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Geological Society, London, Special Publications; 1977; v. 7; p. 163-170;
DOI: 10.1144/GSL.SP.1977.007.01.20
© 1977 Geological Society of London

Volcanogenic mineralization at Avoca, Co. Wicklow, Ireland, and its regional implications

J. W. Platt

Avoca Mines, Ltd., Avoca, Arklow, Co., Wicklow, Ireland

A description is given to the geological setting of the stratiform pyritic orebodies at Avoca in Ireland and a volcanogenic origin is proposed on the basis of comparison with stratiform orebodies located in similar volcanic host rocks in other parts of the Caledonian orogen and elsewhere. The Ordovician volcanic sequence in southeast Ireland is interpreted as a fossil island arc complex, which was formed above a contemporary margin of southeasterly subduction, which locally defined the boundary of the evolving Caledonian orogenic belt in southeast Ireland.

The Avoca ore deposits are located within a sequence of calc-alkaline lavas, pyroclastics and sediments at a level marked by the predominance of felsic eruptives. The stratigraphy of the host rocks is described in some detail and related to the general geology of the Avoca belt. Massive pyritic ores and disseminated sulphide ores of ‘stringer type’ can both be recognized at Avoca. The massive ore shows a compositional gradation suggestive of a primary stratigraphy; a Cu-rich pyritic zone is overlain by a zone of Pb-Zn-Cu pyritic ore.

The interpretation of the genesis of the orebody is complicated by its deformed state, which is typical of other volcanogenic orebodies within the various Caledonian provinces. A genetic model can be proposed, however, by using the descriptions of the undeformed ‘Kuroko’ deposits in Japan as a standard for comparison.

The individual orebodies of Avoca type contain 5 000 000–6 000 000 tons of ore, and these are associated with extensive zones of low-grade disseminated ore in acid pyroclastic host rocks. Sequences of manganiferous and dolomitized sediments are stratigraphically closely related to the ores.

The principal geological features of the Avoca orebodies having been defined, it is deduced that there is a strong possibility that similar orebodies lie hidden in the volcanic rocks of the southeast Irish Caledonides and that an aggressive exploration programme could lead to new discoveries.