DESCRIPTIONS of ORE DEPOSITS
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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.
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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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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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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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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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
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
Telephone: +61 8 8379 7397
Mobile: +61 422 791 776