A stock of trondhjemite intrudes an Archean succession metamorphosed to the greenschist facies at Gutcher Lake, 30 km north of Wawa in Ontario. The stock is 4 km2 in plan and is partly enveloped by an aureole of epidote–amphibole hornfels up to 1 km wide. Within this aureole chlorite partly replaces biotite; chlorite, calcite, and quartz mantle hornblende and epidote; zoned amphibole has a rim of actinolite; and albite varies from 0 to 8% An. The stock has chlorite pseudomorphous after biotite, and feldspar is mottled by white mica and has a clear rim of albite. Fractures filled with quartz, calcite, ankerite, white mica, chlorite, pyrite, and native gold cross-cut the stock and its aureole. Wall rocks to these veins were modified by hydrothermal alteration with addition of Si, Fe, K, H2O + CO2, S, and Rb, leaching of Na, and a shift in Fe2+/Fet from ~0.66 to ~0.90. Primary inclusions in the vein quartz have a solution with a CO2 gas bubble that homogenizes into the liquid at around 300 °C.Initial contact metamorphism of volcanic rocks at T = 450–550 °C and P < 200 MPa (2 kbar) formed an aureole of epidote–hornblende hornfels near the stock. Subsequent regional metamorphism during the Archean at T = 325–450 °C and P = 200–300 MPa (2–3 kbar) retrograded the stock and its contact aureole to a lower greenschist assemblage. The retrogression involved hydration and CO2 fixation in hornfels and trondhjemite by a hot reducing fluid of low salinity. This metamorphic fluid precipitated native gold with quartz and pyrite along fractures in response to cooling and chemical reaction with wall rocks.

You do not currently have access to this article.