Earth Science Conservation Review
| Cashel Rock - Mineralogy - site of local interest | Tyrone |
| Site Type: | Inland exposure |
| Site Status: | local interest |
| Council area: | Omagh District Council |
| Grid Reference: | H589801 |
| Google maps: | 54.66602,-7.08716 |
| Rocks | |
|---|---|
| Rock Age: | Ordovician |
| Rock Name: | Tyrone Volcanic Group |
| Rock Type: | Breccia, Rhyolite |
| Interest | |
| Minerals: | Chalcopyrite, Galena, Pyrite, Sphalerite, Covellite |
| Other interest: | mineral vein, stockwork |
Description:
The location comprises an outcrop (5 x 7m) of silica-veined rhyolite with minor sulphides, and 60m to the west, boulders of a silicified, brecciated rhyolite that is locally rich in sulphides, principally pyrite, chalcopyrite, covellite, galena and iron-rich sphalerite. The latter locality is somewhat sensitive to collection as interesting material is present mostly in boulders, although these are large (1-2m diameter) and are difficult to break even with a heavy hammer.
The Tyrone Volcanic Group (TVG), comprises predominantly volcanic and intrusive granitoid rocks of Ordovician age that were formed in an island arc setting during closure of the Iapetus Ocean. The volcanic sequence comprises basaltic andesite to rhyolitic submarine and subaerial lavas, volcaniclastics with chert horizons and intercalations of graptolitic shales (Hartley, 1933). Arthurs (1976) reported reconnaissance geological and geochemical work in the Sperrins that identified the Tyrone Volcanic Group as metal-enriched. Legg et al. (1985) noted that a prospecting licence was held over the TVG area by Riofinex from 1971 to 1975. Riofinex undertook geophysical and geochemical surveys, trenching, drilling and rock analyses and reported gold assays up to 2.6g/t in pyrite-veined rock float at Cashel Rock. Leyshon and Cazalet (1976) reported on the Riofinex exploration programme and concluded that some of the intrusives are host to significant but sub-economic copper mineralization.
Clifford et al. (1992) followed up the exploration undertaken by Riofinex and reported that gold and base metal enrichment is widespread in the Tyrone Volcanic Group. Mineralization occurs in a variety of styles including stratiform enrichment in cherts and iron formation, stockworks in rhyolitic intrusions, and porphyry-type occurrences associated with tonalite plutons. Exploration at Cashel Rock established that the sulphide-rich breccia is up to 2m thick and that the mineralized zone, intersected in 14 drillholes, extends over a strike length of at least 200m and to a depth of at least 30m. The best intercept from drilling was 3.63m grading 30.5g/t Au including a 1.23m interval assaying 1.14% Cu and 1.85% Pb. Gold occurs as microscopic particles and is typically associated with pyrite, although some types of pyrite are barren, suggesting several generations. Two samples of sulphide mineral separates from the Cashel Rock prospect were subjected to isotopic analysis in the metallogenic study reported by Earls et al. (1996). The results obtained, +12.3 per mil and +10.2 per mil, are consistent with the derivation of sulphur by reduction of sulphate in Ordovician seawater.
The Tyrone Volcanic Group (TVG), comprises predominantly volcanic and intrusive granitoid rocks of Ordovician age that were formed in an island arc setting during closure of the Iapetus Ocean. The volcanic sequence comprises basaltic andesite to rhyolitic submarine and subaerial lavas, volcaniclastics with chert horizons and intercalations of graptolitic shales (Hartley, 1933). Arthurs (1976) reported reconnaissance geological and geochemical work in the Sperrins that identified the Tyrone Volcanic Group as metal-enriched. Legg et al. (1985) noted that a prospecting licence was held over the TVG area by Riofinex from 1971 to 1975. Riofinex undertook geophysical and geochemical surveys, trenching, drilling and rock analyses and reported gold assays up to 2.6g/t in pyrite-veined rock float at Cashel Rock. Leyshon and Cazalet (1976) reported on the Riofinex exploration programme and concluded that some of the intrusives are host to significant but sub-economic copper mineralization.
Clifford et al. (1992) followed up the exploration undertaken by Riofinex and reported that gold and base metal enrichment is widespread in the Tyrone Volcanic Group. Mineralization occurs in a variety of styles including stratiform enrichment in cherts and iron formation, stockworks in rhyolitic intrusions, and porphyry-type occurrences associated with tonalite plutons. Exploration at Cashel Rock established that the sulphide-rich breccia is up to 2m thick and that the mineralized zone, intersected in 14 drillholes, extends over a strike length of at least 200m and to a depth of at least 30m. The best intercept from drilling was 3.63m grading 30.5g/t Au including a 1.23m interval assaying 1.14% Cu and 1.85% Pb. Gold occurs as microscopic particles and is typically associated with pyrite, although some types of pyrite are barren, suggesting several generations. Two samples of sulphide mineral separates from the Cashel Rock prospect were subjected to isotopic analysis in the metallogenic study reported by Earls et al. (1996). The results obtained, +12.3 per mil and +10.2 per mil, are consistent with the derivation of sulphur by reduction of sulphate in Ordovician seawater.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Cashel Rock - Mineralogy - site of local interest, County Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=711. Accessed on 2024-12-26 |
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