New observations on relict volcanic features in medium-grade gneiss of the Montauban group, Grenville Province, Quebec
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Abstract
The Montauban region, south-central Grenville Province, is known for its small, stratiform, polymetallic Au-Ag-Zn-Pb ore deposits. The amphibolite-facies orebodies and hydrothermally altered wall rocks occur within the ca. 1.45 Ga Montauban group, a sequence dominated by quartzofeldspathic gneiss with minor quartzite and amphibolite, locally pillowed metabasalt, and sporadic outcrops of marble and calc-silicate rocks. Occurrences of lapilli tuff are reported, namely from the structural footwall-stratigraphic hanging wall of the orebodies. This is the first unequivocal example of felsic volcanic rocks in the Montauban group. Proximal lapilli tuff and vesicular basaltic pillow lava occur with thinly bedded intermediate to felsic rocks interpreted as distal pyroclastic subaqueous fallout deposits, and laterally intercalated epiclastic sediments. Such volcanic rocks are consistent with deposition of the Montauban group in a shallow submarine environment at the late stage of an andesitic to felsic volcanic cycle, a common occurrence in mature island-arc or backarc settings where Au-rich volcanogenic sulphide deposits are formed.
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