A critical evaluation of the use of hydrocarbon gases in rocks as a pathfinder for base-metal mineralisation in Shannonbridge, central Ireland

This paper discusses the results of a light hydrocarbon gas (LHG) lithogeochemical survey carried out at Shannonbridge in central Ireland. Two methods of LHG extraction (dry grinding and sample solution in EDTA) are compared with a heat extraction method developed by Carter (1981). The results show that due to a more complete breakdown of the sample, LHG contents of up to an order of magnitude greater are released by grinding and sample solution, with no apparent decrease in precision. As a result, a potential problem related to the generation of additional gases by the thermal breakdown of organic matter during heating, can be avoided.Similar LHG component distribution patterns are produced by dry grinding and EDTA extraction suggesting that both methods could be useful alternatives to the heating technique. This is strengthened by the identification of a possible target zone in Shannonbridge based on the distribution of samples depleted in methane and enriched in butyl gases, C4 (butane plus butene plus their associated isomers). However, this empirical interpretation was not supported by the results of heating. Furthermore, subsequent exploration failed to establish the presence of a significant mineral occurrence at Shannonbridge. Evidently, the LHG patterns were either misinterpreted or they reflect hydrocarbon sources which are not related to mineralisation.Further evaluation of the data showed that LHG component distributions are significantly influenced by variations in lithology. Once this effect was reduced, it became apparent that the patterns are principally controlled by regional faulting. Since fault structures would be permeable to hydrocarbon gases formed from a variety of processes (such as maturation of organic matter in the host limestones), no relationship with buried mineralisation is necessary to explain the patterns observed.By using the Shannonbridge data as an example, this paper attempts to emphasise the equivocal nature of data collected using prospecting methods of this type. The aim is not to refute innovative and quality research already carried out in this field because the association of hydrocarbon gases with mineralisation is too frequent to be simply classified as coincidental. However, anomalous levels of LHG can be attributed to other processes which, in many cases, are too easily overlooked.