Gold potential of the Dalradian rocks of north-west Northern Ireland : prospectivity analysis using Tellus data

The Dalradian terrane of north-western Northern Ireland is an attractive target for orogenic vein mineralisation. The current high price of gold and the release in 2007 of high-quality regional geochemical and geophysical datatsets from the Tellus project have stimulated a marked revival of interest in this area. The results of the Tellus project, funded chiefly by the Department of Enterprise Trade and Investment (DETI), have provided major new insights into the geology and mineral potential of Northern Ireland. Significant vein type orogenic gold mineralisation is known at several localities within this region. Previous studies of the most important deposits at Curraghinalt and Cavanacaw, and a review of similar mineralisation elsewhere in the world, allowed the definition of deposit models to underpin systematic prospectivity analysis of the Dalradian terrane in north-western Northern Ireland. Based on these models key exploration criteria were identified and, where appropriate data were available, these were extracted from various multivariate datasets (geology, geochemistry, geophysics and mineral occurrences). These criteria were then assigned significance weightings, zones and styles of influence based on the exploration model and the views of the team undertaking the analysis. A knowledge-based prospectivity analysis using Fuzzy Logic modelling was then applied to map the favourability for the occurrence of deposits of this type within the Dalradian terrane. ESRI’s Arc Spatial Data Modeller version 3.1 (Arc SDM) software was employed for this purpose. The key exploration criteria are certain structural vectors and elevated values for gold and associated elements in Tellus stream-sediment geochemical data. On account of the apparent differences in the importance of structures of various orientations at Curraghinalt and Cavanacaw, and the contrast in the geochemicalmineralogical characteristics of the ores, two separate prospectivity models were applied in this study, one for Curraghinalt and the other for Cavanacaw. Mineral occurrence information from the GSNI MINLOCS database and the occurrence of gold grains observed during panning of stream sediments were also incorporated in the analysis. Regional prospectivity analysis, covering an area of 3074 km2, using the two models identified 22 prospective targets for orogenic vein style gold mineralisation disposed in four groups. Many of the targets coincide with known orogenic gold occurrences while others occur in areas identified as prospective by previous workers. However in some areas the size of the targets has increased and elsewhere new targets have been identified beyond the main Lack – Curraghinalt zone. Of particular interest is a series of targets trending west-north-west, which passes through Curraghinalt and Golan Burn, which may relate to the major fault bounding the southern side of the Newtownstewart Basin. Another series of targets, disposed along a near-linear trend with a similar orientation, is identified to the north of the Omagh-Kesh Basin. A possible interpretation of these results is that both sets of targets are related to major structural features related to a westward extension of the Curraghinalt lateral ramp which is widely regarded as a critical control on the location of the Curraghinalt deposit. The two prospectivity models were also applied to a sub-area of the terrane, the Newtownstewart map sheet, covering 560 km2. This analysis incorporated structural vectors derived from a more detailed evaluation of the Tellus geophysical datasets and from the revised 1:50 000 scale geological map for the Newtownstewart sheet published in 2007 which incorporated modified linework based in part on examination of the Tellus data. Comparison of the results of the regional and detailed analyses shows that, in the detailed analysis, the addition of structural data in areas devoid of such information in the regional analysis has led to an increase in the size of the targets identified. The detailed analysis has also identified extensive additional target areas, including zones of very high prospectivity, in the south-eastern quarter of the map sheet. This study has clearly demonstrated the value of knowledge-based prospectivity analysis in the Dalradian terrane of north-western Northern Ireland. The regional Tellus geochemical and geophysical datasets are critical to the application of this methodology. Further improvement of the technique is possible through implementation of a range of measures. The potential benefits from the use of a sample catchment approach to the geochemical drainage data should be investigated. A more detailed and comprehensive database of bedrock mineral occurrences would help validation of the prospectivity results and would also potentially allow the use of data-driven methods of analysis, removing the subjectivity inherent in knowledge-based methods. Addition of further high-resolution datasets would improve the utility of this approach for follow-up exploration.