FLUID INCLUSION AND STABLE ISOTOPE EVIDENCE FOR TRIASSIC FLUID MIXING FORMING BASE-METAL VEINS IN CONNEMARA, IRELAND

Gawen Jenkin^1,2,3, Cian O'Reilly⁴, Martin Feely⁴ & Tony Fallick¹

Mineralisation is frequently spatially associated with granite intrusions. However, there are a growing number of examples where this mineralisation is clearly much later than the age of intrusion, and therefore cannot be the result of magmatic heat (e.g. cross-course mineralisation in Cornwall; Scrivener et al. 1994). What causes this mineralisation to be spatially related to the granite?

In Connemara, western Ireland, the Caledonian (~400 Ma) Galway Granite exhibits a number of styles of mineralisation (O'Reilly et al. 1997) (Figure 1). Mo mineralisation is clearly related to separation of magmatic fluids from the magma at the end-stages of crystallisation. This was followed by a major influx of meteoric fluid, probably in a meteoric convection system. The last phase of mineralisation (base metal sulphides, quartz, fluorite, calcite, barite), however, is tentatively dated at ~210 Ma; Upper Triassic (Halliday & Mitchell, 1983; O'Connor et al. 1993; Menuge et al. 1997; Jenkin et al. (1998), and is therefore not the result of magmatic heat.

Figure 1. Salinity vs. homogenisation temperature plot for all inclusions measured in and around the Galway Granite. Inclusions associated with Mo-mineralisation have high T_H and moderate salinity and are believed to be magmatic in origin. Inclusions associated with later barren quartz veins have lower T_H and low-moderate salinity and are thought to be derived from a meteoric fluid. Inclusions associated with the Upper Triassic veins have low T_H and moderate to high salinity. Adapted from O'Reilly et al. (1997).

Combined fluid inclusion and stable isotope studies show that the fluid depositing the base metal veins fluctuated in salinity, temperature, ¹⁸O, D, ¹³C and ³⁴S (Figure 2 & Figure 3). Variations in these parameters are correlated, so that the veins are thought to be the result of mixing of two fluid end-members, within the vein system:

	EM1	EM2
T (°C)	205	125
S (wt.% NaCl eq.)	12	21
18O ()	1.2	-3.0
D ()	-17	-45
13C CO2 ()	-4 to 0	-10 to -19
34S SO4 ()	13	20 to 23

EM1 is interpreted to be U. Triassic meteoric water that has gained salinity, either by dissolution of Triassic evaporites, or mixing with evaporated Triassic seawater, or probably both. EM1 ³⁴S and ¹³C are consistent with Triassic marine values.

EM2 is interpreted to be a basinal brine from the adjacent or overlying L. Carboniferous sediments. The low D and ¹³C values are attributed to a component of "organic water". The high ³⁴S values suggest the fluid dissolved L. Carboniferous evaporites.

Given these origins, the higher temperature of the surface-derived EM1 must be explained by this fluid travelling a path to the mixing zone that allowed it to heat up more, either by deep penetration beneath the area, or by traversing Permo-Triassic basins offshore, in which concurrent volcanism took place (Tate & Dobson, 1989). Thus is it suggested that EM1 was rising into the zone of mixing (Figure 4). The cause of fluid mixing is probably incipient North Atlantic opening at this time (Mitchell & Halliday, 1976; Halliday & Mitchell, 1984; O'Connor et al. 1993).

The reason why this mineralisation is concentrated around the granite is probably related to a number of features of granites in general, such as higher heat flow, greater permeability, source of mineralising constituents (e.g. F) and spatial relation to basins.

Figure 4. Click on figure to get full-size A4 landscape version. Cartoon cross section showing proposed fluid-mixing model for generation of Triassic base-metal veins in the Galway Granite, Connemara, Ireland.

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