Saddle reef auriferous veins in a conical fold termination (Oldham anticline, Meguma terrane, Nova Scotia, Canada): reconciliation of structural and age data

Abstract

Many auriferous veins dated at ~370 Ma in the Meguma terrane of Nova Scotia are concentrated in tight domes, and conical fold hinges that are transected by 378 Ma low-pressure and high-temperature metamorphic isograds and 380–370 Ma granitoid plutons. Published experiments suggest that such folds are diachronous initiating at inhomogeneities and propagating both in amplitude and along their length. Thus fold terminations are typically conical in geometry and record only the youngest increment of folding. The northeastern termination of the Oldham anticline is characterized by (1) a conical fold, (2) saddle reef auriferous veins, (3) divergent and convergent cleavage fans, (4) a downdip lineation on the slaty cleavage defined by biotitic mineral aggregates inferred to have developed during fold growth. The age of the biotitic mineral aggregates is bracketed between peak metamorphism at > 600°C, defined by a nearly concordant 378 Ma U–Pb monazite age from a xenolith in lamprophyre dyke, and ~366 Ma 40Ar/39Ar muscovite–biotite ages, recording cooling through ~400–300°C. This suggests that the conical termination of the Oldham anticline grew between 378 and 366 Ma, an observation that reconciles the empirical structural control of the saddle reef auriferous veins with the ~370 Ma age dating of vein minerals. Application of this conclusion to saddle reef auriferous veins in domes suggests that mineralization was related to the youngest increment of fold amplification.