An AMF-PGC International Study Tour
Developed & Managed by Porter GeoConsultancy
Zinc '98
Zinc Deposits of Europe & North America
16th July to 17th August, 1998 - In Three Separate Modules
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DESCRIPTIONS of ORE DEPOSITS

[Navan Mine, Ireland]
Image: Tara Mines headframes, Navan, Ireland.   
This tour, which was developed, organised and managed by Mike Porter of Porter GeoConsultancy Pty Ltd (PGC), on behalf of the Australian Mineral Foundation Inc. (AMF), included:

Module 1 - Europe, Thursday 16 to Friday 24 July, 1998 For information on the remainder of the tour, see the Deposit Descriptions for Module 2 and Module 3

MODULE 1 - EUROPE

This module included a one day seminar in Dublin, Ireland, to provide an overview of the tectonics, geology and metallogeny of Europe, and Ireland in particular, with special reference to base metals.  In addition briefings were given on the Pyrite Belt in southern Portugal and Spain, exploration in the Pyrite Belt and the geology and base metal mineralisation of Cantabria in northern Spain.  Deposits were visited in the following order.

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Neves Corvo

In the Neves Corvo area there is an allochthonous and an autochthonous unit, both encompassing rocks of upper Devonian to lower Carboniferous age.  Both units are divided throughout the Iberian Pyrite belt into a lower Phyllite-Quartzite group, a Volcanic-Siliceous group and an upper Flysch group.  The host Volcanic Siliceous complex is in the autochthonous unit and comprises black shales, siliceous shales, tuffites and cherty rocks with the massive sulphide mineralisation and footwall stockworks with chloritic alteration.  This complex also contains felsic tuffs and related pyroclastics, locally with strong chloritic and quartz-sericite alteration.  Neves Corvo lies on a major lineament that controlled the development of volcanism in the region.  The ore is mineralogically variable, although the dominant minerals are pyrite, chalcopyrite, sphalerite, galena, cassiterite, stannite, tetrahedrite and arsenopyrite, present as 'banded ore', 'massive sulphide', and 'fissure and breccia' ore.  The cupriferous ore, which is the principal exploited ore type, comprises 31 Mt @ 8% Cu, 0.2% Pb, 1.4% Zn, although the associated polymetallic mineralisation constitutes a further 33 Mt @ 0.46% Cu, 1.13% Pb, 5.72% Zn, 40 g/t Ag.  The tin ore zone contained 2.9 Mt @ 2.4% Sn, 13.4% Cu, 1.3% Zn.  Neves Corvo is owned and operated by SOMINCOR, Sociedede Mineira de Neves Corvo, SA, an subsidiary of Rio Tinto.

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Reocin

The Reocin deposit lies within the late Triassic-Jurassic to Cretaceous Basque-Cantabria Basin of northern Spain.  This basin is characterised by shallow to fresh water sediments, saline deposits, with gypsum and clay, marls, limestones, dolomites, sandstones, etc..  The Reocin deposit is immediately hosted by a 150 m thick Cretaceous (Aptian) carbonate bed that has been completely dolomitised.  The main ore zone covers an area of around 3500 x 800 m with four distinctive zones.  Two of these zones are composed of a total of 4 lenses, each of 2 to 6 m in thickness, while the third is 20 m and the fourth 45 to 60 m thick.  The ore occurs immediately above a black dolostone which in turn follows a marly limestone unit.  The host dolomite is overlain by an arenaceous band, which grades laterally into black marls and marly dolomite.  The ore occur as a shallow dipping zone of interconnected stratabound, tabular lenses of 'contorted' banded ore.  It is mined by open cut and by underground at rates of 0.77 & 0.87 Mt per annum in 1988.  The principal minerals are sphalerite, marcasite, galena, melnikovite and sparse pyrite, with a gangue of dolomite and calcite, and secondary smithsonite, melanterite and epsomite.  The ore is both stratabound and controlled by structure, with richer ore following one particular fault line direction.  The total resource at Reocin amounted to 87 Mt @ 11% Zn, 1% Pb, of which proven reserves in 1989 were 29.8 Mt @ 11% Zn, 1.7% Pb.  Reocin is an operation of Asturiana de Zinc, SA.

