Geochemical mass-transfer patterns as indicators of the architecture of a complete volcanic-hosted massive sulfide hydrothermal alteration system, Panorama district, Pilbara, Western Australia

Outstanding exposure across the Panorama district, in the Archean Pilbara b lock of Western Australia, reveals a cross section through a complete volca nic-hosted massive sulfide (VHMS) hydrothermal alteration system in an area of low metamorphic grade and very low strain. Geochemical maps, in combina tion with alteration maps, document mass transfer across this alteration sy stem. Feldspar-bearing rocks at the top of the volcanic pile are enriched i n K and Si, but depleted in Na, in contrast to those at the bottom of the v olcanic pile that are enriched in Na and depleted in K. Sericite-quartz alt ered rocks are typically enriched in Si and depleted in Ca, Na, and Fe, whe reas chlorite-quartz altered rocks, which dominate transgressive feldspar-d estructive alteration zones, are marked by K-Ca-Na depletion and Mg-Fe enri chment. Copper and Zn axe strongly depleted in rocks at the base of the volcanic pi le and, to a lesser extent, in transgressive feldspar-destructive alteratio n zones. The amount of metal leached from the volcanic pile is far greater than that contained in known deposits, and thus a magmatic metal source is not necessary to explain the formation of the Panorama VHMS deposits. Mass-transfer patterns at Panorama have similarities to existing models for regional VHMS hydrothermal alteration systems, including the distribution of alkali elements in feldspar-bearing, semiconformable alteration zones an d the presence of a base metal-depleted zone at the base of the volcanic pi le. However, there are significant differences, both in the architecture of the hydrothermal alteration system and in mass-transfer patterns. These in clude Si enrichment rather than depletion at the top of the volcanic pile, widespread Mg enrichment outside of the upper semiconformable alteration zo nes, the absence of a silica cap rack above the base metal depleted zone, m ore hydrous alteration assemblages in the zones of base metal depletion, an d extensive zones of transgressive feldspar-destructive alteration.