Abstract

The Hercules Pb-Zn deposit consists of a number of isolated, ellipsoidal massive sulfide pods hosted in a tuffaceous slate unit of Middle Cambrian age. These pods are aligned roughly parallel to the main cleavage and have completely discordant to moderately folded bedding. Mineralization is localized in the short limb zones of parasitic F 2 folds from the micro- to the macroscopic scale. Antithetic shearing induced along the bedding during D 3 caused localized dilation and fracturing of some of the short limb zones. This resulted in alteration and replacement late in D 3 such that Sa was overprinted by the mineralization. The following paragenetic replacement sequence exists: pyrite I --> quartz --> chlorite --> pyrite II --> (sphalerite-galena-chalcopyrite). Pb isotope age data are consistent with the derivation of the deposit from dispersed primary sulfides in the Mount Read Volcanics by synmetamorphic mobilization into structural traps at higher levels in the crust.

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