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Navan

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 a shallow water Carboniferous carbonate succession known as the Pale Beds which have a gentle SW dip, and are members of the Navan Group.  Ore within the Pale Beds is characterised by a low Fe content, a fine grain size, contorted and disrupted banding, occasional barite bands, and a large variety of mineral textures.  The maximum development of mineralisation in the Pale Beds occupies a continuous thickness of up to 80 m in sections of the orebody.  An erosional surface cuts the orebody, overlain by a chaotic debris flow unit known as the Boulder Conglomerate which 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 deposit contained a reserve of 77 Mt @ 10% Zn, 2.6% Pb, which has been enlarged recently.  Navan is operated by Outokumpu Tara Mines Limited.

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Lisheen

The Lisheen deposit of 21 Mt @ 12% Zn, 2% Pb, is hosted by lower Carboniferous carbonates within the Rathdowney Trend in central Ireland.  The first discovery of massive sulphide at the prospect was in April 1990.  Within the prospect area there are three main units, the lower Argillaceous Bioclastic Limestone, the Waulsortian Limestone composed of biomicrites, argillites and micrites, and the overlying cherty calcarenites if the Crosspatrick Formation.  The ore is located along the western edge of a broad zone of dolomitisation within the Waulsortian and Crosspatrick Formations along the Rathdowney Trend.  Ferroan dolomite zones over print the dolomitisation and partially envelope, but predate the sulphides.  Massive sulphides are localised immediately above the base of the Waulsortian as a series of stratabound lenses with zones of vein stockworks.  Minor mineralisation extends down into the Argillaceous Bioclastic Limestone as disseminations accompanied by silicification and veining.  Mineralisation is interpreted to represent host rock replacement and open space fill.  A black matrix breccia forms irregular zones, above, below and laterally to massive sulphides.  This breccia is composed of angular to sub-rounded, irregular clasts of dark grey dolostone, set in a very dark grey to black, <1 to 15µm grain sized matrix of ferroan dolomite, non-ferroan dolomite and minor disseminated iron sulphides, with abundant microscopic voids.  The Lisheen mine is being developed by Minorco.

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Galmoy

The Galmoy ore body had an original proven + probable reserve of 6.5 Mt @ 12.9% Zn, 1.26% Pb.  It is hosted by lower Carboniferous carbonates of the Waulsortian Limestone on the Rathdowney Trend in central Ireland, and is approximately 10 km to the north-east of Lisheen.  The zinc-lead sulphide mineralisation is hosted by a grey rock matrix breccia within the Waulsortian Limestone, which at Galmoy is dolomitised throughout.  This breccia is localised stratigraphically, generally commencing within 1 m of the base of the Waulsortian, and has limited lateral extent regionally.  It is composed almost entirely of sub-rounded to sub-angular clasts of dolomite in a dark grey, fine grained dolomitic and argillaceous matrix.  There are two main orebodies, the 700x450x6 m CW-Orebody, and the 450x300x8 m G-Orebody.  There are no significant faults in the vicinity of the CW-Orebody, although the G-Orebody is truncated and displaced by an east-west normal fault.  The stratabound massive sulphides are underlain by a zone of Cu-Ag mineralisation.  Within the main sulphide bands, pale to buff sphalerite and galena occurs as fracture and void fill sulphides, accompanied by sulphide replacement of the host carbonates along clast boundaries and vein margins.  The sphalerite-galena lenses pass gradationally up into a pyrite-marcasite cap which has a sharp upper margin where it has become massive sulphides.  The Galmoy mine is operated by Arcon Mines Limited.

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For more information contact:   T M (Mike) Porter, of Porter GeoConsultancy   (mike.porter@portergeo.com.au)

This tour was designed, developed, organised, managed and escorted by
T M (Mike) Porter of Porter GeoConsultancy Pty Ltd.

Porter GeoConsultancy Pty Ltd
6 Beatty Street
LINDEN PARK, 5065
South Australia
Telephone: +61 8 8379 7397
Mobile: +61 422 791 776



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