Quartz latite rheoignimbrite flows of the Etendeka Formation,north-western Namibia

The Etendeka Formation of north-western Namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the Karoo Igneous Province in southern Africa. The age of the Etendeka Formation is approximately 130–135 Ma. The quartz latites make up a significant proportion of the stratigraphic succession (<25% of the total stratigraphic thickness) and form as much as 60% of the outcrop area in the southern Etendeka. Apart from some systematic differences between pitchstones and devitrified quartz latite, largely explained by alteration processes, individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation. Geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern Etendeka area enabling the reconstruction of the Etendeka Formation stratigraphy in this region. Individual quartz latite units occur as voluminous (400–2600 km³), widespread (up to 8800 km²), sheet-like deposits typically between 40 and 300 m thick. Each unit consists of basal, main and upper zones. The main zone generally constitutes over 70% of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite. In contrast the basal and upper zones of the flow are characterised by flow banding, pitchstone lenses and breccia, with rare occurrences of pyroclastic textures. The quartz latites are sparsely porphyritic (<10% phenocrysts) with glassy or devitrified groundmass textures. The phenocrysts consist of plagioclase, pyroxene, titanomagnetite and rare ilmenite. Pyroxene geothermometry indicates high (1000–1100°C) temperatures of crystallisation which, coupled with the absence or primary hydrous phases, indicates that the quartz latites were relatively hot, H₂O-undersaturated magmas. The quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style. The local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high-temperature rheomorphic ignimbrites (i.e. rheoignimbrites). A combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the Etendeka area.     

An analysis of the dynamic characteristics of flotation circuits by time-series analysis

Time-series analysis techniques are used to investigate the dynamic characteristics of the flotation circuits on the Renison tin concentrator in Tasmania. Routine plant-operating data provide information on the variability of feed streams, the disturbances resulting from normal manual operation, the suitability of certain locations for sitting instruments, and some simple time delays in the circuit. It is also shown how plant upsets and abnormal conditions caused by large perturbations to the operation can prove useful in such studies. Finally, a dynamic experiment is described which shows that disturbances higher in frequency than those in the feed are induced in the roughing and scavenging operations, but that these are filtered out again in the cleaner circuit. The uses and limitations of these techniques are discussed.     

Assessment of the Re decay constant by cross calibration of Re-Os molybdenite and U-Pb zircon chronometers in magmatic ore systems

The past decade has seen renewed interest in ¹⁸⁷Re-¹⁸⁷Os geochronology using a variety of matrices including sulfide minerals, shales and meteorites. The most widely used value of the ¹⁸⁷Re decay constant (λ¹⁸⁷Re) is 1.666 ± 0.005 × 10⁻¹¹ a⁻¹ (±0.31%), which is based on cross calibration of Re-Os and Pb-Pb chronometers for certain meteorites [Smoliar M. I., Walker R. J., and Morgan J. W. (1996) Re-Os isotope constraints on the age of Group IIA, IIIA, IVA, and IVB iron meteorites. Science271, 1099-1102]. However, other recent studies have yielded alternate values of λ¹⁸⁷Re, based upon either direct counting experiments or analysis of meteorites. Here, we provide an independent assessment of λ¹⁸⁷Re, using methodology, sample materials, and preparation of Os standard solutions different from those of Smoliar et al. (1996). Combining Re-Os age data for molybdenite formed in magmatic ore deposits, with the U-Pb zircon age of the magmatic rocks, a refined λ¹⁸⁷Re value is determined by averaging 11 individual cross-calibration experiments spanning ca. 2700 Ma of Earth history. Using the U decay constants of Jaffey [Jaffey A. H., Flynn K. F., Glendenin L. E., Bentley W. C., and Essling A. M. (1971) Precision measurement of half-lives and specific activities of 235U and 238U. Phys. Rev.4, 1889-1906], a value for λ¹⁸⁷Re of 1.6668 ± 0.0034 × 10⁻¹¹ a⁻¹ is determined. Using the λ²³⁸U value of Jaffey et al. (1971) and λ²³⁵U value of Schoene [Schoene B., Crowley J. L., Condon D. J., Schmitz M. D., and Bowring S. A. (2006) Reassessing the uranium decay constants for geochronology using ID-TIMS U-Pb data. Geochim. Cosmochim. Acta70, 426-445], a value for λ¹⁸⁷Re of 1.6689 ± 0.0031 × 10⁻¹¹ a⁻¹ is determined. These values are nominally higher (ca. 0.1 and ca. 0.2%) than the value determined by Smoliar et al. [Smoliar M. I., Walker R. J., and Morgan J. W. (1996) Re-Os isotope constraints on the age of Group IIA, IIIA, IVA, and IVB iron meteorites. Science271, 1099-1102], but within calculated uncertainty. Further refinement of λ¹⁸⁷Re by cross calibrating the molybdenite and U-Pb zircon chronometers should be possible by utilizing high precision, single-grain, chemical abrasion zircon U-Pb analyses.     

