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TitleLow-temperature hydrothermal alteration at the world-class gold-rich VMS LaRonde Penna mine, Abitibi, Québec: An oxygen isotope study
AuthorBeaudoin, G; Mercier-Langevin, P; Dubé, B; Taylor, B E
SourceGeological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Programs with Abstracts vol. 34, 2011 p. 13
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20130564
Meeting2011 GAC-MAC-SEG-SGA Joint Annual Meeting; Ottawa; CA; May 25-27, 2011
Mediapaper; on-line; digital
File formatpdf
NTS32D/01; 32D/08
AreaBousquet; LaRonde; Doyon
Lat/Long WENS-78.4661 -78.4150 48.2753 48.2203
Subjectsgeochemistry; alteration; Archean; volcanogenic deposits; sulphide deposits; gold; oxygen isotopes; isotopic studies; isotope geochemistry; hydrothermal alteration; lithology; structural analyses; mines; metamorphism; LaRonde Penna deposit; Abitibi Subprovince; Blake River Group; Precambrian
ProgramTargeted Geoscience Initiative (TGI-4), Gold Ore Systems
AbstractThe oxygen isotope compositions of metamorphosed, hydrothermally altered, mafic to felsic volcanic and volcaniclastic rocks hosting the world-class Archean LaRonde-Penna gold-rich VMS deposit, Abitibi, Canada, exhibit a large range (?18O = 6.5 to 22.0‰). Basalt and andesite have lower average ?18O values (x = 10.8‰) whereas felsic rocks have higher average ?18O values (x = 11.1 to 13.2‰). A correlation between ?18O values and SiO2 or Zr/TiO2 suggests that only a 2‰ variation in ?18O values can be ascribed to fractional crystallization. Whole rock samples interlayered in massive and semi-massive sulfide zones 5, 6, 20N and 20S have ?18O values ranging from 9.6 to 22.0‰. Despite large ranges in ?18O, the various metamorphosed hydrothermal alteration facies affecting mafic to felsic rocks have average values that are similar (ca. 12.7‰). On a camp scale, a decrease of ?18O values downward in the volcanic sequence, from ~13 to ~10‰, is interpreted to reflect higher temperatures at depth within the hydrothermal system that formed the massive sulfide lenses. An up-section profile, in the immediate footwall of 20N zone, of ?18O values in an undifferentiated rhyodacite-rhyolite unit with constant Zr/TiO2, displays an increase of ?18O values from 13‰ to 14‰, followed by decrease to values near 12‰ beneath the massive sulfide lens. Along this pattern of ?18O values, the concentration of MnO increases gradually in the footwall of the 20N massive sulfide lens. Decrease in ?18O values in Mn-rich altered rocks in the immediate footwall of the sulfide lenses may indicate zones of higher temperature upflow, although there is no correlation between ?18O values and MnO. Water-rock reaction modeling requires temperatures below 100ºC to explain the high ?18O values of altered volcanic rocks assuming oxygen isotope exchange between water with ?18O=0‰ and initial rock with ?18O=7‰. Alternatively, the high ?18O values of altered volcanic rocks were achieved by reaction of a fluid with an initial ?18O up to 5‰ at temperatures below 200ºC. Initial fluid ?18O values up to 5‰ indicate either a fluid reservoir where high temperature exchange under lower water-rock ratios, or mixing of seawater near (0‰) with magmatic water exsolved from felsic magmas (7‰; c.f. Iberian Pyrite Belt). The very high ?18O values (>17‰) quartz-rich volcanic rocks interlayered with massive sulfides likely indicate low temperature (<100ºC) hydrothermal silica precipitation as a result of cooling and mixing with seawater at or near the sea-floor.

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