ORIGIN AND GRANITE ALTERATION EFFECTS OF HYDROTHERMAL FLUID - ISOTOPIC EVIDENCE FROM FLUORITE VEINS, CO. GALWAY, IRELAND

The Costelloe Murvey Granite is a chemically evolved, high heat produc tion, leucocratic component of the 400 Ma old Galway Granite batholith and is host to hydrothermal fluorite-quartz-calcite veins. A previous ly reported clinopyroxene Ar-40-Ar-39 age of 231+/-4 Ma obtained from a pre-mineralization dolerite dyke is reinterpreted as dating this min eralization. The hydrothermal fluid extensively altered its granite wa llrocks, leading to lower Sm and Nd and higher Rb concentrations in al tered granite, disturbing both its Rb-Sr and Sm-Nd isotopic systems. T he Sr-87/Sr-86 ratio of the hydrothermal fluid from which fluorite and calcite precipitated ranged from 0.7101 to 0.7139. These ratios are v ery much lower than in the Costelloe Murvey Granite at the time of min eralization, precluding the granite as a source for more than 2% of th e hydrothermal Sr. The initial Nd-143/Nd-144 ratio varies between fluo rite 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 da ting of fluorite in cases where there are multiple sources of hydrothe rmal Nd. It is proposed that fluorite and calcite precipitated where h ot, dilute fluids rising through the granite mixed with cooler, more s aline fluids of basinal origin migrating through Lower Carboniferous l imestone which then overlay the granite.