Adrian Walker

Reconnaissance-Scale Prospectivity Analysis for Gold Mineralisation in the Southern Uplands-Down-Longford Terrane, Northern Ireland

The Southern Uplands-Down-Longford Terrane in southeast Northern Ireland is prospective for Caledonian-age, turbidite-hosted orogenic gold mineralisation with important deposits at Clontibret in the Republic of Ireland and in Scotland. Geochemical and geophysical data from the DETI-funded Tellus project have been used, in conjunction with other spatial geoscience datasets, to map the distribution of prospectivity for this style of mineralisation over this terrane. A knowledge-based fuzzy logic modelling methodology using Arc Spatial Data modeller was utilised. The prospectivity analysis has identified several areas prospective for turbidite-hosted gold mineralisation, comparable to that at Clontibret and gold occurrences in the Southern Uplands of Scotland. A number of these either coincide with known bedrock gold occurrences or with areas considered prospective and targeted by previous exploration work, validating the predictive capability of the exploration model devised and its translation into a GIS-based prospectivity model. The results of the modelling suggest that as in other parts of the Southern Uplands the coincidence of regional strike-parallel structures and intersecting transverse faults are highly prospective, as these are likely to create zones of anomalous stress for fluid flow and deposit formation. Those areas in which there are no known gold occurrences are considered to be favourable targets for further exploration and should be followed up.

Palaeogene Alpine tectonics and Icelandic plume-related magmatism and deformation in Northern Ireland

The Cenozoic tectonic history of NW Europe is generally attributed to some combination of three principal controlling factors: North Atlantic opening, Alpine collision and formation of the Icelandic mantle plume. Using constraints from the high-resolution Tellus aeromagnetic survey of Northern Ireland, we show that Palaeogene tectonics can be attributed to approximately north–south Alpine-related compression, forming NNW–SSE-trending dextral and ENE–WSW-trending sinistral conjugate strike-slip faults, with the latter defined by kilometre-scale displacements along reactivated Caledonian or Carboniferous faults. This tectonism was, however, punctuated by pulsed magmatic intrusive and extrusive events, including four distinct dyke swarms that are attributed to NE–SW- to east–west-directed plume-related extension. Although this evidence shows, for the first time, that north–south Alpine compression was periodically overwhelmed by the dynamic stresses and uplift associated with pulsed mantle plume-related deformation, associated strike-slip faulting may have controlled the locus of volcanic activity and central igneous complexes, and the location of sedimentary depocentres.

Display and enhancement of gridded aeromagnetic data of the Solway Basin

Abstract The British Geological Survey has completed the conversion of both aeromagnetic and gravity survey records to a digital data base. This paper examines some of the uses of the aeromagnetic data base. Rendering digitized aeromagnetic data as a rectangular grid of interpolated anomaly values allows regional variations in subsurface magnetization to be displayed as images rather than contoured maps. Images of the magnetic field are more appropriate for visual interpretation and can be enhanced using standard methods of digital image processing. The optimum method is a combination of directional first-derivative filtering to impart the illusion of side illumination, with spectral colouring of the 8-bit range of DN, the former being used to modulate the brightness of the latter. As well as many of the known regional magnetic features of the Solway Basin, this method highlights several trends that were unsuspected.

A new high-resolution aeromagnetic dataset over central Ayrshire: insights into the concealed geology

Synopsis High-resolution aeromagnetic data have been acquired over central Ayrshire as part of a multi-component environmental and resource survey. The data were recorded along easterly orientated flight lines at 200 m separation and show a very marked improvement in resolution over the pre-existing aeromagnetic data for the region. The distribution of the extensive volcanic and intrusive igneous rocks in the near sub-surface is defined more accurately, many new features are recognized and features distinguished in the pre-existing aeromagnetic data are now seen to be laterally continuous. Northwesterly trending lineaments are clearly defined and most can be attributed to Palaeogene dykes, the majority of which are reversely magnetized. Depth to source solutions and modelling suggest that even though the dykes have a limited width at outcrop, their widths increase with depth. Permo-Carboniferous easterly trending dykes are not well resolved in the new aeromagnetic data. This is most likely due to a low amplitude resultant magnetic vector that, combined with the easterly-orientated flight lines, results in few large anomalies. One significant northwesterly trending lineament is interpreted as the trace of the Cleveland Dyke across the Midland Valley. These new data should be of value to the extractive and water industries and assist in the planning and sustainable management of these resources.

Chapter 3 The gravity field

The gravity field of Sumatra and the surrounding marine areas is shown in Figure 3.1. Contours in the onshore area of Bouguer gravity, but offshore are of free-air gravity. Terrain corrections have not been applied. Although marine gravity measurements have been made in the forearc basin and elsewhere on a number of research cruises (e.g. Kieckhefer et al. 1981), the data from these generally widely spaced lines have not been used in preparing the maps because free-air gravity values obtained from inversion of satellite radar altimetry provide more systematic coverage and can resolve anomalies with widths of as little as 7 km (Sandwell & Smith 1997). The onshore and satellite-derived offshore data were matched at coastlines without undue difficulty, as should be the case because both free-air and Bouguer corrections are zero at sea level. However, gradients tend to be steep at the coasts in the forearc region, partly because of the change from free-air gravity, which is strongly correlated with local bathymetry, to Bouguer gravity, which is corrected for local topography. Figure 3.2 shows the locations of the onshore stations used in preparing Figure 3.1, but not of the offshore estimates, distributed on a regular 2 minute grid. Onshore data were obtained from a variety of sources, but unfortunately the results of the many detailed gravity surveys carried out by oil companies remain confidential. The largest single available data set was assembled as part of the collaboration between the British Geological Survey (BGS) and the Geological Research and Development