Structural geology, stratigraphy, and gold deposits of the New Britannia Mining District of the paleoproterozoic Snow Lake arc assemblage (Snow Lake, Manitoba, Canada)

Abstract

Orogenic gold deposits in the Snow Lake area in the southeastern Trans Hudson Orogen, Manitoba, include the New Britannia deposit which, with a past production of a 1.4M oz Au (43 699 kg), is the largest Proterozoic gold deposit in Manitoba and Saskatchewan. The deposits are hosted by the ca. 1.89 Ga Paleoproterozoic Snow Lake arc (SLA) assemblage of the Flin Flon Glennie Complex (FFGC). The FFGC is bound by a sedimentary basin to the north, namely the Kisseynew Domain. It represents an oceanic protocontinent within an ancestral ocean, the Manikewan Ocean, which occupied the region between three Archean cratons (Hearne, Sask, Superior). The deposits occur within a sequence of explosive, submarine, bimodal volcanic rocks emplaced during an early episode of rifting and subsidence of the SLA. Deformation of these rocks began with their imbrication along brittle thrust faults during a D1 event. It continued during a ca. 1.84- 1.82 Ga D2 event with thrusting of the Kisseynew basin and FFGC above the colliding Sask craton. The D2 event produced a penetrative regional foliation axial planar to map-scale isoclinal folds (NorAcme Anticline), sheath-like gneiss domes along the Kisseynew-FFGC boundary, and regional southwest-directed ductile thrust faults, such as the McLeod Road Thrust. Final collision of the FFGC and Sask craton with the Superior craton during a ca. 1.83-1.80 Ga D3 event reactivated the ductile thrust faults, folded the gneiss domes, and enabled the development of an orogen-parallel regional stretching lineation during lateral flow parallel to the cold Superior craton. The New Britannia deposit and nearby gold deposits were emplaced in the hinge of the Nor-Acme Anticline early during the D2 event, were folded during tightening of the fold, stretched parallel to the regional stretching lineation, and transposed along a late crosscutting shear zone, the Howe Sound fault, which formed as a transfer fault during thrusting. The deposits formed at amphibolite facies conditions during a prograde metamorphic event that culminated during the collision of the FFGC and Sask craton with the Superior craton during the D3 event. Thus, the deposits are atypical compare to most other Proterozoic and Archean orogenic gold deposits which typically form at greenschist facies conditions.