Origin and granite alteration effects of hydrothermal fluid: isotopic evidence from fluorite veins, Co. Galway, Ireland |
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| Authors: | J F Menuge M Feely C O'Reilly |
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| Institution: | (1) Geology Department, University College Dublin, Belfield, Dublin 4, Ireland, IE;(2) Geology Department, University College Galway, Galway, Ireland, IE |
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| Abstract: | The Costelloe Murvey Granite is a chemically evolved, high heat production, leucocratic component of the 400 Ma old Galway
Granite batholith and is host to hydrothermal fluorite-quartz-calcite veins. A previously reported clinopyroxene 40Ar-39Ar age of 231±4 Ma obtained from a pre-mineralization dolerite dyke is reinterpreted as dating this mineralization. The hydrothermal
fluid extensively altered its granite wallrocks, leading to lower Sm and Nd and higher Rb concentrations in altered granite,
disturbing both its Rb-Sr and Sm-Nd isotopic systems. The 87Sr/86Sr ratio of the hydrothermal fluid from which fluorite and calcite precipitated ranged from 0.7101 to 0.7139. These ratios
are very much lower than in the Costelloe Murvey Granite at the time of mineralization, precluding the granite as a source
for more than 2% of the hydrothermal Sr. The initial 143Nd/144Nd ratio varies between fluorite in different veins due to Nd derivation from local wallrocks, and between fluorite of petrographically
distinct growth phases within a single hand specimen, highlighting the difficulty of Sm-Nd isochron dating of fluorite in
cases where there are multiple sources of hydrothermal Nd. It is proposed that fluorite and calcite precipitated where hot,
dilute fluids rising through the granite mixed with cooler, more saline fluids of basinal origin migrating through Lower Carboniferous
limestone which then overlay the granite.
Received: 3 August 1995 / Accepted: 11 April 1996 |
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