PRE-OBDUCTION STRUCTURES OF THE THETFORD MINES OPHIOLITIC COMPLEX (TMOC), QUEBEC : IMPLICATIONS FOR OCEANIC CRUST FORMATION AND PGE MINERALISATION

The TMOC is divided into two different petrologic and structural units, a NW mantle-dominated massif (Thetford Mines unit) and a SE crustal-dominated massif (Mont Adstock unit). Three types of tectonic fabrics can be identified, pre-, syn- and post-obduction. Post-obduction structural features consist of NE-trending Acadian open folds and reverse faults, Late Silurian- Early Devonian backfolds and normal faults such as the Saint-Joseph fault (SJF).The TMOC occurs in the hanging wall of the SJF. Structures attributed to ophiolite obduction onto the Laurentian Margin consist mainly of a metamorphic sole and related structures at the base of the mantle section. Pre-obduction structures show two different sub-vertical orientations, NS and NW-trending. There are underlined by sheared dunite (mylonite), and the NS-trending set, appears to control the emplacement of mafic-ultramafic bodies into the cumulates. In the upper part of the crust, NS-structures are associated with doleritic dykes, fault breccias, upwardly decreasing throws suggest that they are syn-oceanic grow faults. A Pt-Pd survey shows that chromite-bearing as well as pyroxene-bearing rocks are good prospects for PGE exploration. The greatest PGE concentrations are encountered in discordant chromitite bodies but non-negligible grades have been found in the pyroxenite, near the contact with gabbroic units, and in the bedded chromitite-bearing dunite.