Geological, fluid inclusion and stable isotope studies of Mo mineralization, Galway Granite, Ireland

Abstract

Mo mineralization within the Galway Granite at Mace Head and Murvey, Connemara, western Ireland, has many features of classic porphyry Mo deposits including a chemically evolved I-type granite host, associated K-and Si-rich alteration, quartz vein-(Mace Head) and granite-hosted (Murvey) molybdenite, chalcopyrite, pyrite and magnetite mineralization and a gangue assemblage which includes quartz, muscovite and K-feldspar. Most fluid inclusions in quartz veins homogenize in the range 100 350~ and have a salinity of 1 13 eq. wt.% NaC1. They display Th-salinity covariation consistent with a hypothesis of dilution of magmatic water by influx of meteoric water. CO2-bearing inclusions in an intensely mineralized vein at Mace Head provide an estimated minimum trapping temperature and pressure for the mineralizing fluid of 355 ~ and 1.2 kb and are interpreted to represent a H z O -C O 2 fluid, weakly enriched in Mo, produced in a magma chamber by decompression-activated unmixing from a dense Mo-bearing NaC1-H20-CO z fluid. 6348 values of most sulphides range from c. 0%0 at Murvey to 3 4%0 at Mace Head and are consistent with a magmatic origin. Most quartz vein samples have 6180 of 9-10.3%o and were precipitated from a hydrothermal fluid with 6180 of 4.6-6.7%o. Some have 6180 of 6-7%o and reflect introduction of meteoric water along vein margins. Quartz-muscovite oxygen isotope geothermometry combined with fluid inclusion data indicate precipitation of mineralized veins in the temperature range 360-450 ~ and between 1 and 2 kb. Whole rock granite samples display a clear 6180-6D trend towards the composition of Connemara meteoric waters. The mineralization is interpreted as having been produced by highlyfractionated granite magma; meteoric water interaction postdates the main mineralizing event. The differences between the Mace Head and Murvey mineralizations reflect trapping of migrating mineralizing fluid in structural traps at Mace Head and precipitation of mineralization in the granite itself at Murvey.

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

“…Previously, Hirt et al (1963) analyzed the molybdenite sample from Murvey Granite and obtained the Re-Os age of 365 ± 13 Ma (recalculated using λ ( 187 Re) = 1.666 × 10 -11 yr -1 , Smoliar et al, 1996). Since molybdenites from the three localities were genetically related and were formed contemporaneously from the granitic magma (Gallagher et al, 1992), it is reasonable to expect that they would give harmonious Re-Os ages. However, the Re-Os ages obtained for each locality in this study and by Hirt et al (1963) are not only inconsistent with the ages of the samples from the other localities but also are in disagreement even within each locality (Fig.…”

“…Disseminated and quartz vein-hosted molybdenite mineralization occurs throughout the Galway Granite (Derham, 1986;Max and Talbot, 1986;Derham and Feely, 1988;Feely and Hoegelsberger, 1991;Gallagher et al, 1992;Derham, 1993). The most notable occurrences are at the western end of the batholith, i.e., at Mace Head and Murvey ( Fig.…”

“…Quartz veins in the area also contain Mo-mineralization. Gallagher et al (1992) interpreted the Mo-mineralization in both areas, using geochemical, fluid inclusion and stable isotope (O, H, S and C) data, as having been produced by highly fractionated granite magma. Three localities were sampled in the Galway Granite, i.e., quartz vein-hosted molybdenite from Mace Head and Travore Bay, and granite-hosted molybdenite from Murvey ( Fig.…”

“…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.…”

“…It is now well documented that the Galway Granite has experienced three major fluid events since its intrusion at ca. 400 Ma (Feely and Hoegelsberger, 1991;Gallagher et al, 1992;O'Reilly et al, 1997). The earliest fluid to be trapped was a carbonic fluid of magmatic origin and was responsible for the molybdenite mineralization.…”