Sulfur isotopic systematics of granitoids from southwestern New Brunswick, Canada: implications for magmatic-hydrothermal processes, redox conditions, and gold mineralization |
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Authors: | Xue-Ming Yang David R Lentz |
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Institution: | (1) Department of Geology, University of New Brunswick, P.O. Box 4400, Fredericton, NB, Canada, E3B 5A3;(2) Present address: Ginguro Exploration Inc., 101-957 Cambrian Heights Drive, Sudbury, ON, Canada, P3C 5S5 |
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Abstract: | Bulk δ
34Srock values, sulfur contents, and magnetic susceptibility were determined for 12 gold-related granitoid intrusions in southwestern
New Brunswick, the Canadian Appalachians. The sulfur isotope compositions of sulfide minerals in some of the granitoid samples
were also analyzed. This new dataset was used to characterize two distinctive groups of granitoids: (1) a Late Devonian granitic
series (GS) and (2) a Late Silurian to Early Devonian granodioritic to monzogranitic series (GMS). The GS rocks have a large
range in δ
34S values of −7.1‰ to +13‰ with an average of 2.2 ± 5.0‰ (1σ), low bulk-S contents (33 to 7,710 ppm) and low magnetic susceptibility
values (<10−4 SI), consistent with reduced ilmenite-series granites. The GMS rocks have a relatively narrower variation in δ
34S values of −4.4‰ to +7.3‰ with an average 1.2 ± 2.9‰ but with larger ranges in bulk-S contents (45 to 11,100 ppm) and high
magnetic susceptibility values (>10−3 SI), indicative of oxidized magnetite-series granites. The exceptions for the GMS rocks are the Lake George granodiorite
and Tower Hill granite that display reduced characteristics, which may have resulted from interaction of the magmas forming
these intrusions with graphite- or organic carbon-bearing sedimentary rocks. The bulk δ
34S values and S contents of the GMS rocks are interpreted in terms of selective assimilation–fractional crystallization (SAFC)
processes. Degassing processes may account for the δ
34S values and S contents of some GS rocks. The characteristics of our sulfur isotope and abundance data suggest that mineralizing
components S and Au in intrusion-related gold systems are dominantly derived from magmatic sources, although minor contaminants
derived from country rocks are evident. In addition, the molar sulfate to sulfide ratio in a granitic rock sample can be calculated
from the δ
34Srock value of the whole-rock sample and the δ
34Ssulfide (or δ
34Ssulfate) value of sulfide and/or sulfate mineral in the sample on the basis of S-isotope fractionation and mass balance under the
condition of magmatic equilibrium. This may be used to predict the speciation of sulfur in granitic rocks, which can be a
potential exploration tool for intrusion-related gold systems. |
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Keywords: | |
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