An isotopic (C, O, Sr) study of vein gold deposits in the Meguma Terrane, Nova Scotia; implication for source reservoirs

Abstract

Vein quartz, carbonate, and tourmaline from 19 Meguma gold deposits in the Meguma terrane of Nova Scotia have been analyzed for stable (delta 18 O, delta 13 C) and radiogenic ( 87 Sr/ 86 Sr) isotopes in order to assess the nature and origin of the vein-forming fluids. As with other mesothermal gold provinces, the Meguma gold deposits are well suited to such a study because carbonate is the next most abundant phase after quartz in these mesothermal lode gold deposits. All vein types have been sampled for quartz and carbonate and, in addition, all compositional and textural varieties of carbonate have been sampled. Vein quartz is of uniform isotopic composition with most 18 O values between 10.2 to 17.6 per mil (avg ca. 14 per mil), except for one deposit (West Gore Sb-Au) where values go to 19.4 per mil. There is no systematic variation for quartz within a deposit or position within the Meguma Group stratigraphy. Vein carbonate 18 O values range from 11.8 to 27.5 per mil, with most in the 13 to 16 per mil range. The relatively 18 O-enriched carbonates reflect exchange with low-temperature fluids based on analyses of quartz-carbonate pairs that indicate disequilibrium fractionation (Delta (sub quartz-carbonate) < or =0); this is best illustrated by the strong negative correlation between delta 18 O carbonate and Delta (sub quartz-carbonate) . The 18 O water is estimated at 10+ or -3 per mil for a temperature of vein formation of 350 degrees to 400 degrees C and using the appropriate mineral-water fractionation equations. Whereas delta 18 O water values partly overlap the field for magmatic fluids, the values are wholly consistent with a metamorphic fluid, and it is considered unlikely that a primary magmatic fluid signature has been substantially modified due to wall-rock influences given that mesothermal gold systems are sites of high fluid/rock ratios. Vein carbonate has delta 13 C values of -13.1 to -25.9 per mil, but a slight negative correlation between delta 13 C carbonate and Delta (sub quartz-carbonate) suggests that the primary values lie in the range -20 to -25 per mil. Thus, the delta 13 C values indicate a reduced, biogenic source for the carbon. Oxidation of the reduced carbon, as indicated by CO 2 in fluid inclusions, may have occurred via hydrolysis of graphite or dissolution of carbonate minerals, both of which occur in the wall-rock lithologies of the Meguma Group, the latter of which has the appropriate isotopic composition. The initial 87 Sr/ 86 Sr of the vein fluid, estimated from the analyses of 52 vein carbonates (17 deposits) and four tourmalines (three deposits), ranges from 0.70118 to 0.72284 and within deposits considerable variation is observed. There is insufficient data to quantify the extent of the low-temperature overprint which has modified the C and O isotope data, although it is likely that some influence is present. Nevertheless, the data cannot be reconciled by a source confined exclusively to the Meguma Group, which suggests, therefore, involvement of another reservoir(s). The isotopic heterogeneity can be explained by variable amounts of contamination of a primary fluid with radiogenic Sr derived from Meguma Group lithologies by interaction along the fluid path or at the site of vein formation concomitant with wall-rock alteration; as discussed above, a dominantly magmatic source is not considered feasible. This fluid source is suggested to be within the structural basement to the Meguma Group, and the Liscomb gneisses are the favored source based on the combined results of the Sr isotope data presented herein and previously published Pb isotope data. Collectively, the data indicate that a primary fluid of metamorphic origin has had its isotopic signature variably modified due to interaction with different reservoirs. The most affected isotopic systems are C and S (based on earlier work on delta 34 S values) which are abundant as graphite carbonate and sulfides in the Meguma wall rock, respectively. The range in 87 Sr/ 86 Sr values of the fluid also reflects contamination, but this was quite variable. The uniform delta 18 O water value for the fluid indicates that this was the least affected isotopic system, except for the later exchange of carbonate at low temperatures.