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Navan |
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Ireland |
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Zn Pb
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Navan zinc-lead deposit is located 1 km west of the town of Navan in County Meath, and 45 km NW of Dublin, in Ireland.
The Navan deposit lies within the Midlands basin in Ireland, which was developed during a northward marine transgression across the southern margin of the subsiding continent of Laurussian in the Lower Carboniferous (Courceyan).
A basement of Ordovician siltstones, sandstones and mudstones with intercalated tuffs and volcanic breccias of diverse composition, intruded by syenites and lamprophyres of probable late Caledonian age, is unconformably overlain by the Courceyan Navan Group.
The Navan Group commenced with 0 to 45 m of basal Red Beds, a dark red interbedded unit of interbedded polymictic conglomerate and sandstone. These which upward into a 30 to 45 m thick sequence of fluviatile and shallow marine dark sandstones, siltstones and dark bioclastic limestones, termed the Laminated Beds. These rocks are frequently bioturbated and include a thin, <30 cm, bed of nodular chalcedonic silica, thought to represent a replaced evaporite horizon. The transition to the overlying 10 to 20 m thick Muddy Limestone marks a rapid change from periodic siliciclastic deposition to dominant carbonate sedimentation, composed of generally dark, well-bedded argillaceous and crinoidal limestone with local, strongly bioclastic and coarser microconglomerate.
The Muddy Limestone pass through a variably thick, possibly diachronous transition zone into the overlying Pale Beds, a >200 m thick sequence of micrites, bioclastic and oolitic grainstones with subsidiary sandstones, shales and siltstones, host the known mineralisation. A fine-grained, pale micritic limestone, which marks the base of the Pale Beds, varies from a few to as much as 60 m in thickness, with the thinnest sections corresponding to the greatest thickness of overlying ore, whiule the thicker sections correspond to the margins of the orebody.
The Pale Beds have been divided into a number of lenses, based on separating distinctive marker beds used to classify the main ore layers in the deposit. The lowest of these is No. 5 Lens, composed of poor- to moderately-bedded, fine- to coarse-grained, pale- to medium-grey, bioclastic, locally pelitic and oolitic calcarenite and calcirudite. The base of the lens is marked by the basal pale micritic limestone, and is separated from No. 4 Lens by the Lower Dark Marker, a 5 m thick, dark, irregularly laminated, micaceous siltstone. No. 4 Lens is a medium-grey, moderately-bedded, locally quartzitic calcarenite with minor cross-bedding, which is capped by the Lower Sandstone Marker, a frequently poorly defined, massively bedded calcareous sandstone of variable thickness up to 5 m. No. 3 Lens is composed of moderately well bedded, medium-grey calcarenite with local cross-bedding, sometimes with dark silty horizons or coarser intraclast-bearing calcirudite layers. This lens is overlain by the 5 m thick Nodular Marker which comprises pseudo-nodular, dark silty, locally bioclastic and crinoidal calcarenites. No. 2 Lens denotes a persistent, thin sulphide band found beneath the Nodular Marker. No. 1 Lens contains locally oolitic calcarenite and is terminated by several closely spaced dark shale horizons over a 1 to 3 m interval, known as the Upper Dark Marker. The overlying Pale Beds become slightly paler, more massively bedded and quartzitic, and are largely absent from the mine area as a result of the post-Arundian erosion. About 12 m above the Upper Dark Marker, the 5 m thick Upper Sandstone Marker is a clean, massively bedded sandstone.
The succeeding Shaley Pales consist of ~100 m of varied shaley, bioclastic, locally bryazoan-rich calcarenites and well-bedded shales, which are separated by a thick mudstone horizon from the Argillaceous Bioclastic Limestones, which comprise well-bedded, strongly crinoidal, shaley limestone, which become increasingly crinoidal and paler upwards, until they grade into the more typical reef limestone of the Waulsortian Mudbank Limestones that marks the top of the Navan Group.