Geology, mineralization, and geochronology of the Qianhe gold deposit, Xiong’ershan area, southern North China Craton

The Qianhe gold deposit in the Xiong’ershan area is located along the southern margin of the Archean-Paleoproterozoic North China Craton. The deposit consists of six orebodies that are hosted in Paleoproterozoic andesites to basaltic andesites and structurally controlled by roughly EW-trending faults. Individual orebodies comprise auriferous quartz veins and disseminated Au-bearing pyrite within hydrothermally altered rocks on both sides of, or close to, the veins. Ore-related hydrothermal alteration has produced various mixtures of K-feldspar, quartz, sericite, chlorite, epidote, carbonate, and sulfides. Pyrite is the most important ore mineral, associated with minor amounts of galena, sphalerite, and chalcopyrite. Other trace minerals include molybdenite, arsenopyrite, scheelite, rutile, xenotime, and parisite. Gold occurs mostly as native gold and electrum enclosed in pyrite or along microfractures of sulfides and quartz. Microthermometric measurements of primary inclusions in auriferous quartz suggest that gold and associated minerals were precipitated in the range of 160–305 °C from aqueous or carbonic-aqueous fluids with salinities of 6–22 wt% NaCl equiv. Samples of molybdenite coexisting with Au-bearing pyrite have Re–Os model ages of 134–135 Ma, whereas ore-related hydrothermal sericite separates yield ⁴⁰Ar/³⁹Ar plateau ages between 127 and 124 Ma. The Re–Os and ⁴⁰Ar/³⁹Ar ages are remarkably consistent with zircon U–Pb ages (134.5?±?1.5 and 127.2?±?1.4 Ma; 1σ) of the biotite monzogranite from the Heyu-intrusive complex and granitic dikes in and close to the Qianhe gold mine, indicating a close temporal and thus possibly genetic relationship between gold mineralization and granitic magmatism in the area. Fluid inclusion waters extracted from auriferous quartz have δD values of ?80 to ?72 ‰, whereas the calculated δ ¹⁸O_H2O values range from 3.1 to 3.8 ‰. The hydrogen and oxygen isotopes from this study and previous work indicate that ore fluids were likely derived from degassing of magmas, with addition of minor amounts of meteoric water. Gold mineralization at Qianhe is temporarily coincident with pervasive bimodal magmatism, widespread fault-basin formation, and well development of metamorphic core complexes in the whole eastern North China Craton that have been interpreted as reflecting reactivation of the craton in the late Mesozoic after prolonged stabilization since its formation in the late Paleoproterozoic. It is therefore concluded that the Qianhe gold deposit formed as a result of this craton reactivation event.     

U-Pb zircon geochronology of the Aptian/Albian boundary implies that the GL-O international glauconite standard is anomalously young

A new U-Pb zircon age for the Aptian/Albian boundary (113.1 ± 0.3 Ma) indicates that an alternate Early Cretaceous timescale that is largely devised using the K-Ar date for GL-O glauconite international standard and other K-Ar glauconite geochronology, is inaccurate. Both ⁴⁰Ar/³⁹Ar sandine and U-Pb zircon ages indicate that the K-Ar date for the GL-O international standard does not record the timing of sediment deposition and thus should not be used for timescale calibration. This issue is not solely constrained to the Early Cretaceous, because other geological time intervals also reveal younger K-Ar glauconite ages in comparison to other radioisotopic dating techniques (e.g., U-Pb, Ar-Ar, Re-Os).     

A Matrix‐Matched Reference Material for Validating Petroleum Re‐Os Measurements

This study presents two matrix‐matched reference materials developed for petroleum Re‐Os measurements. We present the Re and Os mass fractions and ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os values (ratio of the number of atoms of the isotopes) for repeatedly measured aliquots (ca. 120–150 mg test portions) of the NIST Research Material 8505 (RM 8505) crude oil, and its asphaltene and maltene fractions, and ~ 90 g of homogeneous asphaltene powder isolated from this oil. Measurements were performed using the Carius tube‐isotope dilution negative‐thermal ionisation mass spectrometry methodology. The RM 8505 crude oil contains 1.98 ± 0.07 ng g^?1 Re and 25.0 ± 1.1 pg g^?1 Os, with Re‐Os isotope amount ratios of 452 ± 6 for ¹⁸⁷Re/¹⁸⁸Os and 1.51 ± 0.01 for ¹⁸⁷Os/¹⁸⁸Os (n = 20, 95% conf.). The homogeneous asphaltene sample contains 16.52 ± 0.10 ng g^?1 Re and 166.0 ± 0.9 pg g^?1 total Os, and possesses isotope amount ratios of 574 ± 3 for ¹⁸⁷Re/¹⁸⁸Os and 1.64 ± 0.01 for ¹⁸⁷Os/¹⁸⁸Os (n = 24, 95% conf.). The intermediate precision of these data makes the RM 8505 whole oil and the (~ 90 g) homogenised asphaltene appropriate petroleum matrix‐matched reference materials for Re‐Os measurements. The asphaltene fraction of the oil is the main carrier of Re and Os of the RM 8505 whole oil, and caution is suggested in using asphaltene and maltene fractions of a single oil for Re‐Os geochronology.