Hypozonal gold mineralization in shear zone-hosted deposits driven by fault valve action and fluid mixing: the Nalunaq deposit, Greenland

Abstract

AbstractThe Nalunaq deposit, Greenland, is a hypozonal, shear zone-hosted, Au deposit. The shear zone has previously been interpreted as having undergone four stages of deformation, accompanied by fluid flow and vein formation. Coupled with previous trapping T estimates, fluid inclusion data are consistent with the trapping of fluids with salinities between 28 and 45 wt% NaCl equiv., from 300 to 475°C during D2 and D3, with pressure varying between c. 800 and 100 MPa. The range reflects pressure cycling during seismic slip-related depressurization events. D4 fluids were lower salinity and trapped from 200 to 300°C, at c. 50–200 MPa during late-stage normal faulting. The variation in major element chemistry is consistent with the ingress of hypersaline, granitoid equilibrated fluids into the shear zone system and mixing with fluids that had reacted with the host metamorphic rocks. D4-stage fluids represent the ingress of meteoric fluids into the system. Gold contents in inclusion fluids range from c. 300 to 10 mg kg−1. These data are consistent with the high-P–T solubility of Au as AuHS(H2S)30 complexes, and Au deposition by decompression and cooling. The high salinities also suggest Au transport as chloride complexes may have been possible. Gold distribution was modified by the release of chemically bound or nanoscale Au during sulfide oxidation at the D4 stage.