A FLUID INCLUSION AND STABLE-ISOTOPE STUDY OF 200 MA OF FLUID EVOLUTION IN THE GALWAY GRANITE, CONNEMARA, IRELAND

Fluid inclusions in granite quartz and three generations of veins indi cate that three fluids have affected the Caledonian Galway Granite. Th ese fluids were examined by petrography, microthermometry, chlorite th ermometry, fluid chemistry and stable isotope studies. The earliest fl uid was a H2O-CO2-NaCl fluid of moderate salinity (4-10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V-1 ) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305-390 degrees C and 0.7-1.8 kbar. Homogeneous quartz delt a(18)O across the batholith (9.5 +/- 0.4 parts per thousand, n = 12) s uggests V-1 precipitation at high temperatures (perhaps 600 degrees C) and pressures (1-3 kbar) from magmatic fluids. Microthermometric data for V-1 indicate lower temperatures, suggesting inclusion volumes re- equilibrated during cooling. The second fluid was a H2O-NaCl-KCl, low- moderate salinity (0-10 wt% NaCl eq.), moderate temperature (270-340 d egrees C), high delta D (-18 +/- 2 parts per thousand), low delta(18)O (0.5-2.0 parts per thousand) fluid of meteoric origin. This fluid pen etrated the batholith via quartz veins (V-2) which infill faults activ e during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloriti zation of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generate d by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5-2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection s ystem during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H2O -NaCl-CaCl2-KCl fluid with variable 205 degrees C), delta D (-17 to -4 5 parts per thousand), delta(18)O (-3 to + 1.2 parts per thousand), de lta(13)C(CO2) (-19 to 0 parts per thousand) and delta(34)S(sulphate) ( 13-23 parts per thousand) that deposited veins containing quartz, fluo rite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V-3 ) Correlations of salinity, temperature, delta D and delta(18)O are in terpreted as the result of mixing of two fluid end-members, one a high -delta D (-17 to -8 parts per thousand), moderate-delta(18)O (1.2-2.5 parts per thousand), high-delta(13)C(CO2) (> -4 parts per thousand), l ow-delta(34)S(sulphate) (13 parts per thousand), high-temperature (205 -230 degrees C), moderate-salinity (8-12 wt% NaCl eq.) fluid, the othe r a low-delta D (-61 to -45 parts per thousand), low-delta(18)O (-5.4 to -3 parts per thousand), low-delta(13)C (<-10 parts per thousand), h igh-delta(34)S(sulphate)(20-23 parts per thousand) low-temperature (80 -125 degrees C), high-salinity (21-28 wt% NaCl eq.) fluid. Geochronolo gical evidence suggests V-3 veins are late Triassic; the high-delta D end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evapori tes, whereas the low-GD end-member is interpreted as a basinal brine d erived from the adjacent Carboniferous sequence. This study demonstrat es that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the fina l stages of crystallisation, through a meteoric convection system, pro bably driven by waning magmatic heat, to much later mineralisation, co ncentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long a fter magmatic heat has abated.