The sulphide mineralization in the Allihies Region, County Cork, Eire.

Sulphide-bearing quartz veins occur within the Devonian sediments near Allihies, in the western part of the Beara peninsula, Co. Cork. A local facies change trends east-northeast, coincident in part with a major fault line. Folding and fracturing are different in the sandstone group to the north and the slate group in the south, since these processes are functions of lithology and the thickness of the beds comprising that lithology. The major fold trends west-south-west, hut smaller folds diverge from this trend, and may he conoidal in geometry. Three phases of jointing are proposed, hut emphasis is placed on the early-formed systematic joints which trend east- south-east. The cross-joints between these are occupied by quartz veins, so that Z-shaped bends occur. The main ore- shoots lie in these deflections. Isofracture maps show that these occur in low density areas within hands of high density fracturing. It is proposed that the major quartz veins were introduced before the folds were tightened up, so that deformation of the veins seen in the field and in thin section was the result of this folding phase. The mineralogy of the veins is discussed in length because of the recognition of molybdenite and mariposite. A number of parageneses is proposed, so that the conventional paragenetic sequence shows overlap in the order of deposition of most of the minerals, which include chalcopyrite, bornite, chalcocite, tetrahedrite, pyrite and specularite. Secondary minerals are also described. Exsolution lamellae of bornite in chalcopyrite and vice versa are seen. Propylitic and sericitic alteration haloes occur around most of the ore-hearing veins. Isopach maps are constructed of the thicknesses of these zones related to the thickness of the veins and lodes. Quantitative studies by X-ray diffractometry involved the preparation of standard calibration graphs for quartz, albite, muscovite, chlorite and dolomite, and results are presented. Similar coloured rocks to those adjacent to the lodes occur on a regional scale. Mineralogical comparisons are drawn between these and unmetamorphosed red beds elsewhere. Trace element analyses show a greater distinction between these beds. The origin of the ore bodies is discussed in the light of the evidence presented. If the quartz and metals come from an unseen deeper source, the source rocks must be acid igneous rocks to account for the molybdenite, unless the ore solutions only use the same channelways as the igneous rocks, and rise from the mantle. If they are derived by lateral secretion from the country rocks, movement of the appropriate solutions must have occurred in late diagenesis or early metamorphism when silica was released by the formation of chlorite from other clay minerals.