Tectonic and metallogenic importance of an Archean composite high- and low-Al tonalite suite, Western Superior Province, Canada

Abstract

Emplacement of the ca. 2735–2720 Ma Sturgeon plutonic suite (SPS) into 2774–2745 Ma oceanic arc crust of the Savant-Sturgeon Lake greenstone belt, western Superior Province, occurred contemporaneously with South Sturgeon caldera complex formation and deposition of its contained volcanogenic massive sulfide (VMS) deposits and subeconomic Cu–Mo porphyry system. The SPS, which includes both large (∼1400 km 2) mid-crustal intrusions and smaller, shallower-level mafic and felsic plutons, is interpreted as a series of stratiform-coeval intrusions that represent the magmatic roots and feeder system to a continental margin arc-rift sequence. New Nd and O isotopic results, plus unpublished VMS Pb and published granitoid zircon Hf data, suggest derivation of felsic SPS phases from juvenile (<2.9 Ga) crustal materials and mantle-derivation of mafic SPS phases. Felsic SPS phases include: (a) a high-Al, HREE-depleted tonalite–trondhjemite–granodiorite (TTG) subsuite and (b) a low-Al, HREE-enriched tonalite subsuite. Initially arc rifting resulted in mantle-decompression-melting and crustal under- and intra-plating with mafic magmas, inducing sequential dehydration partial melting of amphibolitic lower to middle crustal protoliths. High-Al SPS TTG-type phases were derived at >30 km depth under garnet-stable P– T conditions, whereas low-Al SPS felsic phases were likely derived at 10–18 km depth, leaving residual amphibole and plagioclase. SPS dioritic phases represent basaltic magma fractionation products. Coeval lower to upper crustal emplacement and eruption of SPS magmas formed a crustal-scale magmatic pumping system that generated and sustained a long-lived (∼15 Ma) thermal corridor. This rift-related thermal corridor was intimately related in time and space, first to: (a) hot and dry, low-Al intrusive and eruptive units, characterized by FII rhyolite chemistry, that initiated and supported a VMS-generating seafloor hydrothermal seafloor system at ca. 2735 Ma; and subsequently to (b) subvolcanic devolatilization of more volatile and K-rich high-Al intrusions that produced porphyry-type mineralization 14 Ma later.