Geochemistry and geodynamic setting of volcanic and plutonic rocks associated with Early Archaean volcanogenic massive sulphide mineralization, Pilbara Craton

Abstract

Early Archaean volcanic rocks and syn-volcanic granites at Strelley, east Pilbara Craton, Western Australia, were formed during an extensional tectonic phase associated with an active convergent continental margin. The volcanic sequence comprises a bimodal suite of fractionated sub-alkalic tholeiitic basalts, andesites and dacites, siliceous laminites and volcanogenic massive sulphide (VMS) deposits in the footwall and hangingwall calc-alkaline dacites. The footwall rocks are characterized by Th/La pm>1, variably fractionated LREE and relatively high Y and Yb (>33 and >3 ppm, respectively). Negative normalized anomalies of Nb, P, Ti, are consistent with derivation via slab dehydration fusing the mantle wedge. The co-magmatic and syn-volcanic Strelley Granite is enriched in LILE relative to HFSE, and has distinctively low Al 2O 3 (11–14) and [La/Yb] n (4–10) suggesting a relatively mature and cool subducted oceanic lithosphere as for the majority of Phanerozoic arc granitoids and convergent margin magma compositions. The chemical composition of the footwall basaltic to dacitic rocks and the sub-volcanic Strelley Granite, display signatures highly prospective for VMS mineralization (large -ve Eu anomalies, low Zr/Y ratios, high level magmatic fractionation), comparable with the Noranda FIIIa-type suite. The hangingwall dacites have more pronounced HREE fractionation than footwall counterparts, and lower Y and Yb (<21 and <1.6, respectively), consistent with slab melting. The hangingwall dacitic rocks display non-prospective VMS signatures, and are unmineralized. Accordingly, Archaean magmatic arcs may have varied between slab dehydration and slab melting. This change in variation of melting and dehydration may be related to key temporal stages in a geodynamically evolving convergent margin. The chemistry of the footwall volcanic rocks and granite reflect typical Phanerozoic conditions of steep subduction, hydrous metasomatism in the mantle wedge, and fractionation. Similar characteristics are also evident in plutonic–volcanic suites associated with VMS deposits in the Late Archaean, as at Noranda, Canada. Early extension with nascent back-arc development coupled with steep subduction is considered critical to the development of VMS deposits.