The onset of widespread extension during the late Courceyan to early Chadian, led to the development of syn-depositional faulting, influenced by the reactivation of old basement structures. The orebody is spatially associated with several of these major NE to ENE-trending extensional faults, an area of low angle slides and major erosion, and fault inversion of likely Variscan (Late-Carboniferous) age. This faulting generated high permeability pathways for migrating hydrothermal fluids, which were responsible for sulphide mineralisation. The faulting locally reflects a transition from early Chadian, NW-dipping normal faults, to a rift-related episode of complex SE-directed faulting and low-angle slides on the NW flank of the developing Dublin Basin. These structural processes resulted in extensive fracturing and minor faulting in the central part of the deposit. The transition from early NW-dipping faults and a fractured relay ramp, to SE-directed faulting and sliding in the central part of the deposit, appears to coincide with the main period of mineralisation. The location and geometry of the faults and slides are consistent with the structural degradation (sliding/slumping) of an uplifted footwall on the NW side of a major SE dipping normal fault.
The Courceyan rocks of the Navan Group were truncated by this series of low angle slides and erosional surfaces caused by this faulting. The resulting structural degradation surface, displaying a relief of >500 m, resulted in the removal of >300 m of overlying sedimentary cover from above the zone of mineralisation, and the deposition of submarine debris flows and fault talus deposits that blanket the downthrown SE side of the structure, and constitutes the Boulder Conglomerate. This conglomerates, which overlies the Waulsortian and Pale beds above a late Courceyan-early Chadian erosional surface, comprises erratically sorted, crudely layered, angular to sub-rounded clasts (from pebble size to boulders of >5 m) of Pale Beds, Shaley Pales and Reef Limestone within a dark argillitic matrix. It hosts Fe-rich mineralisation (Conglomerate Group Ore, comprising ~3% of the resource) and are succeeded by a thick sequence of Arundian to Asbian calc-turbidites of basinal character, locally termed the Upper Dark Limestones. The Upper Dark Limestone is largely composed of thick-bedded alternations of thin dark shales and more massive beds of graded calcarenites and calcirudites of turbiditic origin
The host carbonate rocks of the Navan Group contain typical early marine calcite cementation (except the basal Pale Beds micrite which shows evidence of emersion and early meteoric cementation). This primary phase is overprinted by texturally-destructive dolomitisation, interpreted to have been related to both seawater-derived brines and hydrothermal fluids. Generally, the host rocks proximal to ore are partially- to well-dolomitised, with the ore having been emplaced prior to the total occlusion of porosity. Stratabound dolomite bodies commonly follow sandy and silty oolitic horizons in the Pale Beds, locally forming distinct hanging-walls to some ore lenses, while a 'plume-like' mass of saddle dolomite within the Pale Beds occurs above a zone of extensional faulting below the Main Orebody.
The bulk of the ore at Navan occurs as a series of superimposed, overlapping and interconnected tabular lenses which are generally concordant. The main hosts are the shallow water Carboniferous carbonate succession of the Pale Beds. The deposit occupies a continuous thickness of up to 80 m in sections of the Main Orebody, and occurs as a NE-trending 5 x 1 km ellipsoid, dipping at 15°SW from the surface and flattening with depth. The distribution of ore is erratic with rapid changes in thickness, grade and location within the host sequence. Mineralisation within the Main Orebody is most intense near the base of the Pale Beds, but best developed in the upper Pale Beds in the SW Extension, while to the east of the Main Orebody multiple, superimposed, stratabound ore lenses are found. Fault control is evident, although ore distribution is not governed by a single structure. Strong stratigraphic controls also exist with mineralisation preferentially located below dolomitic siltstones/shales and in bioclastic grainstones having remnant porosity at the time of mineralisation. The principal ore minerals are sphalerite and galena, with iron sulphides and barite, accompanied by minor silver and antimony bearing sulphosalts. Although significant iron sulphides are present, the Pale Beds are characterised by a low Fe content overall. Ore textures are very complex and variable, comprising varying proportions of disseminated, replacement, fracture-fill, breccia, vein and massive styles that exhibit repetitive disruption, contorted banding, cyclic fine-grained sulphide deposition and open space filling and occasional barite bands.
