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TitleThe thermal and chemical evolution of hydrothermal vent fluids in shale hosted massive sulphide (SHMS) systems from the MacMillan Pass district (Yukon, Canada)
 
AuthorMagnall, J M; Gleeson, S A; Blamey, N; Parradis, S; Luo, Y
SourceGeochimica et Cosmochimica Acta vol. 193, 2016 p. 251-273, https://doi.org/10.1016/j.gca.2016.07.020
Image
Year2016
Alt SeriesEarth Sciences Sector, Contribution Series 20150418
PublisherElsevier BV
Documentserial
Lang.English
Mediapaper; on-line; digital
File formatpdf
ProvinceBritish Columbia; Northwest Territories; Yukon
NTS94B; 94C; 94D; 94E; 94F; 94G; 94J; 94K; 94L; 94M; 94N; 94O; 95B; 95C; 95D; 95E; 95F; 95G; 95J; 95K; 95L; 95M; 95N; 95O; 96B; 96C; 96D; 96E; 96F; 96G; 104; 105; 106A; 106B; 106C; 106D; 106E; 106F; 106G; 106H; 114; 115A; 115B; 115G; 115H; 115I; 115J; 115O; 115P; 116A; 116B; 116G; 116H
Lat/Long WENS-140.0000 -122.0000 66.0000 56.0000
Subjectsgeochemistry; igneous and metamorphic petrology; metallic minerals; sedimentology; hydrothermal deposits; submarine hydrothermal vents; carbon isotopes; oxygen isotopes; isotopes; sulphide deposits; ankerite; mudstones; stockworks; siderite; thermal gradients; europium; organic carbon; sedimentary ore deposits
Illustrationslocation maps; photographs; photomicrographs; tables; geochemical plots
ProgramTargeted Geoscience Initiative (TGI-4) Sedimentary Exhalative Ore Systems
ProgramTargeted Geoscience Initiative (TGI-4) Sedimentary Exhalative Ore Systems
AbstractAt Macmillan Pass, hydrothermal vent complexes for two shale-hosted massive sulphide (SHMS) deposits (Tom, Jason) are well preserved within Late Devonian strata. These deposits provide a unique opportunity to constrain key geochemical parameters (temperature, salinity, pH, fO2, SS) that are critical for metal transport and deposition in SHMS systems, and evaluate the interaction between fluids and the mudstone host rock. This has been achieved using a combination of detailed petrography, isotopic techniques (d34S, d13C, d18O), carbonate trace element analysis (LA-ICP-MS), fluid inclusion analysis (microthermometry, gas analysis via incremental crush fast scan mass spectrometry), and thermodynamic modelling.
Two main paragenetic stages are preserved in both vent complexes: (Stage 1) pervasive ankerite alteration of the organic-rich mudstone host rock and crosscutting stockwork ankerite veining (± pyrobitumen, pyrite and quartz) and (Stage 2) main stage massive sulphide (galena-pyrrhotite-pyrite ± chalcopyrite-sphalerite) and siderite (± quartz and barytocalcite) mineralization. Co-variation of d18O and d13C values in ankerite has been modelled, and can be described by temperature dependent fractionation and fluid rock interaction; together with fluid inclusion microthermometry, this provides evidence of a steep thermal gradient (300 to ~100 °C), temporally and spatially constrained within the paragenesis of both vent complexes, developed under shallow lithostatic (< 1km; 250 bars) to hydrostatic (< 400m; 40 bars) conditions. There is evidence of mixing between diagenetic and hydrothermal fluids recorded in chondrite-normalised rare earth element (REE) profiles of ankerite and siderite; middle REE enrichments and superchondritic Y/Ho ratios (> 28), characteristic of diagenetic fluids, are coupled with positive europium anomalies and variable light REE depletion, which are more consistent with hot (> 250 °C), acidic hydrothermal fluids. In this shallow sub-seafloor setting, thermal alteration of organic carbon in the immature, chemically reactive mudstones also had an important role in the evolution of fluid chemistry; reduced sulphur generation via thermochemical reduction of Late Devonian seawater sulphate produced positive d34S values in sulphide minerals (+7.5 to +19.5 ¿), coupled with a suite of volatile components (CO2, CH4, C1-C4 hydrocarbons, N2) trapped in Stage 2 quartz.
Many of these geochemical features developed during the final stages of fluid ascent, in a retrograde system close to the site of mineralisation. Using this information, we have modelled the metal transporting capacity of the deep hydrothermal fluid, which even at modest salinities (6 wt. % NaCl) was high (>> 100 ppm Pb, Zn), owing to the combined effects of high temperature and low pH (= 4.5). Therefore in SHMS systems, enhanced geothermal gradients and rapid fluid ascent (with minimal fluid cooling) are considered to be the most important factors for transporting high concentrations of base metals to the site of mineralisation.
Summary(Plain Language Summary, not published)
At Macmillan Pass, hydrothermal vent complexes for two shale-hosted massive sulphide (SHMS) deposits (Tom, Jason) are well preserved within Late Devonian strata. These deposits provide an opportunity to constrain key geochemical parameters (temperature, salinity, pH, fO2, SS) that are critical for metal transport and deposition in SHMS systems, and evaluate the interaction between fluids and the mudstone host rock. This has been achieved using a combination of analytical techniques.
GEOSCAN ID297552

 
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