TRACING THE TRANSITION OF GOLD FROM SOURCE TO SPONGE TO SINK

Abstract

The 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, sieve-textured arsenopyrite and pyrite are gold rich and tend to yield the least radiogenic Pb isotope compositions (high 207 Pb/ 206 Pb and 208 Pb/ 206 Pb 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 207 Pb/ 206 Pb and 208 Pb/ 206 Pb 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 206 Pb/ 204 Pb and 207 Pb/ 204 Pb 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.