| Title | Open system sulphate reduction in a diagenetic environment - Isotopic analysis of barite (d34S and d18O) and pyrite (d34S) from the Tom and Jason Late Devonian Zn-Pb-Ba deposits, Selwyn Basin,
Canada |
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| Author | Magnall, J M; Gleeson, S A; Stern, R A; Newton, R J; Poulton, S W; Paradis, S |
| Source | Geochimica et Cosmochimica Acta vol. 180, 2016 p. 146-163, https://doi.org/10.1016/j.gca.2016.02.015  Open Access |
| Image |  |
| Year | 2016 |
| Alt Series | Earth Sciences Sector, Contribution Series 20150422 |
| Publisher | Elsevier BV |
| Document | serial |
| Lang. | English |
| Media | paper; on-line; digital |
| File format | pdf |
| Province | Yukon |
| NTS | 105O |
| Area | Selwyn Basin |
| Lat/Long WENS | -132.0000 -130.0000 64.0000 63.0000 |
| Subjects | economic geology; geochemistry; metallic minerals; barium geochemistry; isotope geochemistry; hydrothermal deposits; sulphur geochemistry; sulphur isotope ratios; pyrite; lead zinc deposits; sulphide
deposits; barite; shale hosted massive sulphide deposits; sulphate reduction; hydrothermal base metal sulphides; sulphur isotopes |
| Illustrations | cross-sections; sketch maps; photographs; tables; photomicrographs; graphs; schematic diagrams |
| Program | Targeted Geoscience Initiative (TGI-4) Sedimentary Exhalative Ore Systems |
| Abstract | Highly positive d34S values in sulphide minerals are a common feature of shale hosted massive sulphide deposits (SHMS). Often this is attributed to near quantitative consumption of seawater sulphate,
and for Paleozoic strata of the Selwyn Basin (Canada), this is thought to occur during bacterial sulphate reduction (BSR) in a restricted, euxinic water column. In this study, we focus on drill-core samples of sulphide and barite mineralisation from
two Late Devonian SHMS deposits (Tom and Jason, Macmillan Pass, Selwyn Basin), to evaluate this euxinic basin model. The paragenetic relationship between barite, pyrite and hydrothermal base metal sulphides has been determined using transmitted and
reflected light microscopy, and backscatter electron imaging. This petrographic framework provides the context for in-situ isotopic microanalysis (secondary ion mass spectrometry; SIMS) of barite and pyrite. These data are supplemented by analyses of
d34S values for bulk rock pyrite (n = 37) from drill-core samples of un-mineralised (barren), siliceous mudstone, to provide a means by which to evaluate the mass balance of sulphur in the host rock. Three generations of barite have been identified,
all of which pre-date hydrothermal input. Isotopically, the three generations of barite have overlapping distributions of d34S and d18O values (+22.5‰ to +33.0‰ and +16.4‰ to +18.3‰, respectively) and are consistent with an origin from modified Late
Devonian seawater. Radiolarian tests, enriched in barium, are abundant within the siliceous mudstones, providing evidence that primary barium enrichment was associated with biologic activity. We therefore propose that barite formed following
remobilisation of productivity-derived barium within the sediment, and precipitated within diagenetic pore fluids close to the sediment water interface. Two generations of pyrite are texturally associated with barite: framboidal pyrite (py-I), which
has negative d34S values (\'0123‰ to \'0128‰; n = 9), and euhedral pyrite (py-II), which has markedly more positive d34S values (+8‰ to +26‰; n = 86). We argue that stratiform pyrite and barite developed along diagenetic redox fronts, where the
isotopic relationships (d34Spyrite \'03 d34Sbarite) are explained by anaerobic oxidation of methane coupled to sulphate reduction (AOM-SR). Furthermore, the relatively narrow distribution of d34Sbarite values is consistent with an open system model
of sulphate reduction, in which reduced sulphur generation occurred with a reduced isotopic fractionation (e34S = <15‰) linked to higher rates of sulphate reduction and AOM-SR. Importantly, hydrothermal sulphides (pyrite, sphalerite and galena) all
post-date this diagenetic barite-pyrite assemblage, and textural and mineralogical evidence indicates barite replacement to be an important process during hydrothermal mineralisation. Neither the textures nor the documented isotopic relationships can
be produced by processes operating in a euxinic water column, which represents a major departure from the conventional model for SHMS formation at Macmillan Pass. We suggest that positive d34S values in sulphides, a common feature of SHMS systems
both in the Selwyn Basin and throughout the geologic record, could be linked to AOM-SR. At Macmillan Pass, positive d34Spyrite values developed during open system diagenesis, which was critical for rapid sulphur cycling and the development of an
effective metal trap. |
| Summary | (Plain Language Summary, not published)
Highly positive d34S values in sulphide minerals are a common feature of shale hosted massive sulphide deposits (SHMS). Often this is attributed to near
quantitative consumption of seawater sulphate, and for Paleozoic strata of the Selwyn Basin (Canada), this is thought to occur during bacterial sulphate reduction (BSR) in a euxinic water column. In this study, we focus on drill-core samples from
Late Devonian SHMS mineralisation (Tom and Jason deposits, Macmillan Pass, Selwyn Basin). The paragenetic relationship between barite and base metal sulphides has been determined using various analytical methods. |
| GEOSCAN ID | 297568 |
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