The world-class Irish Zn-Pb(-Ag) deposits occur within one of the world's major metallogenic provinces. While it has been well documented that these orebodies are structurally controlled, exactly how fluids migrated from source to trap is still poorly understood. Using 3-D modeling techniques, the current study investigates metal distribution patterns at the Silvermines and Lisheen deposits to gain insights into fluid pathways and structural controls on mineralization. Distinct points along segmented normal faults are identified as the feeders to individual orebodies, allowing hot, hydrothermal, metal-bearing fluids to enter host rocks and form 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 the feeders. Metal distributions are initially controlled by major and minor normal faults and subsequently affected by later oblique-slip dextral and strike-slip faults. High-tonnage areas without typical feeder signals are interpreted to be structural trap sites, which are distal to fault-controlled feeder points. This study highlights both the importance of a well-connected plumbing system for metal-bearing fluids to reach their basinal traps and the control that an evolving structural framework has on spatial distribution of metals.Koen Torremans is a structural geologist who specializes in mineral deposits in sedimentary basins. He is currently a Marie Skłodowska-Curie postdoctoral fellow based at iCRAG, the Irish Centre for Research in Applied Geosciences in University College Dublin, Ireland. Koen received his B.Sc., M.Sc., and Ph.D. (2016) degrees from KU Leuven University in Belgium. He has worked extensively on base metal deposits in the Central African Copperbelt and the Irish ore field, applying a wide variety of techniques to elucidate ore deposit formation, fluid flow, and structural controls. This includes structural analysis, basin analysis, geochemistry, and geologic and numerical modeling techniques.

Normal faults commonly represent one of the principal controls on the origin and formation of sedimentary rock-hosted mineral deposits. Their presence within rift basins has a profound effect on fluid flow, with their impact ranging from acting as barriers, causing pressure compartmentalization of basinal pore fluids, to forming conduits for up-fault fluid flow. Despite their established importance in controlling the migration and trapping of mineralizing fluids, we have yet to adequately reconcile this duality of flow behavior and its impact on mineral flow systems within basinal sequences from a semiquantitative to quantitative perspective. Combining insights and models derived from earthquake, hydrocarbon, and mineral studies, the principal processes and models for fault-related fluid flow within sedimentary basins are reviewed and a unified conceptual model defined for their role in mineral systems. We illustrate associated concepts with case studies from Irish-type Zn-Pb deposits, sedimentary rock-hosted Cu deposits, and active sedimentary basins. We show that faults can actively affect fluid flow by a variety of associated processes, including seismic pumping and pulsing, or can provide pathways for the upward flow of overpressured fluids or the downward sinking of heavy brines. Associated models support the generation of crustal-scale convective flow systems that underpin the formation of major mineral provinces and provide a basis for differences in the flow behavior of faults, depending on a variety of factors such as fault zone complexities, host-rock properties, deformation conditions, and pressure drives. Flow heterogeneity along faults provides a basis for the thoroughly 3D flow systems that localize fluid flow and lead to the formation of mineral deposits.

Halloysite, a phyllosilicate clay mineral chemically similar to but structurally different from kaolinite, occurs in a variety of mineral deposit types. It is, however, difficult to identify and dehydrates quite readily. When identified in a porphyry or epithermal environment, halloysite is often interpreted to be a low-temperature polymorph of kaolinite and an indication of supergene processes. The occurrence of halloysite in the Cerro la Mina Au (Cu-Mo) prospect, Chiapas, Mexico, was investigated to determine if halloysite in porphyry-epithermal deposits might also be the product of hypogene processes. The prospect consists of Quaternary volcaniclastic breccias intruded by monzodiorites and trachyandesites crosscut by a volcanic-hydrothermal breccia pipe. Gold-copper-molybdenum mineralization at Cerro la Mina is structurally and lithologically controlled by matrix-rich breccia pipes hosting all of the significant alteration and mineralization drilled to date. Average grades within the prospect are 0.4 ppm Au, 0.16% Cu, and 131 ppm Mo. A total of 100 samples were obtained from 22 diamond drill holes over a range of stratigraphic levels. X-ray diffraction (XRD) and SWIR techniques were employed to identify the different clay minerals found within these samples, including halloysite, kaolinite, dickite, and illite. Identification of halloysite and kaolinite were facilitated by the use of formamide intercalation to produce distinctive peaks for the two minerals in X-ray diffractograms. Halloysite and kaolinite were found to occur from the current erosional surface to depths of over 800 m. Halloysite occurs as both the hydrated 10Å and dehydrated 7Å forms within the deposit. The morphology of the halloysite present is that of well-developed tubes and spheroids. Results of the study show that halloysite associated with gypsum and jarosite is of supergene origin, whereas halloysite occurring with quartz, alunite, dickite, kaolinite, and pyrite is of hypogene origin.

Faults are important structures in the formation of many mineral deposits, often acting as conduits for oreforming fluids and sometimes providing, or generating, the bounding structures to associated mineralizing sites. Using 3-D analysis and modeling of the Lisheen and Silvermines deposits within the Irish ore field, we investigate the geometry of normal fault systems and their implications on the origin and nature of associated deposits. These Irish-type deposits are carbonate hosted and developed within the hanging walls of normal faults arising from an Early Carboniferous episode of north-south rifting, with relatively limited amounts of later deformation. Structural analysis of high-quality mine datasets indicates that fault segmentation is ubiquitous with left-stepping segments arising from north-south stretching developed above generally ENE-NE-trending fault arrays, which are subparallel 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 structural evolution, with larger scale segments and intervening relay ramps defining distinct orebodies within deposits and smaller scale segments and relays potentially providing paths for upfault fluid flow. The difference in behavior is attributed to the integrity of associated relay ramps where intact ramps represent orebody-bounding structures, and smaller breached ramps provide enhanced associated hydraulic properties and act as vertical conduits. Hanging-wall deformation along the rheological boundary between host-rock limestones and underlying shales has an important control on the localization of earlier dolomitization and/or brecciation and later mineralization adjacent to this contact, and on the migration pathways for basinal brines and mineralizing fluids.Roisin Kyne is a structural geologist specializing in the formation of sedimentary rock-hosted base metal deposits. After receiving her HB.Sc. degree from Lakehead University (2009), Roisin completed a Ph.D. at CODES, University of Tasmania (2014), followed by a post-doc at iCRAG, University College Dublin (2018). Her work focused on structural controls of Zn-Pb-Cu deposits in Cobar, NSW, and the Irish ore field and implications for formation, fluid flow, and mineralization. In her current role as project geologist at Teck Resources, Roisin is applying her knowledge of 3-D modeling, basin analysis, and deposit formation to explore for Zn-Pb deposits in North America.

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