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The Stirling Group consists of ca. 680 Ma volcanic and volcaniclastic rocks, interpreted to have been deposited in and around a trough flanked by felsic volcanic centres. The volcaniclastic rocks are dominated by lapilli tuff and tuffaceous litharenite sequences. The lapilli tuffs show some evidence of subaqueous reworking but most may have originated as air-fall deposits. Sedimentary structures in the litharenites suggest subaqueous mass transport into the trough and deposition below wave base. Pyritic siltstone-chert-carbonate laminites, which are locally associated with the litharenites along the southeastern side of the trough adjacent to felsic volcanic centres, probably represent a mixture of fine-grained distal turbidites and chemical precipitates from subaqueous volcanic exhalations. The volcanic rocks of the Stirling Group do not show convincing evidence for subaqueous deposition: the felsic rocks appear to form subvolcanic intrusions and domes flanked by felsic crystal-rich lapilli tuff and rare welded(?) tuff; the mafic volcanic rocks are not pillowed but are closely associated with breccias which may have formed partly by quench fragmentation. The overall facies assemblage and its association with VMS Zn-Pb-Cu-Ag-Au mineralization are compatible with generation in an intra-arc basin. As the Stirling Group is significantly older than the volcanic belts which flank it, it may represent a tectonically displaced remnant of a pre-existing arc.  RESUME  Le Groupe de Stirling consiste en roches volcaniques et volcanoclastiques d'environ 680 Ma, interpretees comme ayant ete deposees a l'interieur et autour d'une fosse bordee de centres volcaniques felsiques. Les roches volcanoclastiques sont dominees par des seequences de tufs a lapilli et de litharenites tuffacees. Les tufs a lapilli montrent quelques evidences de remaniement subaqueux mais la plupart pourraient etre des depots de retombees aeriennes. Les structures sedimentaires dans les litharenites suggerent des mouvements de masse subaqueux dans la fosse et une deposition sous la base des vagues. Des laminites de siltstone pyriteux, chert et carbonates, qui sont localement associees aux litharenites le long du flanc sud-est de la fosse I proximite des centres volcaniques felsiques, representent probablement un melange de turbidites distales a grain fin et de precipites chimiques provenant d'exhalaisons volcaniques sous-marines. Les roches volcaniques du Groupe de Stirling ne montrent pas d'evidences convaincantes de depot sous-marin: les roches felsiques semblent former des intrusions subvolcaniques et des domes bordes de tufs a lapilli felsiques riches en cristaux et de rares tufs soudes (?); les roches volcaniques mafiques ne sont pas coussinees mais sont etroitement associees a des breches qui ont pu en partie se former par fragmentation relive a la trempe. L'assemblage general de facies et son association avec de la mineralisation en sulfures massifs volcanogenes de Zn-Pb-Cu-Ag-Au sont compatibles avec une formation dans un bassin intra-arc. Comme le Groupe de Stirling est nettement plus ancien que les ceintures volcaniques qui le bordent, il pourrait representer un vestige d'un arc preexistant deplace tectoniquement.  [Traduit par la redaction]

Geological setting and depositional environment of the Stirling Group of southeastern Cape Breton Island, Nova Scotia