The value of a combined approach: Innovative mineral exploration techniques in the Irish Zn-Pb orefield

Abstract

The Early Carboniferous stratigraphy of the Irish Midlands contains one of the world?s major carbonate-hosted zinc-lead orefields covering a region of approximately 8000 km2. The large-scale nature of the sedimentary fluid flow systems that produce these ore deposits suggest it is necessary to understand not only the nature of mineralisation, but also the nature of the basin in which it is hosted. In this study we integrate typical methods of stratigraphic analysis (core and outcrop logging) with recently acquired seismic reflection data and gamma ray logs to ascertain the relationship between the stratigraphic and structural setting of the Ballinalack Zn-Pb deposit and the wider host basin (Dublin Basin). Geometric relationships between layers as revealed by seismic reflection, in conjunction with detailed lithological analysis has shown extensional tectonism was the main driving force of sedimentation in the Dublin Basin. A period of significant extensional tectonism first occurred in the late Tournaisian (Moathill Event, ~348 Ma) and was accompanied by faulting and regional subsidence. This was followed by a period of relative tectonic quiescence, before a second period of tectonism during the lower Viséan (Tober Colleen Event, ~345 Ma) resulted in regional subsidence, but without significant faulting. Major subsidence associated with these events produced strong transgressions in the Dublin Basin, interspersed with periods of regression. This view of the tectonic history of the Irish Carboniferous basin contrasts with previous interpretations that suggest the Lower Carboniferous represents an overall deepening sequence. The suggested earlier timing of fault movement has implications for arguments about the origin of Irish-type Zn-Pb deposits and the necessity (or not) for having active faulting during mineralisation. These novel results reveal the value of a combined approach to sedimentary basin analysis.