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
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.
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.