Multi-scale fluid-flow path analysis: Calibration and modelling using
Andy Barnicoat University of Leeds
Characterisation of structural controls on mineralisation using grade data, mine
plans and underground work
Grade distribution at the mine scale marked by a very low-grade domain in the NW and
very high grades in the SE. This does not appear to correlate directly with facies.
Zn/Pb values correlate with total mineralisation level at a scale smaller than the
NE and NW trending elements occur in the grade distribution. These correlate
only patchily with the B/T faults. Features in these orientations can be seen at a
wide range of different scales and over areas with differing drilling profile
orientations. They thus seem to be real and not an artefact of the gridding
NE: NW features occur in the approximate frequency ratio 5:3.
Cross sections of the grade data show that rather than occurring as massive,
uniform stratabound mineralisation, metal grades displays centres with relatively
uniform spacing, in particular on NE-SW lines. Grades drop substantially
between these centres.
The mapped fault pattern in the 5 lens can be stripped into three main suites of
structures with differing orientations. The pattern may be interpreted as revealing
the capture of Caledonian trends during reactivation in the Carboniferous.
One family of structures (called the ‘A’ family but awaiting another name)
parallels the NE-SW grade lineaments. Very rare faults trend NW on the map,
but calcite-bearing ‘joints’ mapped underground have a pronounced cluster
5 lens isopachs show NW and NE trending rapid thickness changes, which may
be due to the reactivation of basement structures during sedimentation. Further
reactivation of such trends is thought to be responsible for the patterns seen in
the grade data.
Underground, NW and NE striking sulphide –filled veins (faults) are very
common. Individual elements are generally 0.1 – 1m long, rarely up to 2 m.
These veins feed pods and layers of sulphide in zones 10 –30 m wide.
These observations combine to suggest that the vertical migration of
hydrothermal fluids is extremely important, and that lateral fluid movement
leading to ore formation occurred within the damage zones of these structures
within the limestone stratigraphy. Extensive layer-parallel fluid migration is hard
to reconcile with the detailed grade patterns and underground observations. This
does fit with the comments of various people that most of the stratigraphy was
‘tight’ prior to mineralisation, in particular where dolomitisation had occurred.
Note, however, that the veins represent only a small proportion of the total
The orientation of veins underground matches that of the lineaments picked
from the grade database.
Topography of the stratigraphy (derived from survey peg data) shows that
displacements of < 2 m are present in the mine across unmapped small faults.
Typical displacement gradients imply that maximum displacements of ca. 1-2 m
are likely. Such features will in general be captured on routine mine mapping.
Veins are cut by faults: this and relationships with the conglomerate group ore
are critical constraints on timing. The general feeling was that ore formation
probably occurred more-or-less simultaneously in all of the affected stratigraphy
at the time of the development of the unconformity beneath the conglomerate
group. This implies a depth of perhaps 600 m or so.
Collation of old mine plans at Leadhills shows a rather similar pattern to that at
Regional-scale relay zones developed in the Carboniferous by the reactivation of
basement faults looks to be the critical structural pattern responsible for the
system at Navan.
Preliminary results of fluid inclusion work at Tara Mine
Inclusions are small (most < 10 μm, more typically 3-4 μm) and relatively sparse.
Minerals used include calcite, dolomite, sphalerite and barite. There is no
correlation of inclusion size and homogenisation temperature (other than
perhaps in barite) suggesting that necking and stretching have not occurred.
Temperatures are thus likely to be unaffected by any post-filling adjustment.
In general, two populations of inclusions are observed – earlier higher
temperature (120- 200 ºC) lower salinity inclusions (5 – 5 wt% NaCl equiv.) and
later inclusions lower T (100- 150 ºC) and higher salinity (18 –25 wt% NaCl
equiv.). Three is some suggestion that there is a progressive change in inclusion
chemistry between the two (in terms of spatial distribution) which if corroborated
is strong evidence in support of fluid mixing.
The general characteristics of the populations are similar to that in other Irish ore
deposits, though at lower temperatures.
Early planar dolomite (5 lens dolomite) contains more dilute, higher T inclusions.
There is a suggestion in a dolomitised sample containing a dolomite- calcite vein
that chemistry changes from higher T to lower T brines then T increases again.
CL signatures correlate with inclusion populations.
The significant variation in inclusion chemistry documented in small samples
means that only in some case can bulk crush-leach techniques be used. It is likely
to be possible that at least a few samples with the end-member inclusion
populations will be available. Key indicators to look at will be halogen ratios (to
characterise fluid sources), alkali ratios (to characterise equilibration temperatures
with the basement where appropriate) and Li contents (to characterise the degree
of interaction with basement lithologies). Redox state information may be
available from Fe/Mn values, though only in minerals lacking those elements
(any quartz? Perhaps only in basement veins).
Microanalytical techniques can probably look at inclusions down to about 15 μm
diameter (Zurich) – we need to try and select/characterise such sample where
There is no sign of any hydrocarbon in the inclusions. Given the nature of the
host rocks and the frequency of hydrocarbon shows elsewhere in similar rocks,
this may be a significant observation implying that likely hydrocarbon has been
Discussion was wide ranging, but probably the key topic addressed was the
nature and pathway of the ‘second’ fluid, the low T brine.
The bacteriogenic sulphur signal must have developed in an open system (i.e. one
where the sulphate reservoir was large and in communication with the site of
reduction). Either the reduction would have been in situ or waters containing
reduced S were transported to the site of ore formation.
Open system processes imply that the bulk of the S in the water was present as
sulphate, hence the waters must have been relatively oxidised (fO2 > hm/mt).
Note that pyrite formed between ore-forming events has a 34S of -30 - -40‰:
this potentially represents the background signal.
The pathway for this modified seawater reaching ore sites is not yet well defined.
It may have been able to migrate down faults that at depth had hydrothermal
fluid migrating up them. Alternatively, fluid migration may have occurred along
stratigraphy: many of the likely aquiclude layers in the stratigraphy are not
continuous over enormous areas (though they may well blanket he mine area).
Petrographic work shows that vuggy porosity in carbonates (including dolomites)
and the sandier horizons was filled with zoned carbonates identical to those
found in the ore. Apparent porosities of ~20% have been found in some
samples. This implies that stratigraphic permeability probably did exist at the time
of mineralisation. Characterisation of samples from the same stratigraphic
horizons away from the ore bodies is hence desirable.
The low T brines were probably partially evaporated seawater derived from brine
pools (shallow potentially barred areas, potentially even emergent). Comment
was made that these fluids appear absent from small ore showings elsewhere and
may be critical in the development of significant ore bodies. This implies the
possibility of a regional palaeogeographic control on the location of big ore
stable isotope work at SURRC – 2 tranches either side of Christmas/New Year.
Ion probe time to characterise the18O signatures of the various generations of
dolomite (and calcite – but lees important to the story) mid-2001
Micro-drilling of carbonates for Sr analysis to be scheduled after laser and ion
probe work (sample destruction).
Regional fieldwork in January focusing on (a) Longford Down quarries/coastal
exposures etc. to look at basement fractures populations and veins and (b) the
stratigraphy between the mineralisation and the Conglomerate group
unconformity to look for evidence of the migration of brines.
Next group meeting in February to discuss stable isotope results and regional