Beaudoin, Georges, Gagnon, Yan

MEDEF,
Département de Géologie et de Génie géologique, Université Laval, Québec, Canada G1K 7P4

The Val-d'Or camp, Abitibi sub-province, Québec, comprise steeply-dipping, south-facing, ultramafic to mafic submarine volcanic complexes that are overlain by calc-alkaline intermediate to felsic volcanic rocks (Val-d'Or Formation) and intruded by the subvolcanic Bourlamaque batholith at about 2700 Ma. Rocks of the Val-d'Or Formation contain several volcanogenic massive sulfide deposits (ex. East Sullivan, Louvicourt). The structural facing of the rocks allows to investigate the VMS hydrothermal system in cross-section using the oxygen isotope composition of the host rocks.
A systematic sample grid (1 sample/4 km2) has been applied to obtain a regular distribution for the 123 samples. d18O(VSMOW) values in volcanic rocks display similar ranges and averages for all formations (Dubuisson: 4.0-11.2, av. 7.5±1.7; Héva: 4.4-11.3, av. 8.3±1.7; Jacola: 5.7-15.3, av. 7.9±2.2; Val-d'Or: 2.0-13.3, av. 8.4±2.4). Values for the Bourlamaque batholith range from 4.6 to 13.1 with an average of 7.0±1.8. The spatial distribution of d18O values is deciphered by smoothing the isotope composition gradients using the kreiging technique based on a geostatistical analysis. The anisotropy ellipse indicates that the range of influence is shorter N-S (about 2km) and somewhat longer in the ENE-WSW direction (about 3.5 km), whereas the value of the variogram function shows no significant anisotropy. Using these parameters, the map of krieged values displays a pattern comprised of three undulating, south-pointing corridors emanating from the Bourlamaque batholith with values below 7.5 separated by zones of higher values near the top of the Val-d'Or Formation. The western corridor is towards the city of Val-d'Or whereas the central and eastern corridors point towards the East Sullivan and Bevcon plutons, respectively. These corridors represent major upflow zones of high temperature hydrothermal fluids in the volcanic sequence. The East Sullivan massive sulfide deposit sits near the southern end of the central corridor. The Louvicourt deposit, however, is located between the central and eastern corridors, perhaps a consequence of the more diffuse hydrothermal flow within its fragmental hostrock.
The volcanic rocks oxygen isotope map displays no obvious features that can be related to the infiltration of auriferous hydrothermal fluids during D2 deformation. That is despite a well defined pattern of oxygen isotope variation defined from auriferous vein quartz, which suggest low fluid to rock ratios during mesothermal gold vein formation.