GOLD PROSPECTIVITY IN THE CALEDONIDES OF SOUTHEAST IRELAND - APPLICATION OF THE UPPER-CRUSTAL RESERVOIR MODEL

Recent models for the genesis of lode-gold deposits in volcano-sedimen tary successions have emphasized the role of upper-crustal stratigraph ic units as potential reservoirs for gold and its pathfinders, Sb and As. Gold mineralization is known to occur in a variety of settings in the Cambrian-Silurian volcano-sedimentary sequences of the paratectoni c Caledonides of southeast Ireland. Bedrock gold is most widely associ ated with Caradocian volcanic rocks of the Duncannon Group, particular ly where they host important Cu-Fe-Zn-Pb mineralization, but it is als o found in the dominantly sedimentary successions of the older Bray an d Ribband Groups and the younger Kilcullen Group (Silurian). Regional lithogeochemical and drainage-sediment data have been integrated with existing geological data and information on mineral occurrences to ass ess the potential of individual lithological units in southeast Irelan d as reservoirs for Au, Sb and As. The abundance of Au in spatially de termined lithostratigraphic units suggests that only the Duncannon Gro up is prospective for gold in the context of a reservoir model: the hi ghest median Au value recorded in stream sediments is 4 ppb (slightly above the average crustal abundance of 3 ppb) for both the Kilmacrea F ormation (Duncannon Group) and the underlying Oaklands Formation (Ribb and Group). In rocks only the Avoca Volcanic Formation (Duncannon Grou p) has a median value (2.5 ppb) above the detection limit of 2 ppb. Ne ither value suggests that these rocks contain primary enrichments of A u. Both Sb and As are enriched in rocks and stream sediments relative to their average crustal abundances, but only at some locations are th ey spatially correlated with Au: Sb shows good correlation with Au in some parts of the Duncannon Group and in the immediately underlying an d contiguous Oaklands Formation of the Ribband Group, reinforcing the conclusion that these units are probably the most prospective for gold . The spatial distribution of As is largely independent of Au and Sb d espite the fact that arsenopyrite is a common associate of gold minera lization in the region. Overall, the levels of the element recorded in rocks and stream sediments suggest that upper-crustal reservoirs for Au may not exist in the volcano-sedimentary sequences of southeast Ire land. It is considered most likely that the enhancement of gold observ ed in volcanic sequences of the Duncannon Group is a product of volcan ogenesis and reflects the deep, upper mantle source of these rocks. De spite these observations, an abundance of recently discovered gold dep osits in southeast Ireland suggests that prospectivity in the region i s high. Other models of gold metallogeny are therefore likely to be mo re relevant in the region.