Plenary Address

The Science and the Discovery of Volcanogenic Massive Sulfide Deposits
By Harold Gibson
Laurentian University

Volcanogenic massive sulphide (VMS) deposits formed at or immediately below the seafloor in spatial, temporal and genetic association with volcanism. They represent a significant source of Cu, Zn, and Pb; Au, Ag, as well other elements (e.g. Sn, In), are common by-products. A total of 155 VMS deposits have been mined in Canada and most of these are Archean and Paleoproterozoic in age. The episodic distribution of VMS deposits from the Paleoproterozoic onwards has been linked to periods of supercontinent assembly and the preferential preservation of arc environments during accretionary events (e.g. Nuna and VMS deposits of the Trans-Hudson orogen).

VMS deposits are, perhaps, the most thoroughly researched deposit type. Since recognition of their syngenetic origin in the mid-1950s and the discovery in 1979 of actively forming VMS deposits on the seafloor, ancient deposits and their modern seafloor analogues have been the focus of decades of intensive geological, geochemical, isotopic and geochronological research. This has resulted in the development of arguably one of the best genetic models for any deposit type, which along with the relatively high value and multi-metal nature of their ores has, up until the mid 90’s, fuelled exploration for this deposit type.  Exploration criteria and strategies have changed as the genetic model for these deposits continuously evolved over the last hundred years. In Canada, for the period from 1900 to present, distinct lows in VMS discovery rates correspond to periods of low metal prices (recessions) and less exploration. Most pre-1950 discoveries are attributed to prospecting using exploration criteria based on an epigenetic model. New technologies, particularly the onset of commercial airborne electromagnetic surveys in the 1950s to 60s, and subsequent new technologies (e.g. Input AEM in the 70s) resulted in a significant increase in the discovery rate. Smaller peaks followed widespread adoption of a syngenetic model in the 1970s and refinements to that model from research done on modern seafloor analogues. This is particularly true for exploration in established mining districts where continued discovery, often at increasingly greater depths, was guided by exploration criteria based on a refined syngenetic model coupled with the use of new technologies (borehole EM, lithgeochemistry). The steady decrease in VMS discovery rate since the mid 1970s and, in particular, since the mid 1990s reflects, in part, the challenges resulting from increasing depth of discovery, the extensive surficial cover of shield areas, and a shift by many primary base metal companies to explore for larger deposit types that are more amenable to open pit mining operations. Increasing energy costs and environmental concerns associated with open pit mining may refocus exploration efforts on VMS deposits. However, future exploration will require new criteria based on the seamless integration of the geological, geophysical and geochemical attributes of a VMS ore system into a predictive, holistic model.