The geological environment of post-caledonian base-metal mineralization in Ireland

Abstract

The Dlnantian host and wall rocks to the Ballyvergin, Gortdrum, Oola, Carrickittle and Tynagh base-metal deposits were analysed for a variety of trace elements with a view to establishing a local sedimentary syngenetic contribution of metals. Against expectation all the trace element aureoles examined proved the epigenetic nature of the sulphide mineralization. The aureoles are of two kinds corresponding to the sulphide deposit types. The copper deposits in the Lower Limestone Shales and the argillaceous Lower Limestones; Ballyvergin, Gortdrum and Oola, are fringed with enrichments of arsenic and lead, whereas the Waulsortian wall rocks to the Tynagh and Carrickittle lead-zinc deposits contain uneven enrichments of many trace elements. A reconnaissance survey in the Waulsortian mud bank complex to the west north west of Tynagh revealed what may be an extensive syngenetic manganese aureole to the Tynagh chert-hematite deposit. The exhalative origin proposed by Derry, Clark and Gillatt (1965) for the Tynagh iron deposit is supported by chemical analysis. Thus hot springs were in existence at Tynagh in mid-Dinantian times. This was probably the case too at Silvermines (Graham 1970). The iron deposit at Keel as well as the thick developments of chert at Silvermines and Aherlow are taken here as additional evidence for a mid-Dinantian age for the onset of mineralization. Although the local structural controls to the sulphide deposits may be related to the Armorican Orogeny, the distribution of the ore deposits is more easily explained in terms of north-south geofracturing caused by the tensile stresses which eventually led to the formation of the Atlantic (Rockall Trough) margin. The recent discovery of the Navan sulphide deposit was broadly predictable by this theory.