Facilitation of Pb-Zn mineralisation by diatremes in the Irish Orefield?
Abstract
The Limerick Basin in southwest Ireland is an important part of the Pb-Zn Orefield within the Irish Midlands [1]. A recently discovered cluster of basaltic diatremes within the Stonepark area, spatially associated with mineralization, challenges previously accepted theories that the carbonate hosted Pb-Zn deposits were not related to Lower Carboniferous volcanics within the area. Thus, the genetic relationship between mineralization and magmatism is poorly understood and highly controversial. The diatremes emplaced into the Lower Carboniferous carbonate stratigraphy, are attributed to increased heat flow and magmatism during extensional tectonic activity in the Tournasian [2]. The preferred NE-SW trend of diatremes suggests that they might have been fault controlled. In close proximity to the diatremes a series of extra-crater basaltic lava flows and pyroclastics (Knockroe Formation) is commonly interbedded with argillaceous and crinoidal limestones formed in a shallow marine environment (Lough Gur Formation) [3]. Analysis of trace elements from juvenile material from both the extra-crater and diatreme deposits have confirmed that the two are related, following a broadly linear fractionation trend. The Knockroe sequence is likely sourced from the diatreme eruptions, apparently thickening toward the diatremes, and share compositional and textural similarities with the diatremes. The deposits record at least two different magmatic phases, the first contains fragmented crystals and a very low degree of vesiculation. The second contains no phenocrysts; the lapilli are more altered and have a higher vesicularity (21-42 area %). The low vesicularity of juvenile lapilli (5-26 area %), interbedding of diatreme-sourced Knockroe pyroclastic material and shallow marine deposits, and other large-scale geological relationships indicate that the eruptions were largely phreatomagmatic. However, the occurrence of highly vesiculated lapilli and late stage crosscutting dykes indicate that eruptions most likely consisted of phreatomagmatic and magmatic phases. Base metal mineralization occurs as replacement of Black Matrix Breccias (BMB), formed by the passage of hydrothermal fluids through the limestone country rock. Analysis of sulphur isotopes by in situ laser analysis of polished blocks and conventional analyses on separated sulfides is presented to explain the process of sulfide formation in this geological setting. Signature mineral phases in the BMB are calcite, quartz and dolomite. The occurrence of diatreme clasts within mineralized BMB suggests that mineralization post-dates or is contemporaneous with magmatic activity. The presence of dolomite within the lowest part of the diatremes can be explained by the passing of BMB hydrothermal fluids through lower sections of the diatremes before entering the adjacent country rock. [1] Redmond, P. (2010). The Limerick Basin: An Important Emerging Subdistrict of the Irish Zn-Pb Orefield. SEG newsletter, 82, 20-25 [2] Muchez, P., Heijlen, W., Banks, D., Blundell, D., Boni, M., and Grandia, F., (2005). Extensional tectionics and the timing and formation of basin-hosted deposits in Europe. Ore Geology Reviews, 27, 241-267 [3] Somerville, I.D., Strogen, P., and Jones, G. (1992). Biostratigraphy of Dinantian limestones and associated volcanic rocks in the Limerick Syncline, Ireland. Geological Journal, 27, 201-222
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V41D2833E
- Keywords:
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- 8404 VOLCANOLOGY Volcanoclastic deposits;
- 8424 VOLCANOLOGY Hydrothermal systems;
- 8427 VOLCANOLOGY Subaqueous volcanism;
- 8428 VOLCANOLOGY Explosive volcanism