A CRITICAL-EVALUATION OF THE USE OF HYDROCARBON GASES IN ROCKS AS A PATHFINDER FOR BASE-METAL MINERALIZATION IN SHANNONBRIDGE, CENTRAL IRELAND

This paper discusses the results of a light hydrocarbon gas (LHG) lith ogeochemical survey carried out at Shannonbridge in central Ireland. T wo 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 sampl e, 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 additio nal gases by the thermal breakdown of organic matter during heating, c an be avoided. Similar LHG component distribution patterns are produce d by dry grinding and EDTA extraction suggesting that both methods cou ld be useful alternatives to the heating technique. This is strengthen ed by the identification of a possible target zone in Shannonbridge ba sed on the distribution of samples depleted in methane and enriched in butyl gases, C4 (butane plus butene plus their associated isomers). H owever, this empirical interpretation was not supported by the results of heating. Furthermore, subsequent exploration failed to establish t he presence of a significant mineral occurrence at Shannonbridge. Evid ently, the LHG patterns were either misinterpreted or they reflect hyd rocarbon sources which are not related to mineralisation. Further eval uation of the data showed that LHG component distributions are signifi cantly influenced by variations in lithology. Once this effect was red uced, it became apparent that the patterns are principally controlled by regional faulting. Since fault structures would be permeable to hyd rocarbon 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 usin g the Shannonbridge data as an example, this paper attempts to emphasi se the equivocal nature of data collected using prospecting methods of this type. The aim is not to refute innovative and quality research a lready carried out in this field because the association of hydrocarbo n,eases with mineralisation is too frequent to be simply classified as coincidental. However, anomalous levels of LHG can be attributed to o ther processes which, in many cases, are too easily overlooked.