The Boulder Conglomerate, which comprises a chaotic debris flow unit, contains clasts of mineralised Pale Beds, and an associated development of pyritic 'matrix', disseminations and banded massive sulphide. Cross-cutting mineralisation occurs as discrete veins and breccia zones developed with a NE- to ENE-trend.
The zones of high Zn+Pb within the Main Orebody are developed parallel to NE-SW structural trends and are coincident with major faults or localised zones of minor extensional fracturing. The Zn:Pb ratio increases westwards and downwards, and Fe increases to the NE and upwards, with the strongest Fe concentrations in the pyritic, stratigraphically highest lens, the Conglomerate Group Ore. In the SW Extension, lateral enrichment of Zn+Pb has a strong correlation with early NW-dipping faults, although the zonation of Zn:Pb is indistinct. Lithogeochemical studies indicate irregular halos in the Pale Beds and Upper Dark Limestones characterised by enrichments of Zn, Pb, Mn, Fe, As, Sb, Ba and Mg.
The deposit contained an original pre-mining resource of:
69.9 Mt @ 10.1% Zn, 2.6% Pb, at a 4% combined Zn+Pb cut-off (Ashton et al.,, 1986).
Production + resources in 2010 are quoted (Barker and Menuge, 2010) as:
~105 Mt @ 8.1% Zn, 2.0% Pb, composed of
Production to end 2009: 73.1 Mt @ 8.3% Zn, 1.9% Pb (Ashton et al., 2010), with
JORC compliant Mineral Resources of 11.8 Mt @ 7.1% Zn, 1.8% Pb + Ore Reserves of 17.0 Mt @ 7.2% Zn, 1.8% Pb.
This summary is largely based on descriptions and interpretations in Ashton et al., 1986, Blakeman et al., 2002 and Ashton et al., 2010 and references quoted therein.
The most recent source geological information used to prepare this decription was dated: 2010.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Anderson I K, Ashton J H, Boyce A J, Fallick A E, Russell M J 1998 - Ore depositional processes in the Navan Zn-Pb deposit, Ireland: in Econ. Geol. v93 pp 535-563
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Ashton J H 1995 - Guide to the geology of the Navan orebody: in Anderson K, Ashton J, Earls G, Hitzman M, Tear S (Eds.), Irish Carbonate Hosted Zn-Pb Deposits SEG Guidebook Series, Littleton, Colorado, USA v21 pp 150-169
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Ashton J H, Black A, Geraghty J, Holdstock M, Hyland E 1993 - The geology setting and metal distribution patterns of Zn-Pb-Fe mineralization in the Navan Boulder Conglomerate: in Bowden A A, Earls G, O Connor P G, Pyne J F (Eds.), The Irish Minerals Industry 1980-1990 Irish Association for Economic Geology, Dublin pp 171-197
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Ashton J H, Blakeman R, Geraghty J, Beach A, Coller D, Philcox M, Boyce A and Wilkinson J J, 2010 - The Giant Navan Carbonate-Hosted Zn-Pb Deposit – A Review: in Archibald S M (Ed.), 2010 Proceedings of the Zinc2010 Meeting, Cork 2010, Irish Association for Economic Geology pp. 102-107
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Ashton J H, Downing G T, Finlay S 1986 - The geology of the Navan orebody: in Andrew C J, Crowe R W A, Finlay S, Pennell W M, Pyne J F (Eds.), Geology and Genesis of Mineral Deposits in Ireland Irish Association for Economic Geology, Dublin pp 243-280
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Barker G J and Menuge J F, 2010 - The geological significance and mining related problems associated with framboidal pyrite-rich ore at Navan, Ireland: in Archibald S M (Ed.), 2010 Proceedings of the Zinc2010 Meeting, Cork 2010, Irish Association for Economic Geology pp. 38-41
