Skip to main content

Numerical modelling of the structural controls on Irish Zn-Pb ore deposits

Periodic Reporting for period 1 - MODIO (Numerical modelling of the structural controls on Irish Zn-Pb ore deposits)

Reporting period: 2017-04-01 to 2019-03-31

Sustainable access to mineral resources is of critical importance to our society. Ireland is a major producer of zinc and lead metal in Europe. These metals are essential components in car batteries, construction alloys or dyes and are extracted from the subsurface. But because the depth at which we find these resources increases year after year, we have to get smarter at finding them. MODIO aimed to understand where these metals can be found in the Irish subsurface, and thereby help mineral explorers to find them. For this, we used 3D visualisations of many different types of subsurface data and advanced computer modelling.

The Irish Lower Carboniferous (ILC) rocks are host to significant Zn-Pb mineral deposits that are both stratigraphically and tectonically controlled. These ore deposits are found in rocks of Lower Carboniferous age. Much of the metals occurs in the hanging wall of major normal faults through the rocks, by replacing the carbonate rocks or carbonate breccias. MODIO combined and integrated top-quality data from many Irish mining industry partners and government agencies. It then used this integrated data to investigate the links between the fault systems and orebodies. Another objective was to capture the fundamentals of how the fault systems actually developed through time, and how this had an influence on where the metals were able to be trapped.
"We have constrained the fault network and its relationship to mineralisation in several Irish mineral deposits and made 3D geological models in collaboration with other researchers at the Irish Centre for Research in Applied Geosciences in University College Dublin. As a result, we now have better understanding of fluid flow and metal distribution processes in Irish Zn-Pb deposits. Through an integrated workflow of seismic interpretation, potential-field geophysics and drilling and surface mapping constraints, we now also have a better understanding of the combined tectonic and stratigraphic evolution of the Lower Carboniferous onshore Ireland. These results have world-wide implications for sedimentary rock-hosted basins and for fault-related fluid-flow in sedimentary basins.

More specifically, using 3D analysis and modelling, we investigated the geometry of normal fault systems and its implications for the origin and nature of associated mineral deposits. Structural analysis of high quality mine datasets indicates that fault segmentation is ubiquitous, with left-stepping segments arising from N-S stretching developed upon generally ENE-NE trending fault arrays which are sub-parallel to older Caledonian penetrative fabrics and structure within underlying Silurian and Ordovician rocks. Fault segments occur on different scales and have a profound impact on the structural evolution, with larger scale segments and intervening relay ramps defining distinct ore bodies within deposits and smaller scale segments and relays potentially providing paths for up-fault flow. The difference in behavior is attributed to the integrity of associated zones that transfer displacement from one segment to another, ""relay ramps"", with intact ramps representing ore body bounding structures, and smaller breached ramps providing enhancing associated hydraulic properties and acting as vertical conduits.

Metal distribution patterns identify these conduits as distinct points along the segmented normal fault array that acted as feeders to individual orebodies, allowing hot, hydrothermal, metal-bearing fluids to enter host lithologies and precipitate orebodies. These points are characterized by highly localized and elevated Ag, Cu, Co, Ni and As concentrations as well as low Zn/Pb ratios which increase away from these feeders. The localization of mineralisation at the boundary of massive host rock limestones and underlying shales is partly attributed to hangingwall deformation localized along this rheological boundary and but also to the dense nature of associated brines. These deposits exemplify the importance of fault control, from source to trap, on the localization and deposit-scale distribution of mineralisation within host rock lithologies.

We also defined the geometric and tectono-stratigraphic framework of the Lower Carboniferous onshore Ireland utilizing geological, paleontological, 2D and 3D reflection seismic, drillhole, and geophysical data at various scales. Extension was initially distributed across the Irish Midlands basin through a dense network of segmented normal faults, the localization of which is often related to pre-existing Caledonian basement structures and heterogeneities. With increasing extension, fault system growth is characterized by the death of small normal faults and the concentration of extension on fewer but faster-growing and larger displacement faults. These faults have displacements in excess of several hundred meters, and strongly control the internal architecture of sub-basins and the distribution of sedimentary facies.

During the project the results and work were presented, amongst others, at three invited seminars, and two invited short courses. The results were also published in three research articles so far, with several other planned in the future. The project results have also been disseminated by the fellow at over 10 conferences and workshops. The fellow had major contributions to the successful application for four other projects, involving governmental and industry partners. But most importantly, due to the applied nature of the research and the societal and industry importance of the work, the results of the project were continuously put in a two-way transfer with industry and governmental partners of the project, through frequent site visits and face-to-face meetings. Over 40 individual knowledge-exchange workshops with mineral exploration companies were done to put these research results into practice in each company’s own mineral exploration campaign."
Through a better understanding of fault geometries and basin architecture, and by building high quality state-of-the-art integrated datasets on structures in the Irish Lower Carboniferous, the work carried out has allowed companies in the metals industry to de-risk exploration investments and provided them with new tried-and-tested exploration tools for exploration under increasingly deeper cover. The advanced research training in the structural controls on mineral deposits provided to the fellow, and in turn to other researchers and industry enhanced their collective innovation capacity and will aid the fellow and others in finding solutions to the challenges posed in sustaining development of often mature ore-fields in the EU, as the depth and geological complexity of new ore deposits increases yearly.