A fluid inclusion and stable isotope study of 200 Ma of fluid evolution in the Galway Granite, Connemara, Ireland

O’Reilly, Ciaran; Jenkin, Gawen R.T.; Feely, Martin; Alderton, D. H. M.; Fallick, Anthony E.

doi:10.1007/s004100050327

Cited by 30 publications

(36 citation statements)

References 76 publications

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“…This is indicated by the proximity of: i) a significant late Palaeozoic (Dinantian) carbonate basin (<10 km east: Graham et al, 1989) and; ii) fluid inclusion salinities more typical of basinal brines associated with Carboniferous and later mineralization in the British Isles (e.g. Ireland: Samson and Russell, 1987;Banks and Russell, 1992;Banks et al, 2002;O'Reilly et al, 1997;Wilkinson, 2010;Devon: Shepherd et al, 2005;Scotland: Samson and Banks, 1988;Curtis et al, 1993;Baron and Parnell, 2005). These two contrasting scenarios are discussed below.…”

Section: Discussionmentioning

confidence: 99%

“…However, O'Reilly et al (1997) propose that base metal-bearing veins, containing pyrite with carbonate, barite and minor chlorite/clay hosted within the Galway Granite, to the south of Bohaun are associated with a basinal brine from a Carboniferous basin. These veins are also comparable to Bohaun in terms of containing brecciated wall rock fragments, comb quartz and vugs.…”

Section: Late Stage Brinesmentioning

confidence: 98%

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Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

et al. 2011

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The Bohaun gold deposit in County Galway, western Ireland is located in the Caledonian orogenic belt, proximal to a major accretionary boundary. The mineralization is hosted in a normal extensional fault, representing a zone of repeated extension and vein infilling of open cavities, within clastic sedimentary rocks, which have only reached the late diagenetic zone of low-grade metamorphism.The mineralization consists of undeformed veins, stockworks and breccias. Quartz dominates all the veins and commonly displays multiple growth stages and a variety of classic epithermal textures, including comb, plumose and banded, indicative of low confining pressures. The veins have a simple mineralogy comprising quartz-sericite-chlorite with minor sulfides, hematite, rare visible gold, with a high silver content (up to 41 wt %), and late barite and dolomite.Vein quartz is associated with two fluids compositions: Type I with moderate to high salinity (8.0-23.6 wt % NaCl equiv) and Type II with low to moderate salinity (0.0-7.8 wt % NaCl equiv). CO 2 contents in both fluid types are <3.5 wt percent. The majority of fluid inclusion homogenization temperatures range from 126 to 257 o C and geologically constrained depth estimates (≤5 km) indicate maximum fluid inclusion trapping temperatures between 175 and 245°C.The geodynamic setting of Bohaun, which comprises a deformed continental margin of an allochthonous terrane, is typical of orogenic gold deposits. However, an orogenic association for the Bohaun mineralization is discounted on the basis of fluid inclusion evidence, atypical vein textures, gold grain composition and undeformed veins. While vein textures locally resemble those found in epithermal quartz-adularia mineralization, there is no obvious magmatic association and alteration is restricted to the veins and immediate host rocks. The origin of the gold at Bohaun remains enigmatic. However, we favor an association with widespread Carboniferous or later mineralizing fluids and a model which involves transport and deposition of gold from a basinal brine type fluid, in a similar fashion to red-bed Au-Pd mineralization. Bohaun provides further evidence that low temperature brines may be capable of generating economic grade gold mineralization, independent of other fluids.3

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Section: Discussionmentioning

confidence: 99%

Section: Late Stage Brinesmentioning

confidence: 98%

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

et al. 2011

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“…This event took place at least 100 Ma after granite consolidation and maybe as late as the Triassic, because field evidence reveals that these fluoritebearing veins cut a 300 Ma dolerite dyke which intruded the granite. Both the first and second fluids are never found in these late fluorite veins, while fluid inclusions from all three events occur in quartz samples from the molybdenite veins and mineralized granite (Feely and Hoegelsberger, 1991;Gallagher et al, 1992;O'Reilly et al, 1997).…”

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confidence: 99%

“…Suzuki et al (2000) revealed experimentally that the Re-Os system in molybdenite can behave as an open-system when immersed in warm aqueous NaCl or NaHCO 3 bearing fluid. Detailed fluid inclusion studies of molybdenite vein quartz and granite quartz (Gallagher et al, 1992;O'Reilly et al, 1997) have shown that the Galway Granite was the locus for repeated fluid events, i.e., 1. Expulsion of aqueous carbonic magmatic fluids during the final stages of crystallization, 2.…”

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confidence: 99%

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Disturbance of the Re-Os chronometer of molybdenites from the late-Caledonian Galway Granite, Ireland, by hydrothermal fluid circulation.

Suzuki

Feely

O’Reilly³

2001

Geochem. J.

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Re-Os ages were determined for molybdenites from the late-Caledonian Galway Granite in the west of Ireland. Molybdenites from three localities within the batholith, Mace Head, Murvey, and Travore, yield respective Re-Os ages of 425.2 ± 3.1 Ma, 383.2 ± 8.1 and 426.1 ± 5.5 Ma, and 338.9 ± 5.6 and 464 ± 28 Ma. The molybdenites from the three localities are believed to be contemporaneous. However, the Re-Os ages obtained for each locality are not only in disagreement with the ages of the samples from the other localities but also are inconsistent within a single locality. Replicate analyses of a molybdenite standard (HLP-5) characterized by AIRIE using our method (Suzuki et al., 1992) yields reproducible Re-Os ages, which are in agreement with the results of Markey et al. (1998). Therefore, the age discrepancies in Galway molybdenites in this study should not be attributed to analytical errors. Suzuki et al. (2000) revealed experimentally that the Re-Os system in molybdenite can behave as an open-system when immersed in warm aqueous NaCl or NaHCO 3 bearing fluid. Detailed fluid inclusion studies of molybdenite vein quartz and granite quartz (Gallagher et al., 1992;O'Reilly et al., 1997) have shown that the Galway Granite was the locus for repeated fluid events, i.e., 1. Expulsion of aqueous carbonic magmatic fluids during the final stages of crystallization, 2. Meteoric fluid convection probably driven by waning magmatic heat and 3. Incursion of basinal brines associated with fluorite mineralization at least 100 Ma after granite consolidation. We suggest that younger hydrothermal fluids perhaps associated with fluorite mineralization may have perturbed the Re-Os systematics of molybdenites from the Galway Granite.

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Extensional Veins and Pb-Zn Mineralisation in Basement Rocks: The Role of Penetration of Formation Brines

Gleeson

Yardley

2002

Water-Rock Interaction

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A fluid inclusion and stable isotope study of 200 Ma of fluid evolution in the Galway Granite, Connemara, Ireland

Cited by 30 publications

References 76 publications

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

Disturbance of the Re-Os chronometer of molybdenites from the late-Caledonian Galway Granite, Ireland, by hydrothermal fluid circulation.

Extensional Veins and Pb-Zn Mineralisation in Basement Rocks: The Role of Penetration of Formation Brines

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