within a radius of 3 mm, display a remarkable similarity in focal mechanisms irrespective of the cycle or time within a cycle at which they occur. The polarity observations of 48 of 116 acoustic emission events could be fit with the simple quadrupole indicating that the microfracturing processes even in uniaxial compression often simplistically portrayed as propagating axial cracks are indeed quite complex.     

Terrestrial,meteoritic, and lunar titanium isotopic ratios revaluated: evidence for correlated variations

Variations in the titanium isotopic ratios of various solar system materials have been reported by three laboratories. In this work we compare these data (including some new Allende inclusion values reported here) on a common basis. The data of all three groups for terrestrial samples have been found to be consistent within about ten parts per ten thousand for all ratios. The considerable disagreement in reported titanium isotopic anomalies for Allende materials has been investigated by renormalization of all published high-precision titanium isotopic ratio data to various bases in order to assess the nature, number, and loci of possible anomalies. On this basis, variations appear to be present in⁴⁶Ti and⁴⁷Ti, as well as in⁵⁰Ti. All materials reported to date can be divided into two main groups, with the only exceptions being about 10% of Allende inclusions. All but one of the materials reported to date are interpretable as mixtures of a material similar to C-1 and varying amounts (up to about 0.5% of⁵⁰Ti) of a material characterized by⁵⁰Ti/⁴⁶Ti?6 and⁴⁶Ti?⁴⁷Ti+⁴⁸Ti+⁴⁹Ti. Inclusion EK 1-4-1 requires a further component (up to about 0.5%⁴⁶Ti) characterized by⁴⁶Ti/⁴⁷Ti? 1.3and⁴⁷Ti?⁴⁸Ti+⁴⁹Ti+⁵⁰Ti.     

Incipient magma chamber formation as a result of repetitive intrusions

An analytical solution for periodic magma intrusions in conduits was developed to study the onset of shallow magma chamber formation. The solution is based on determining when a repetitive series of intrusions can cause the wall rock of a conduit to reach its melt temperature. The results show that magma chamber formation in conduits is a strong function of the volume rate of intrusion and that magma chamber formation is likely when the intrusion rate exceeds 10^?3 km³/ yr. which agrees with observations by other investigators. Once this critical value of intrusion rate is reached, magma chambers are likely to begin forming after only a few intrusive pulses (less than ten). Results for both cylindrical conduits and dikes show cylindrical conduits are more favourable for the formation of shallow magma chambers.     

Corundum- and kyanite-bearing anatexites from the Precambrian of tanzania

In the Precambrian near Morogoro (East Tanzania) there are anatectic gneisses which are composed of two different domains: (1) Medium grained gneiss with the assemblage (+ rutile + baddeleyite): albite (ab₉₅an₀₃or₀₂) + muscovite + phlogopite_ss + corundum, and albite (ab₈₈an₀₉or₀₃ + kyanite or sillimanite + phlogopite_ss. (2) Coarse grained nests of corundum and antiperthite with minor albite, muscovite, phlogopite, rutile, baddeleyite, and tourmaline. The bulk composition of the antiperthite id ab₇₅an₀₂or₂₃, the composition of the Na-phase ab₉₅an₀₃or₀₂, that of the K-phase ab₁₃an₀₀or₈₇. Phase relationship of the rock can be modelled in the Na-rich, Si-undersaturated part of the KNASH system. P-T conditions are close to the intersection of the melting curve muscovite_ss + H₂O = corundum + liquid with the kyanite-sillimanite transition, i.e. 7.7 kb/695°C assuming a_H2O = 1.