At Lac Rocher, the upper portions of the Granite Gneiss Group and locally, the lower portions of the overlying Sericite Schist Group are dominated by biotite-anthophyllite- and cordierite-rich gneiss and schist. The Mg-Fe enrichment and Na-Ca-K depletion shown by these members (Table II) indicates that they may represent the highly altered footwall rocks typical of voicanogenic deposits (Sangster, 1972; Sangster and Scott, 1976). These altered units, though only roughly outlined to date, typically do not show strong discordancies to foliation and may in fact represent altered stratigraphic units and not the more typical pipe-shaped feeder system. Alternatively, such feeder pipes may now be difficult to recognize, having been partially transposed into the plane of gneissosity during development of the Mattagami Synform.
Regardless of their present distribution, these Mg-rich gneisses are considered to represent a high-grade metamorphic equivalent of primary "chloritic" alteration zones developed in the footwall units by the initial ore-forming solutions.
Pods and lenses of these Mg-rich units are locally present in the overlying biotitic, sillimanitic and siliceous members of the Sericite Schist Group. Na-K depletion continues through these upper schists but Ca depletion is not evident and the overall Mg-Fe enrichment. though present, is less intense. The high silica content of these upper schists may indicate that these units have been highly silicified.
Although the parent material of the schists cannot be established definitely, their spatial association with, and broad chemical similarities to the overlying biotitic quartzites indicate that they may be related to the quartzite unit. Quartzite flanking the Sericite Schist Group is locally sericitic, and distinguishing between the two units is sometimes difficult. These quartzites represent the lateral equivalents of a sequence of quartz-feldspar-gneisses outlined by Milne (1969, 1974). Figure 3 shows the lateral distribution of these felsic-gneisses in the synform. The high alumina content of this group indicates that they were not likely formed as chemical precipitates. Rather, we consider these siliceous gneisses to represent highly metamorphosed felsic volcanic or detrital volcanogenic sedimentary rocks. Thickening rapidly westward, these gneisses attain maximum thicknesses in the hinge area of the syncline near Chabazouille.
Over-thickening of the siliceous gneisses in this area is considered in part a primary depositional feature and may indicate relative proximity to a major volcanic centre. Despite its strongly developed alteration, thinning of this unit near the Lac Rocher deposit may indicate that the Lac Rocher orebody formed at a greater distance from a volcanic centre than did the other deposits of the camp (Timms and Marshall, 1959; Chown, 1957).
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