Sedimentary Cycles and Selective Dolomitization in Limestones Hosting the Giant Navan Zinc-Lead Ore Deposit, Ireland

The Navan zinc-lead ore deposit (70 million tonnes) is hosted principally (97%) by the Meath Formation (Lower Carboniferous, Courceyan, Navan Group), comprising a lower, mainly carbonate mudstone unit, the Stackallan Member, and an upper grainstone-dominated unit. The Stackallan Member, about 60 m thick, comprises about 35 peritidal cycles including an oolitic grainstone interval. Grainstones forming the upper part of the formation, about 150 m thick, comprise at least six shallowing-upward ramp cycles. Emergence horizons are indicated by subaerial dissolution surfaces, in situ breccias, and a single palaeosol. Most dolomitization at Navan is confined to the Meath Formation. The linear dolomite body, trending NE-SW, has a flattened, laterally limited, tabular geometry in cross-section, suggesting that dolomitizing fluids initially rose vertically, cross-cutting stratigraphic and sedimentological boundaries, although fluid flow was subsequently controlled by these features. Three stages of dolomitization are indicated by textural relationships, cathodoluminescence (CL), oxygen isotopes, and fluid inclusion data. Stage 1 crystals, with generally dully luminescing cores and up to six overlying subsidiary zones, are the main replacive phase. Crystal surfaces are commonly corroded and overlain by bright stage 2 rhombs (which also form new nuclei lining molds and fractures). Three subsets of zones are present in stage 2 crystals: bright, non luminescent, and dull; these are locally separated by additional corrosion surfaces. Stage 3 comprises relatively large baroque crystals, generally dully luminescent but with up to seven sub-zones. Isotopic and fluid inclusion data suggest growth of successive stages from waters which became progressively hotter (60°–160°C).
The distribution of the ores follows the dolomite trend. Ores were precipitated between Stages 1 and 2 of dolomite growth, following Stage 2, and also during the nucleation and growth of Stage 3 crystals. Dolomitization and mineralization appear to have been temporally and genetically related. The cyclic sedimentation of the host rocks, which accentuates lithological and petrographical contrasts between depositional units, controlled dolomitization. The relationship between dolomitization and mineralization, which is closely associated with faults, provides powerful exploration criteria. The recognition of these features in limestones elsewhere in the Irish ore field may point toward unrecognized zinc-lead prospects.
Keywords: Navan zinc-lead ore deposit, Meath Formation, Mudstone, Upper grainstone-dominated unit
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Summary: A consistency-driven pairwise comparisons method for mineral potential assessment is presented, using a simplified case of volcanic-associated massive sulfide type deposits as an example. Geological, geochemical, and geophysical criteria are considered on two levels: local and regional. The local geological criteria are subdivided into stratigraphy, lithology, alteration and/or mineralization, and structure. The concept of geomerit index and a procedure for computing this index are introduced...
Publication: Exploration & Mining Geology
Author(s): WALDEMAR W. KOCZKODAJ: Centre in Mining and Mineral Exploration Research Laurentian University, Sudbury, Ontario, Canada P3E 2C6; WILLIAM O. MACKASEY: WOM Associates, 140 Crater Crescent, Sudbury, Ontario, P3E 5Y8
Keywords: Mineralogy, Geological criteria, Geochemical criteria, Geophysical criteria
Issue: 1
Volume: 6
Year: 1997
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Summary: The Sudbury Structure consists of three major components: 1) the Sudbury Basin; 2) the Sudbury Igneous Complex which surrounds the basin as an elliptical collar; and 3) an outer zone of shatter-coned and intensely brecciated footwall rocks. Although the Sudbury Event which formed the structure may be unique, the area has, in fact, been shaped by a series of tectonic, magmatic and mineralization events which can be considered in terms of two Wilson cycles of continental extension and closure....
Publication: Exploration & Mining Geology
Author(s): DON H. ROUSELL, HAROLD L. GIBSON: Mineral Exploration Research Centre Department of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada P3E 2C6, and
IAN R. JONASSON: Mineral Deposit Division, Geological Survey of Canada 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8
Keywords: Mineralization, Sudbury structure, Tectonic events, Magmatic events
Issue: 1
Volume: 6
Year: 1997
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Summary: Measurement of natural radioactivity has been used in both a qualitative and a quantitative way in mineral exploration, particularly in the search for uranium. In the last five years, the Kernfysisch Versneller Instituut (KVI) and British Geological Survey (BGS) have designed, built and tested a new detector system that greatly improves quantitative applications in mineral exploration, especially on the seafloor and in the nearshore zone. The new system is an enhancement of an earlier BGS...
Publication: Exploration & Mining Geology
Author(s): D.G. JONES, P.D. ROBERTS: British Geological Survey, Keyworth, Nottingham NG12 5GG, United Kingdom; A. ROZENDAAL, W.G. MACDONALD: Department of Geology, University of Stellenbosch, P.O. Box X01, Matieland 7602, Stellenbosch, South Africa
Keywords: Radioactivity, KVI, BSG, Mineral exploration, Mineral Processing, Heavy-mineral concentration, On-line quality control
Issue: 1
Volume: 6
Year: 1997
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Summary: The Amazon region is characterized by the development of deep chemical weathering represented by widespread lateritic covers and soils. Two distinct periods of laterite formation can be distinguished: mature laterites from the Eocene-Oligocene (and locally from the end of the Cretaceous), and immature laterites from the Pleistocene. The older laterites occur on plateau landscapes, and the younger ones on widespread hilly to flat lowlands. Both types show complete or truncated profiles,...
Publication: Exploration & Mining Geology
Author(s): MARCONDES LIMA COSTA: Geosciences Center, Federal University of Pará, C.P. 1611 66075-110, Belém-PA, Brazil.
Keywords: Amazon Region, Laterite formation, Mineral deposits.
Issue: 1
Volume: 6
Year: 1997
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