The silver deposits at Cobalt and Gowganda, Ontario. II: An experiment in age determinations employing radiometric and paleomagnetic measurements

Abstract

U–Pb analyses of primary baddeleyite and vein-related secondary rutile, separated from a Nipissing diabase sill at the Castle mine, Gowganda, yield ages of 2217.5 ± 1.6 and 2217.0 + 6 Ma, respectively. The data allow for the possibility that (i) the vein formed at essentially the same time as diabase intrusion, defined within the limits of the intercept age [Formula: see text], or (ii) the vein could have formed up to 8.1 Ma later than intrusion. Paleomagnetic analyses of the diabase suggest that it is part of the older (N2) phase of this widespread intrusive event. Paleomagnetic analyses of a pre-vein granite dike, intrusive into the diabase, provide a minimum time interval of 20 000 – 50 000 years separating intrusion and vein formation.40Ar–39Ar analyses of silicates obtained from ore veins in the Cobalt camp yield highly disturbed age spectra. Paleomagnetic analyses of altered wall rocks to ore veins in both the Castle mine and the Cobalt camp provide a wide range of apparent ages, inconsistent with the U–Pb data. The 40Ar–39Ar and paleomagnetic data are interpreted as indicating a prolonged history of post-vein disturbances during regional tectonism.