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Title: Lithogeochemical aureoles to Irish mineralisation
Author: Gray, Gary John
ISNI:       0000 0001 3512 4298
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 1986
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Examination of the Mn, Fe, Zn and Mg content of host limestones around a number of Irish base metal deposits (Silvermines, Tynagh, Ballinalack, Keel, Moate, Moyvore, Ballyvergin, Aherlow, Courtbrown and Mallow) has shown that all the deposits studied, with the possible exception of Mallow, are accompanied by elevated levels of Mn and Fe in nearby Waulsortian and equivalent limestones, Zinc is also enriched more locally around most of these deposits. Background values of Mn and Fe are less than 100 ppm in the southwest of the country, rising to over 250 ppm in the north Midlands, and below 10 ppm throughout for Zn. The enrichments are interpreted as the products of hydrothermal exhalation onto the Waulsortian seafloor, with primary incorporation of trace elements into carbonate sediments. Careful sampling of host rock is required to avoid secondary (or epigenetic) contamination, or masking by random lithological variation, and analysis is by atomic absorption spectrophotometry, following dissolution of powdered samples in 2M hot acetic acid for 1 hour. Tynagh has the best developed aureole, with Mn and Fe enriched to 6 km distant and Zn (and Sr) to 5 km, of which over 80% of samples contain anomalously high levels within these limits. Enrichments around the other deposits extend to between 2 and 6 km distant, and are best developed around Ballinalack, Silvermines, Aherlow and Courtbrown, and relatively poorly developed around Ballyvergin, Keel, Moate and Moyvore. Vertical profiling of boreholes at Silvermines indicates that peak values occur near the base of the Waulsortian, in the same stratigraphic position as the stratiform ore horizon. Enrichments of Mn, Fe and Zn extend up to 100 m into the hanging wall above the ore horizon, and are also present in the immediate footwall sediments, Possible weak basal enrichment of mudbank limestone is also indicated at Keel, but no stratigraphic control is displayed within the host rocks at Ballinalack or Tynagh. The presence of apparently primary enrichments of Mn and Fe in Waulsortian limestones near to epigenetic deposits at Aherlow, Moate, Moyvore and Ballyvergin indicates that emplacement of sulphide was approximately contemporaneous with mudbank growth. In the case of Silvermines, sever post-depositional modification (including recrystallisation, dolomitization, pressure solution and stratigraphic attenuation) has considerably altered the original primary trace element patterns, possibly enhancing the absolute levels of Mn and Fe enrichment in much of the hanging wall dolomite breccias. The extent of diagenetic modification around the other deposits is less well known but probably much less severe. Application of lithogeochemistry is most useful in early reconnaissance stages of exploration, allowing recognition of areas of local enrichment from possible hydrothermal sources. Stratigraphic profiling of exploration boreholes may also highlight favourable horizons within a sequence. As part of this study, reconnaissance lithogeochemistry in a number of areas of Waulsortian outcrop has led to the discovery of Pb-Zn mineralisation associated with possible exhalative centres in north County Cork and the Nenagh area in County Tipperary.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Limestone deposits in Ireland