The geology and geochronology of Al Wahbah maar crater, Harrat Kishb, Saudi Arabia

Al Wahbah is a large (∼2.2 km diameter, ∼250 m deep) maar crater in the Harrat Kishb volcanic field in western Saudi Arabia. It cuts Proterozoic basement rocks and two Quaternary basanite lava flows, and is rimmed with an eroded tuff ring of debris from the phreatomagmatic explosion that generated the crater. A scoria cone on the northern wall of the crater was dissected by the explosion and exposes a dolerite plug that was intruded immediately prior to crater formation. The dolerite plug yields a <sup>40</sup>Ar/<sup>39</sup>Ar age of 1.147 ± 0.004 Ma. This is the best possible estimate of the time Al Wahbah crater formed. It is a few tens of thousand years younger than the age of the lower and upper basalt flows, 1.261 ± 0.021 Ma and 1.178 ± 0.007 Ma respectively. A dolerite dyke exposed within the basement in the wall of the crater is dated at 1.886 ± 0.008 Ma. This is the most precise age so far determined for the initiation of basaltic volcanism of Harrat Kishb, and confirms that it is significantly younger than the other post-rift volcanic provinces in the region. This study provides constrains the timing of humid climatic conditions in the region and suggests that the Quaternary basaltic volcanism that stretches the length of the western side of the Arabian peninsula may prove to be useful for establishing palaeoclimatic conditions

Exploration for volcanogenic sulphide mineralisation at Benglog, north Wales

Exploration for volcanogenic sulphide mineralisation around Benglog is one of three investigations designed to assess the metallogenic potential of the Ordovician Aran Volcanic Group. Detailed geological mapping in the Benglog area enabled an interpretation of the volcanic environment, critical to such an assessment, to be made. The eruptive rocks are acid and basic in composition; the acid rocks are mostly ash-flow tuffs derived from outside the area, whereas the basic rocks have a local derivation. They are all interbedded with dark grey or black silty mudstone and were probably erupted in a submarine environment. Contemporaneous dolerite sills were intruded into wet sediment. This environment was suitable for volcanogenic exhalative sulphide deposits to form and indications of a metallogenic horizon were found at the top of the Y Fron Formation in the form of abundant pyrite, minor pyrrhotite and minor base metal enrichment. Soil samples, analysed for copper, lead and zinc, were collected and geophysical surveys were carried out along eleven east-west trending traverse lines 300 m apart across the volcanic succession. Indications were found of minor vein mineralisation at dolerite intrusion margins and locally along faults. Very high chargeability and low resistivity anomalies over mudstones did not spatially coincide with geochemical anomalies in soil, but the secondary redistribution of metals in soils and variable thickness of overburden precluded confident interpretation of the source of many soil anomalies. Geochemical drainage data, in conjunction with rock analyses, show strong barium enrichment in mudstones which could be volcanogenic in origin but related to two separate eruptive episodes. The findings of the survey were inconclusive. An environment suitable for the formation of volcanogenic exhalative sulphide deposits was established, but the geochemical and geophysical surveys located only minor vein mineralisation and tenuous indications of other styles of mineralisation. Recommendations are made for further work

Strumble-Preseli ancient communities and environment study (SPACES); Sixth report 2007-08

This paper reports the results of field surveys and geophysical surveys in the eastern Preseli Hills of Pembrokeshire, Wales, together with petrological descriptions of rock samples taken from key outcrops and quarry sites

Application LANDSAT imagery to geologic mapping in the ice-free valleys of Antarctica

The author has identified the following significant results. Studies in the Ice-Free Valleys are resulted in the compilation of a sizeable library of maps and publications. Rock reflectance measurements were taken during the Antarctic summer of 1973. Spectral reflectance of rocks (mostly mafic lava flows) in the McMurdo and Ice-Free Valleys areas were measured using a filter wheel photometer equipped to measure reflectances in the four Landsat bands. A series of samples were collected at regular intervals across a large differentiated, mafic sill near Lake Vida. Chemical analyses of the sample suggest that the tonal variations in this sill are controlled by changes in the iron content of the rock. False color images were prepared for a number of areas by the diazo method and with an optical multispectral biviewer. These images were useful in defining boundaries of sea ice, snow cover, and in the study of ablating glaciers, but were not very useful for rock discrimination

Frictional experiments of dolerite at intermediate slip rates with controlled temperature: Rate weakening or temperature weakening?

A rotary shear apparatus has been newly set up in Chiba University which can control the temperature near a sliding surface, T_meas, up to 1000°C independently from slip rate, V. Frictional experiments at 0.010 m/s, 1 MPa normal stress, and variable T_meas for dolerite have revealed a remarkable effect of temperature on the friction coefficient, f. With increasing T_meas, f starts from 0.7 to 0.8 at room temperature (RT), decreases down to 0.5–0.6 at 400°C, increases until 800°C, and then decreases again. We have also conducted XRD analyses of the wear materials (mainly submicron particles) and investigated microstructures of the sliding surfaces developed at different temperatures T_meas, and we found that there is a negative correlation between f and the amount of amorphous material except at RT and 1000°C. The generation of the amorphous phase probably causes the weakening. There is no amorphous phase recognized for a sample at 1000°C which is an aggregate of rounded crystals. EBSD analyses show that the material on the sliding surface at 1000°C contains randomly oriented hematite grains, which together with the observed microstructural features suggests that granular flow was taking place. We have also demonstrated that f depends not only on the instantaneous value of temperature, but also on its history. By comparing with conventional rotary shear friction experiment for the same dolerite without temperature control, we conclude that strong “rate weakening” as recently observed in high-velocity frictional experiments without an active control of the temperature has a significant amount of contribution from the temperature effect

Geophysical surveys near Strontian, Highland Region

Reconnaissance VLF-EM and magnetic surveys have bean carried out over Ba- Pb-Zn prospects in an area near Strontian in the Highland Region of Scotland. Rather than attempting to detect the economic minerals directly, which is unlikely to be practicable by geophysical methods, the trials concentrated on exploration for the crush zones and associated Permo-Carboniferous basic dykes which act as hosts to mineralisation. The results are encouraging, with the VLF-EM method proving effective in delineating crush zones while magnetic traverses detected the basic dykes. To the east of Bellsgrove mine a crush zone and dyke extend eastwards along the strike of the Strontian Main Vein; however, to the west of the Whitesmith mine the evidence of a westward extension of the Main Vein is insubstantial. A number of crush zones and associated dykes have been identified in the Corrantee-Whitesmith area. Probable ext_ensions are indicated to a number of known veins in the vicinity of the Fee Donald mine. The rss~llts merit geophysical, geological and possibly geochemical follow-up

The Geology as an indispensable tool for optimizing the exploration of dimension Stones

Like any Geological Resource, the Dimension Stones can only be exploited where they occur. How they occur is a reflection of geological history that presided over its formation. Their nature determines the mode of deposit and therefore it’s potential to be exploited as ornamental stone. The geological setting of the Portuguese marbles became a key factor in the optimization of its exploitation, two ductile deformation phases must be consider, which originates the complex folded metamorphic layering. At least three fracturing systems are responsible for the high segmentation of the marbles in fragile deformation conditions and that must also be considered. Together these geological constraints lead to a really low production ratio (3% - 12%). These low efficiency productions emphasized the emergence of the geological knowledge of individual quarries in order to optimize is exploitation. Until recent years, with high prices for the Portuguese marbles the companies haven’t been concerned with the geological knowledge of its quarries, but now that an economic crisis is installed in the sector, basic geological studies in the quarries are mandatory and could save thousands of euros for each company