| Geological and high-field-strength element (HFSE) relationships for Chester Group felsic volcanic rocks and corresponding sub-volcanic intrusives in the south-east Swayze greenstone belt (SESGB) indicate deposition in an dynamic volcanic-arc tectonic setting, from two separate magmas with calc-alkaline and HFSE-depleted trace element signatures respectively. Chester Group, Yeo Formation rocks have undergone a regionally extensive sericite-quartz (type 1) alteration, involving the loss of 3.25 wt.% Na2O, addition of 12 wt.% SiO2 and 2.2 wt.% K2O, and 13% net mass-gain, indicating water/rock ratios of at least 6. Chlorite-sericite ± biotite ± garnet (type 2) alteration at paleo-hydrothermal vent sites was localized along synvolcanic fault structures. Type 2 alteration was accompanied by a loss of 2.44 wt.% Na2O and 8.69 wt.% SiO2, and a gain of 5.90 wt.% Fe2O3, requiring water/rock ratios of at least 432.;Hydrothermal sediments or iron formation (IF) within the SGB are composed largely of Si and Fe. Base-metal mineralization in the SESGB and other localities in the SGB is hosted by brecciated hydrothermal sediments, overlain and flanked by, more broadly distributed pelitic IF, enriched in large ion lithophile elements (LILE), Al2O3, TiO2 and HFSE, with distinct Fe2O3/TiO2 versus Al2O 3/(Al2O3 + Fe2O3 + MnO) ratios indicative of mixing with locally derived volcanic material. Well-laminated BIF-type hydrothermal sediments are distributed widely throughout the SGB and are enriched in MnO, depleted in LILE, HFSE and transition elements, and have a wide range of Ce/Ce* values reflecting diverse redox conditions in paleodepositional environments distal to hydrothermal venting. Thermodynamic calculations suggest that hydrothermal fluids responsible for generation of hydrothermal sediments have lower temperatures (250–300°C) but similar pH (∼4) to polymetallic volcanogenic massive-sulphide ore-forming fluids. |
