THE SETTING, STYLE AND TIMING OF LATE NEOPROTEROZOIC HIGH-SULFIDATION AND LOW-SULFIDATION EPITHERMAL SYSTEMS OF THE AVALON ZONE, NEWFOUNDLAND, CANADA

The Avalon Zone of Newfoundland is host to numerous examples of both high- and low-sulfidation styles of epithermal mineralization developed within volcano-plutonic arc complexes and immediately overlying siliciclastic sedimentary basins. This mineralization is often proven or inferred to be late Neoproterozoic in age, and is affected by varying degrees of post-mineral deformation. Within the eastern portion of the Avalon Zone in Newfoundland the effects of this post-mineral deformation are relatively minor, and well preserved high- and low-sulfidation style epithermal systems exist in relatively close proximity to one another (ca. 1 mi lateral distance). Here pyrophyllite−diaspore advanced argillic alteration, which is essentially barren with respect to precious metal mineralization, occurs within ca. 585 Ma subaerial felsic volcanic rocks. The same volcanic sequence is also host to the development of crustiform-colloform, adularia-bearing low-sulfidation veins, which are locally host to bonanza gold grades.

Epithermal systems along the western margin of the Avalon Zone in Newfoundland are commonly more strongly deformed, but are observed to contain the characteristic textures and mineralogy indicative of both high- and low-sulfidation styles of epithermal mineralization. Regionally extensive epithermal-related alteration extends for some 60 mi along the Burin Peninsula region, hosting variably developed zones of silica, pyrophyllite, alunite, dickite, muscovite and locally topaz and diaspore alteration. Within these zones locally developed vuggy silica alteration is host to bonanza-grade gold mineralization in association with enrichment in Ag, As, Cu, Hg, Sb, Se, Sn and Te. Alteration developed in relation to the low-sulfidation style mineralization in the region is more discrete, consisting largely of variably developed silicification, but also containing minor associated clay alteration.

Despite their relatively shallow levels of formation, these Neoproterozoic epithermal systems demonstrate excellent preservation within the geological record, in part due to their rapid submergence followed by the deposition of post-mineral sedimentary cover sequences.