Fluid origin and evolution of Cu-Pb-Zn mineralization in rhyolite breccias in the Lón area, southeastern Iceland

Abstract

Iceland, the landward extension of the Mid-Atlantic Ridge, hosts dilute, predominantly meteoric hydrothermal systems that rarely form base metal (Cu-Pb-Zn) mineralization. One occurrence of Cu-Pb-Zn mineralization in intrusive rhyolitic breccias is in the Lón area of southeastern Iceland. Petrographic, electron-probe, fluid inclusion, stable isotope, and U-Pb zircon dating analyses on samples from Lón constrain the conditions and timing of sulfide mineral formation.Observations of outcrops and hand samples suggest that hydrothermal fluids precipitated chalcopyrite, sphalerite, galena, quartz, epidote, chlorite and calcite in rhyolitic breccia pipes and adjacent basalt flows. The mean salinity of liquid-dominated, multi-phase fluid inclusions in quartz coeval with chalcopyrite inclusions in quartz is 4.2wt% NaCl, and the mean trapping temperature is 332°C, which is consistent with the results of δ34S geothermometry of coexisting galena and chalcopyrite. Calculated δ18O and δD values of fluids in equilibrium with epidote coeval with chalcopyrite range from −5.2 to −2.7±0.1‰ and from −35.9 to −27.7±0.1‰, respectively. The δ18O values of fluids in equilibrium with quartz coeval with chalcopyrite are up to 5‰ larger than those of fluids in equilibrium with late stage quartz precipitated after chalcopyrite. The U-Pb crystallization age of magmatic zircons in the rhyolite breccia is 2.6±0.1Ma, significantly younger than the proximal 3.7 to 7.3Ma silicic intrusions of southeastern Iceland.Our results indicate that early-stage, mineralizing fluids derived from a mixture of meteoric water, seawater, and a minor magmatic water component exsolved from an evolved anatexis-produced melt. Late-stage fluids were derived exclusively from meteoric waters. Although anatectic dehydration melting of altered basalt produced millions of years of felsic magmatism in southeastern Iceland, only hydrothermal fluids that derived from a mixture of meteoric water, seawater, and brine exsolved from a highly evolved melt concentrated base metals in significant quantity to produce base metal sulfide mineralization.