Abstract

Outstanding exposure across the Panorama district of the Archean Pilbara block of Western Australia reveals a cross section through a ca. 3.24 Ga massive sulfide-bearing volcanic pile and an underlying coeval subvolcanic intrusion, in an area of low metamorphic grade and very low strain. This has provided a rare opportunity to map a complete regional-scale hydrothermal alteration system associated with volcanogenic massive sulfide mineralization and to assess the role of a subvolcanic intrusion in driving such a system.Four major alteration facies can be defined in the Panorama district. These are feldspar-bearing background alteration and feldspar-sericite-quartz alteration, and completely feldspar-destructive sericite-quartz and chlorite-quartz alteration. These alteration facies affect all of the volcanic pile and the upper parts of the underlying subvolcanic intrusion. Regional semiconformable alteration zones, which are predominantly feldspar-bearing, are crosscut by broad corridors of transgressive feldspar-destructive alteration. A convective hydrothermal model is invoked to explain this distribution of alteration facies, where semiconformable alteration zones represent seawater recharge, and transgressive feldspar-destructive alteration zones mark the path of evolved seawater discharging back to the sea floor. The interpreted discharge sites are coincident with base metal sulfide deposits at the top of the volcanic pile.Geologic relationships, and the distribution of alteration zones, clearly implicate the Strelley Granite as the subvolcanic intrusion which drove the hydrothermal alteration systems. Granophyric textures and miarolitic cavities in the upper parts of the intrusion suggest that a magmatic fluid phase was exsolved early in its crystallization history. Granite-hosted copper-zinc-tin veins, near the top of the intrusion, are interpreted to be a product of this magmatic fluid. This raises the possibility of a substantial contribution of magmatically derived metal to the overlying volcanogenic massive sulfide hydrothermal alteration systems.

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