Tectonometamorphic evolution, fluid production, and evaluation of gold liberation in the Quetico metasedimentary belt, Canada

Abstract

The Quetico subprovince is a prominent 1200 x 100 km tectonometamorphic belt in the north-central Superior Province and represents one of the largest exposures of siliciclastic metasedimentary rock globally. It has historically been a key feature in advancing subductionaccretion geodynamic models for Neoarchean crust, which many recent studies have challenged in favour of plume-sagduction models. Part of the confusion stems from poor constraints on the regional architecture of these belts and the nature and timing of tectonometamorphic events. Recent studies have also suggested that metasedimentary basins may represent sources of goldbearing fluids for nearby orogenic gold deposits through metamorphic devolatilization. Yet, the pressure-temperature-composition (P-T-X) conditions for fluid release and the breakdown of auriferous pyrite in these basins are poorly defined. This thesis demonstrates that the architecture of the Quetico is largely symmetrical: pre-Quetico lithotectonic units have similar composition, age, and distribution on the N and S margins; sediment layers vary from calc-semipelite-rich at the margins to pelite-rich in the interior; stratigraphy is largely outward facing; the sequence, geometry and relative timing of isograds are mirrored; and strain fabrics are consistently subvertical. Quetico sediments are richer in Ca and Na and poorer in K and Al than the typical pelite and mainly derived from a single felsic volcanic source at ~2.7 Ga. The metamorphic sequence of chlorite, biotite, garnet, staurolite + andalusite, cordierite, sillimanite, in situ melt, K-feldspar, and orthopyroxene is consistent with a single protracted clockwise P-T path that is syn-to post-regional deformation, D2. The subsolidus P-T history is characterized by burial to ~6.5 kbar/550 °C, followed by heating and decompression to ~3 kbar/600 °C. The simultaneous breakdown of calcite and muscovite in biotite zone semipelites (470–500°C/~4.5 kbar) produced a major pulse of CO2-rich fluid (XCO2 = ~0.3). Under these conditions, gold was likely liberated in the pyrite to pyrrhotite transition for weakly oxidized bulk compositions (XFe3+ ≈ 0.05–0.08). Under more oxidizing conditions, pyrite breakdown is predicted above 550 °C, and under less oxidizing conditions, pyrite is consumed at T < 350 °C, outside of any significant fluid production window. Only a small amount of H2S is added to the fluid (XH2S < 0.005) during devolatilization, as excess S from pyrite readily reacts with Fe in silicates to form additional pyrrhotite. Garnet Lu-Hf and U-Pb monazite ages constrain prograde metamorphism through the garnet to sillimanite zones from ~2680 to 2660 Ma in the western Quetico. In the eastern Quetico, metamorphism in equivalent zones appears to be ~10 Ma younger, and peak temperatures in the migmatite zones were reached simultaneously with those in the subsolidus zones. These spatiotemporal features indicate a geodynamic setting dominated by near-field sedimentation, subsidence, and advective ductile flow in the middle-upper crust, characteristic of ultra-hot orogens (UHOs). The finding providing new belt-scale insights into the thermal structure and fluid evolution of Archean transpressive UHOs.