The Irish Midlands host one of the world's major zinc orefields.
The Irish zinc deposits occur in a transgressive sequence of Lower
Carboniferous marine carbonate rocks lying above a wedge of Upper
Devonian continental red beds. The deposits have enough shared
characteristics, as well as differences from other carbonate-hosted
zinc-lead deposits worldwide, to have been given the sobriquet
"Irish-type." The Irish deposits share the following
features:
(1) They occur preferentially in the stratigraphically lowest,
non-argillaceous carbonate unit.
(2) They occur along, or immediately adjacent to, normal faults
which formed conduits for ascending hydrothermal fluids.
(3) Sphalerite and galena are the principal sulfides. Iron sulfides
occur in variable amounts; some deposits are dominated by iron
sulfides while others contain very minor amounts. Barite is present
in all the deposits, ranging from a dominant phase to a minor
constituent. Many deposits contain minor tennantite, chalcopyrite,
and/or Pb-Cu-Ag-As sulfosalt minerals.
(4) They are stratabound and many display large-scale stratiform
morphologies.
(5) They display complex sulfide textures ranging from replacement
of host rock by fine-grained, anhedral and colloform sulfides
to infill of solution cavities by fine-grained, colloform and
medium- to coarse-grained crystalline sulfides. Layered sulfide
textures, other than colloform banding, are restricted to geopetal
cavity fillings.
(6) They formed from the mixing of metal-bearing, moderately saline,
slightly acidic, relatively sulfur-poor fluids with relatively
sulfur-rich fluids that appear to have been derived from Carboniferous
seawater.
The Irish orefield is regionally zoned. Copper and silver are most common in deposits located within the southern portion of the country. Pre-mineralization dolomitization is also largely restricted to southern deposits. The age of mineralization is known with certainty only for the Navan deposit which formed several million years after deposition of its host sediments; geologic relationships suggest that the other Irish deposits formed at approximately the same time as the Navan deposit. This period is marked in the Irish Midlands by the establishment of a complex facies mosaic consisting of fault-controlled carbonate basins and high-standing platforms indicating an extensional tectonic environment. Extension was relatively modest and was related to continental collision (the Hercynian Orogeny) occurring to the south of Ireland.
The apparent contemporaneity of mineralization and tectonism, together with the regional zoning of metals and dolomitization, suggests that the Hercynian Orogeny was a fundamental driving force for mineralization in the Irish orefield. Topography-driven flow related to the uplift of Hercynian highlands to the south of Ireland produced a hydraulic head that drove formation waters northward through the confined Upper Devonian red bed aquifer. Along the flow path these formation waters increased in temperature due to burial and they leached metals. Fluids were focused into the area of present-day Ireland by a high-standing basement block to the east and by the northward thinning of the red bed aquifer. The Irish zinc deposits formed where normal faults tapped the confined red bed aquifer and focused flow of hydrothermal solutions upwards into the Lower Carboniferous carbonate sequence. This focusing allowed the development of discrete thermal anomalies capable of initiating thermal convection cells which mixed formation water from within the Carboniferous sequence with seawater from the overlying ocean.
Fluid inclusion studies indicate that the hydrothermal fluid had temperatures of between 150° and 240°C and salinities of between 10 and 23 weight percent NaCl equivalent when it reached the sites of sulfide precipitation. Limited fluid inclusion data suggests that the water pulled into the system from above was significantly cooler (<120°C) and less saline (<10 weight percent NaCl equivalent). Sulfide precipitation occurred as the metal-rich, sulfur-poor, mildly acidic hydrothermal fluids reacted with carbonate sediments causing an increase in fluid pH. Sulfur isotope studies indicate that sulfide precipitation was increased due to the mixing of hydrothermal fluids with the cooler, sulfate-rich water.
The host rocks and mineral textures of the Irish deposits are similar to many Mississippi Valley-type deposits. They differ, however, in having a metal suite which includes more copper, silver, and iron than most MVTs and in containing extensive zones of truly massive, often highly iron sulfide-rich, sulfide. These differences are probably the result of higher hydrothermal fluid temperatures which allowed higher metal contents in the fluids and increased reactivity.
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