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

The Nalunaq deposit, Greenland, is a hypozonal, shear zone-hosted, Au deposit. The shear zone has previously been interpreted to have undergone 4 stages of deformation. Each stage was accompanied by fluid flow recorded by vein formation and fluid inclusion trapping. Coupled with previous trapping T estimates fluid inclusion microthermometric data are consistent with trapping of fluids with salinities between 28-45 wt. % NaCl eq., from 300-475°C during D2 and D3, with pressure varying between ~800 and 100Mpa. The range reflects pressure cycling during seismic slip related depressurisation events. D4 fluids were lower salinity and trapped from 200-300°C, at ~50-200Mpa during late stage normal faulting. The variation in major element chemistry is consistent with 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 ingress of meteoric fluids into the system. Gold contents in inclusion fluids range from ~300-10mg/kg. These data are consistent with the high P-T solubility of Au as AuHS(H2S)30 complexes, and Au deposition by decompression and cooling. Gold distribution was modified by the release of chemically bound or nanoscale Au during sulphide oxidation at the D4 and subsequent stages.