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FIG. 15. Model for mineralization at Lisheen. (a). Stage 1: Preore
dolomitization. Early diagenetic regional dolomitization of the Waulsortian
Limestone driven by brine reflux from the shelf area bordering the Leinster
massif predated or may have been partly synchronous with brecciation of the
Waulsortian Limestone. Lisheen is located at the northwestern fringe of a zone
of pervasive dolomitization, probably close to a shelf basin break controlled by
propagation of Caledonian basement structures up into the overlying cover in the
late Courceyan. (b). Stage 2: Fluid flow into the ore-forming environment. The
high intergranular matrix porosity developed as a result of synsedimentary
and/or the onset of hydrothermal brecciation was a critical ground preparation,
producing a permeable lithology into which later mineralizing fluids were
focused. Brecciation coupled with sufficient development of fault-related
permeability resulted in connectivity between the two main fluid reservoirs
(deep hydrothermal and shallow H2S-rich brine). This allowed the two
fluids to penetrate into, and mix within, the breccia units. Alteration and
stripping of Cu, As, Co, and Ni from the Old Red Sandstone adjacent to the main
east-west–trending fault array was likely the result of focusing and upward
flow of relatively reduced lower Paleozoic-equilibrated metal-bearing fluids
within fault-related damage zones. Flow into the basal Waulsortian breccias was
via the main Lisheen fault array and northwest-trending faults in the hanging
wall. Downward flow of low-temperature saline brines occurred by migration down
the upper parts of the Lisheen fault array or laterally along stratigraphy,
within the dolomitized Waulsortian, the Lisduff Oolite, and also possibly along
the regional dolomite front. Brine flow could have been driven by local
convection induced by thermal anomalies associated with the upwelling,
high-temperature fluids or by simple density contrast. (c). Stage 3: Fluid
mixing. The onset of fluid mixing within the breccias led to rapid precipitation
of sulfides in isotopic disequilibrium in any available pore space. Acid
generated by sulfide deposition enabled dolomite dissolution, thereby creating
additional porosity. The volume reduction associated with dolomite replacement
by sulfide could have caused as much as a 25 percent increase in porosity,
enabling further mixing and ore deposition. This, together with the elevated
initial porosity and nonstoichiometry of the matrix dolomite, may explain why
the lithology was such a favorable host rock to ore. Abundant evidence for
multiple stages of sulfide deposition, reworking, and brecciation suggests a
protracted evolution of the system in tandem with active faulting. Zone-refining
processes may have led to some of the metal zonation and textural complexity
observed. White matrix breccia may have developed above the ore zones due to
limited downward collapse caused by the volume loss associated with sulfide
replacement. (d). Stage 4: Hydrothermal sealing and progressive burial. Sulfur
isotope data suggest that mineralization terminated due to exhaustion of the
main H2S source and brine reservoir or blocking of brine infiltration
pathways, possibly by deposition of supra-Waulsortian argillaceous sediments.
The late ore stage involved choking of residual porosity by precipitation of
calcite, dolomite, and sulfides from the principal ore fluid alone. Sulfide
deposition at this time was probably driven by simple cooling. The late-stage
calcite (C4) and dolomite (D4) veins and breccias may
reflect overpressuring of the principal ore fluid in the main-flow conduits
caused by progressive burial and hydrothermal sealing. A late overprint by
low-temperature, high-salinity fluids resulted in remobilization of preore or
main ore-stage sulfides and precipitation of D5 pink dolomite. It is
suggested that these fluids represent evolved basin formation waters, mobilized
as a result of the onset of Variscan compression to the south of Ireland in the
Mid- to Late Carboniferous.
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34S values. Preore sulfides (mainly
pyrite) are characterized by very low 
), shown in gray boxes. Late to postore-stage values are shown in
black boxes. Higher values for both main- and late ore-stage sulfides are
observed near the main east-west–trending faults and/or northwest-trending
structures, suggesting that these are the main conduits for fluids transporting
hydrothermal sulfur from depth. Two principal subvertical conduits for
hydrothermal fluid ingress into the ore zones are suggested, one in the east
Main zone and one in the west Derryville zone.