Evidence of upgrading of gold tenor in an orogenic quartz-carbonate vein system by late magmatic-hydrothermal fluids at the Madrid Deposit, Hope Bay Greenstone Belt, Nunavut, Canada

Abstract Evidence of secondary gold enrichment due to the addition of new gold into an earlier orogenic quartz-carbonate vein deposit by magmatic-hydrothermal fluids is strongly suggested for the Madrid Deposit, hosted in the Hope Bay Greenstone Belt in Nunavut, Canada. The conclusion is based on an extensive in situ microanalytical protocol (SEM, confocal Raman microspectroscopy, microthermometry, decrepitate mound analysis, LA-ICP-MS, cathodoluminescence, SIMS) not previously applied to gold systems. This approach was used to characterize the mineralogy and fluid inclusion systematics associated with the upgrading event. Mineralization comprised of only Ag-bearing gold (“electrum”; 89.8 at. % Au avg.; n  = 8) is present throughout all investigated laminated and brecciated orogenic quartz veins. However, in high-grade vein intersections where gold grades are locally elevated (up to 122 g/t), assemblages containing tennantite-tetrahedrite + chalcopyrite + electrum (80.7 at. % Au avg.; n  = 15) ± Ag Pb Au tellurides occur that are texturally late-stage relative to electrum-only mineralization. Subdomains of quartz coeval with this later assemblage are optically- and texturally-distinct from earlier orogenic quartz. This late mineral assemblage is absent in all low-grade vein intersections (∼1 g/t Au avg.) examined where only electrum is identified. Quartz-hosted fluid inclusions (H 2 O NaCl ± CO 2 ) of intermediate salinity (16.7 ± 1.2 wt.% NaCl equiv.; n  = 93) were identified only in the high-grade vein samples and are present along healed planes associated with tennantite-tetrahedrite + chalcopyrite + electrum ± Ag Pb Au telluride assemblages. In situ SIMS δ 18 O analyses of quartz, combined with temperature constraints from mineral equilibria, show that early orogenic vein quartz and late quartz subdomains associated with gold upgrading precipitated from fluids with similar δ 18 O H2O values of 4.5–12.4‰ ( n  = 10) and −5.5 to 11.8‰ ( n  = 13), respectively. Isotopic data suggests that meteoric water was a negligible component in fluids responsible for gold precipitation. Microthermometry and Raman spectroscopy show that upgrading fluids were distinct in composition compared to the earlier metamorphic fluids (H 2 O NaCl CO 2  ± CH 4  ± N 2 ; 4.6 ± 1.6 wt.% NaCl equiv., n  = 33) and later Canadian Shield basement brines (H 2 O NaCl; 22.4 ± 1.2 wt.% NaCl equiv., n  = 12) which have also been identified in the fluid inclusion record at Madrid. Laser ablation ICP-MS analyses indicate the gold upgrading fluids are enriched in As Sb Zn Pb and LILE (Cs Ba Rb Sr); these data are consistent with late fluids derived from an evolved magmatic-hydrothermal system. Trace element mapping of pyrite, coupled with principle component analysis of the data, confirms a strong correlation between Au and Ag Te Sb Bi W( As) in upgraded veins, whereas only Au and As strongly correlate in low-grade veins. The study suggests that gold upgrading, either involving newly introduced gold or remobilization of existing gold, can be linked to the incursion of a late magmatic-hydrothermal fluid that post-dated formation of the main orogenic-type auriferous quartz vein system. The geological setting and mineral-chemical features suggest an intrusion-related (i.e., porphyry), or intermediate-sulfidation epithermal mineralization style for the later event. This work provides another example of the importance of compositionally distinct cumulative hydrothermal events in the development of high-grade gold deposits in orogenic settings.