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TitleTracing the transition of gold from source to sponge to sink
AuthorLawley, C J M; Jackson, S; Yang, Z; Davis, W; Eglington, B
SourceEconomic geology and the bulletin of the Society of Economic Geologists vol. 112, no. 1, 2017 p. 169-183, https://doi.org/10.2113/econgeo.112.1.169
Year2017
Alt SeriesEarth Sciences Sector, Contribution Series 20160072
PublisherSociety of Economic Geologists
Documentserial
Lang.English
Mediapaper; on-line; digital
File formatpdf
ProvinceNunavut
NTS55J/12; 55J/13; 55K/09; 55K/16; 55K/10; 55K/15; 55N/01; 55N/02; 55O/03; 55O/04
AreaMeliadine gold district
Lat/Long WENS -94.0000 -90.0000 64.0000 62.0000
Subjectsgeochemistry; gold; mineral deposits; pyrite; metamorphic rocks; lead isotope ratios; lead geochemistry; isotopic studies; sulfides
Illustrationslocation maps; geological sketch maps; photomicrographs; tables; diagrams; graphs
ProgramGold Ore Systems, Targeted Geoscience Initiative (TGI-4)
Released2017 01 09
AbstractThe late timing of gold is a characteristic feature of orogenic gold deposits. For many of these deposits, gold and other metals were liberated from the host greenstone belt and/or early sulfides concomitant with the syndeformational and fluid-assisted metamorphic transition of pyrite to pyrrhotite and remobilized into late, high-grade ore shoots. However, this metamorphic-driven gold-upgrading model is usually inferred, at least in part, from mineral textures, which can be equivocal in the absence of geochemical and/or isotopic constraints. Herein, we report new bulk mineral separate and in situ Pb isotope results for minerals in key textures in the Paleoproterozoic
Meliadine gold district. Arsenopyrite and pyrite porphyroblasts occur in Paleoproterozoic quartz ± ankerite veins and are enveloped by remobilized pyrrhotite, chalcopyrite, and galena crystals that accompany gold in late, microtextural sites, a texture that forms the basis for the gold-upgrading model. Early, sievetextured arsenopyrite and pyrite are gold rich and tend to yield the least radiogenic Pb isotope compositions (high 207Pb/206Pb and 208Pb/206Pb ratios), suggesting these relict domains escaped complete recrystallization
during later reworking. Late, inclusion-free arsenopyrite and pyrite overgrowths scatter to more radiogenic Pb isotope compositions (low 207Pb/206Pb and 208Pb/206Pb ratios) and are typically gold poor, which we attribute to reworking and precipitation from a younger and more radiogenic fluid prior to and/or during the precipitation of free gold in variably plunging high-grade ore shoots at 1.90 to 1.85 Ga. Remobilized sulfides are the youngest sulfide phases, but locally yield low 206Pb/204Pb and 207Pb/204Pb ratios and, thus spurious old model "ages" which overlap with the least radiogenic, sieve-textured arsenopyrite/pyrite domains. The isotopic signature of these remobilized sulfides was likely inherited from Pb liberated during reworking of early arsenopyrite and pyrite and a second, crustal Pb source that was introduced during late remobilization. Our results highlight the importance of element recycling from early sulfides and/or the Neoarchean deposit host rocks during significantly younger Paleoproterozoic reworking and gold remobilization. The in situ Pb isotope results of early and remobilized sulfides document the transition of gold from source to sponge to sink, which is likely a feature that
occurred during the formation of other orogenic-style gold deposits.
Summary(Plain Language Summary, not published)
Many of the world's largest gold deposits are often conceptualized in terms of a simple transition from source to sink. In this contribution, we present Pb isotopic analyses for a variety of sulphide minerals that accompany gold at the Meliadine gold district. Our results suggest that some sulphide minerals act as transient, gold-rich reservoirs that can release metals prior to the precipitation of gold during younger metamorphic events. This intermediary step to the classic source-to-sink mineral exploration paradigm is likely a key feature of other world-class gold deposits.
GEOSCAN ID298813