Geochronology and stratigraphic relationships of the Sulphur Springs Group and Strelley Granite: a temporally distinct igneous province in the Archaean Pilbara Craton, Australia

Rocks from the Sulphur Springs Group, a newly identified stratigraphic succession hosting volcanogenic massive sulphide (VMS) base-metal deposits in the Soanesville Belt, Pilbara Craton, northwestern Australia, have been analyzed by sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon mass-spectrometry to determine their age and tectonostratigraphic relationships. Most volcanism occurred between 3238 and 3235 Ma, distinctly different from the time interval spanned by the Wyman Formation (type section is 3315 Ma, other dates ∼3325 Ma) with which it was previously correlated. Outlying volcanic rocks with similar geochemistry at the Bernts and Obelix prospects also produce dates from 3240 to 3235 Ma and are, therefore, best interpreted as structurally-detached fragments of the Sulphur Springs Group. The Strelley Granite, an intrusion which shares geochemical characteristics with the overlying volcanic rocks, also yields similar ages, thus extending the period of temporally-linked extrusive–intrusive silicic magmatism in the evolution of the Pilbara Craton. Model Pb–Pb dating of galena from the Zn–Cu mineralization at the various VMS prospects has yielded ages consistent with the zircon U–Pb ages of the host rocks, indicating that the hydrothermal mineralizing systems were contemporaneous with and apparently initiated by igneous activity. An epicontinental tectonic setting for the Sulphur Springs Group during VMS mineralization is implied by the broad range of old zircon ages obtained from a tuff at the base of the volcanic pile. As the igneous episode coincides with the peak regional metamorphic event recorded in other east Pilbara supracrustal rocks and with voluminous intrusive activity in nearby granitoid complexes, it marks a major event in the crustal evolution of the eastern Pilbara Craton.