Abstract

The style of extensional quartz veins changed during deformation and metamorphism of greywacke–mudstones near Yellowknife, with successive types of veins accompanying a progression from steeply plunging, predominantly macroscopic F1, and F2 folds to mesoscopic F3 folds and subvertical S3 axial planar cleavages. (A) Early-stage multiple bedding-parallel veins are confined to pelitic units. Some form saddle reefs around hinges of F1 and a few later folds, whereas others are unrelated to fold hinges. Emplacement of most bedding veins during or before F1 folding is suggested. (B) Echelon veins pre-dominantly lie stratabound within pelites. Folding about S3 cleavage indicates a pre-F3 or early F3 origin. (C) Foliation veins more commonly cross sandstones than pelites and follow S3 or F3 axial surfaces. Boudinage of the veins indicates emplacement before S3 was fully developed. (D) Sporadic veins, discordant to bedding and cleavage, vary from undeformed to folded about S3. Evidently these veins were emplaced towards the end or after F3 folding.The change from pelite as the preferred host rock for bedding and echelon veins to sandstone for foliation veins could reflect migration of loci of hydraulic fracturing as fluid was produced and lost during metamorphism. Bedding and foliation veins apparently followed weakness planes (bedding fissility and cleavage), whereas the echelon vein arrangement suggests that during an intermediate stage of deformation pelites acted as shear zones. All veins could have formed during variably directed, subhorizontal, tectonic compression.

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