Global Irish – Diversity of the diaspora

Abstract

The spectrum of zinc-lead deposits formed in basinal mineral systems encompasses VMS (volcanogenic massive sulphide), SHMS (shale-hosted massive sulphide), Irish-type and MVT (Mississippi valley Type) deposits. The platform carbonate-hosted part of that spectrum, the Irish-type and MVT deposits, has created the greatest challenges to pigeonholing approaches and the Irish Midland deposits have been variably considered as unique, “SedEx” variants, or MVT variants. In fact, the Irish-type spectrum of deposits can be considered as a global diaspora of diverse deposits that, nonetheless, show a number of distinct and economically significant characteristics in style and setting. For this reason, they warrant consideration as a discrete deposit type, though not in a neat pigeonhole, that is best considered in a mineralizing system context. The distinguishing features of Irish-type mineral systems can be considered in terms of source, trigger, pathway, trap, and preservation. The key features that distinguish Irish-type from ‘typical’ MVT and SHMS deposits are related to basin type and setting, timing of the mineralization event, mineralization style and chemistry, and deposit geometry. Empirically, these characteristic basin to deposit scale features overlap both MVT and SHMS but, together, are unique to Irish-type systems. This gives rise to criteria that can be applied to determine prospectivity of basins for Irish-type deposits and to target deposits within these basins. It is important to distinguish Irish-type from MVT systems because their economic characteristics are different. However, it is also important to recognise that there is great variability within the broad basinal carbonate-hosted zinc-lead deposit family and that each basin, and indeed each trend and deposit, are to some extent unique. It is therefore extremely important to avoid model-driven exploration and to develop a targeting understanding that acknowledges the model framework but is based on actual observations and data. To understand this diversity and targeting context, it is pertinent to consider the wide range of carbonate-hosted deposits that do not fit into the published MVT pigeonhole, including the Irish Midlands deposits; the Alpine deposits; deposits in the Basque-Cantabria Basin; deposits on the Gondwana margin including a number of deposits in North African,, southeast Turkey, Iran, and Duddar in Pakistan; the Early Cretaceous deposits on the Atlantic margin in Gabon; the Ordovician of the Sibumasu terrane (Tibet to Southeast Asia); Polaris in the Franklinian Basin; Nanisivik in the Borden Basin; and the Devonian Lennard Shelf deposits of Western Australia. All of these deposits occur in rift-sag basins with carbonate platforms, in some cases with multiple rift-sag cycles or with successor basins and, where constrained, the mineralization event is syn-basinal and typically related to early extension or inversion events. The deposits are stratabound and mostly tabular and continuous, often show strong direct control by extensional structures, are typically dominated by replacement, and commonly have significantly higher grades than ‘typical’ MVT deposits such as those in the mid-continent US, Silesia (Poland) and Pine Point (Canada).