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Blakeman R J, Ashton J H, Boyce A J, Fallick A E, Russell M J 2002 - Timing of interplay between hydrothermal and surface fluids in the Navan Zn + Pb orebody, Ireland: evidence from metal distribution trends, mineral textures and 34S analyses: in Econ. Geol. v97 pp 73-91
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Fallick A E, Ashton J H, Boyce A J, Ellam R M, Russell M J 2001 - Bacteria were responsible for the magnitude of the world-class hydrothermal base metals sulfide orebody at Navan, Ireland: in Econ. Geol. v96 pp 885-890
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Gagnevin D, Menuge J F, Kronz A, Barrie C and Boyce A J, 2014 - Minor Elements in Layered Sphalerite as a Record of Fluid Origin, Mixing, and Crystallization in the Navan Zn-Pb Ore Deposit, Ireland: in Econ. Geol. v.109 pp. 1513-1528
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Gagnevin D, Menuge J F, Murphy M A, Barrie C D, Boyce A J and Blakeman R J, 2010 - Genesis of the Navan Zn-Pb ore body (Ireland): insight from Fe, Zn, Pb and S isotopic variations in sphalerite: in Archibald S M (Ed.), 2010 Proceedings of the Zinc2010 Meeting, Cork 2010, Irish Association for Economic Geology pp. 50-53
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Hitzman M W 1995 - Mineralisation in the Irish Zn-Pb-(Ba-Ag) Orefield: in Anderson K, Ashton J, Earls G, Hitzman M, Tear S (Eds.), Irish Carbonate Hosted Zn-Pb Deposits SEG Guidebook Series, Littleton, Colorado, USA v21 pp 25-61
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Hitzman M W 1995 - Geological setting of the Irish Zn-Pb-(Ba-Ag) Orefield: in Anderson K, Ashton J, Earls G, Hitzman M, Tear S (Eds.), Irish Carbonate Hosted Zn-Pb Deposits SEG Guidebook Series, Littleton, Colorado, USA v21 pp 3-23
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Johnston J D 1999 - Regional fluid flow and the genesis of Irish Carboniferous base metal deposits: in Mineralium Deposita v34 pp 571-598
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LeHuray A P, Caulfield J B D, Rye D M, Dixon P R 1987 - Basement controls on sediment-hosted Zn-Pb deposits: a Pb isotope study of Carboniferous mineralization in central Ireland: in Econ. Geol. v82 pp 1695-1709
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Peace W M, Wallace M W, Holdstock M P, Ashton J H 2003 - Ore textures within the U lens of the Navan Zn-Pb deposit, Ireland: in Mineralium Deposita v38 pp 568-584
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Phillips W E A, Sevastopulo G D 1986 - The stratigraphy and structural setting of Irish mineral deposits: in Andrew C J, Crowe R W A, Finlay S, Pennell W M, Pyne J F (Eds.), Geology and Genesis of Mineral Deposits in Ireland Irish Association for Economic Geology, Dublin pp 1-30, xvi
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Symons D T A, Smethurst M T, Ashton J H 2002 - Paleomagmatism of the Navan Zn-Pb deposit, Ireland: in Econ. Geol. v97 pp 997-1012
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Walshaw R D, MenugeJ F and Tyrrell S, 2006 - Metal sources of the Navan carbonate-hosted base metal deposit, Ireland: Nd and Sr isotope evidence for deep hydrothermal convection : in Mineralium Deposita v41 pp 803-819
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Wilkinson J J 2003 - On diagenesis, dolomitisation and mineralisation in the Irish Zn-Pb orefield: in Mineralium Deposita v38 pp 968-983
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Wilkinson JJ, 2010 - A Review of Fluid Inclusion Constraints on Mineralization in the Irish Ore Field and Implications for the Genesis of Sediment-Hosted Zn-Pb Deposits : in Econ. Geol. v105 pp. 417-442
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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