科研之友
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摘要
Sampling of quartz vein material from two gold deposits of similar geological setting but different ages (The Ovens, 408 Ma; Dufferin, 380 Ma) in the Meguma Terrane of Nova Scotia has been done to compare and contrast their delta O-18(quartz) signatures. Despite different ages of formation, quartz from all vein types in each of the deposits (i.e. saddle-reef, bedding-concordant leg reefs, en echelon, discordant) shows limited intra-deposit variation with similar average delta O-18 values of +15.2 +/- 0.9aEuro degrees (n = 16) for The Ovens and +15.7 +/- 0.6aEuro degrees (n = 12) for Dufferin. Using an average delta O-18 value of +15.4aEuro degrees for the two deposits, the corresponding delta O-18(H2O) values, calculated for 400A degrees C and 350A degrees C based on constraints from mineral assemblages and fluid inclusion studies, indicate averages of 11.4 A +/- 0.2aEuro degrees and +10.2 A +/- 0.2aEuro degrees, respectively. The isotopic data indicate that: (1) the vein-forming fluids have a metamorphic signature, (2) all vein types in the two deposits represent formation from a single, isotopically homogeneous fluid, and (3) a fluid of similar isotopic signature was generated by two contrasting tectono-thermal events in the Meguma Terrane that were separated by 30 Ma. Integration of these results with previously published data for 14 Meguma gold deposits indicate a general stratigaphic dependence in delta O-18(H2O) values for deposits when arranged in their relative stratigraphic position such that delta O-18(H2O) values increase upwards in the stratigraphy. This apparent trend cannot be explained by models involving either fluid mixing or cooling of the vein-forming fluids, but instead is modelled using fluid/rock interaction taking into account a change in the modal mineralogy of the metasedimentary rocks upwards in the stratigraphy.
- 出版日期2011-3
