Plot of δD vs δ18O of Brandberg West Mine samples (muscovite), regional samples (whole rock), and Damara granites (biotite). Also shown is the field for “primary magmatic” water (Taylor 1977) and quartz vein fluid inclusion (δD by direct measurement, δ18O estimated from quartz δ18O value and quartz–water fractionation factor of Matsuhisa et al. 1979). The shaded band represents the range of values of meteoric water (−39±13 ‰, 1 SD) found at the present day at the equivalent latitude to Brandberg West at 530 Ma (see text). The shaded field indicates water in equilibrium with muscovite from BW and the Frans prospect, calculated assuming Δmusc–water (H)=−22.5 at 455°C (Suzuoki and Epstein 1976) and Δ musc–water (O)=0.5 (Chacko et al. 1996)
The fluids in equilibrium with the greisen muscovite from the
Brandberg West mine have a range of δ D compositions
between −45 and −38‰ (Fig. 8 ), calculated using the
muscovite–fluid fractionation equation of Suzuoki and Epstein (
1976 ) and a temperature of 455°C.
The δ18 O vs δ D plot (Fig. 8 )
compares the composition of minerals from the Brandberg West
deposits to those of granite, and the composition of the regionally
metamorphosed siliciclastic country rocks are also shown.
This is the best estimate of the likely isotope composition of
meteoric water and corresponds to a δ18 O
value of −6.1‰ on the global meteoric water line (Fig. 8 ).
However, the hydrogen isotope data do not support significant
involvement of meteoric water at Brandberg West (Fig. 8 )..
However, as discussed above and illustrated in Fig. 8 ,
significant quantities of unmodified meteoric water were not
involved in the genesis of the Brandberg West ore deposit..
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