Recognition of metamorphosed hydrothermal systems in high grade metamorphic terrains : Diagnostic geological tools and implications for regional exploration, a DIVEX contribution
CIM Montreal 2003
Kenneth Williamson, Louise Corriveau, Anne-Laure Bonnet, Sharon Parsons,
The discovery of new ore deposits constitutes the starting point of a healthy mineral industry. Historically, it has led to the development of prosperous mining camps mostly in greenstone belts such as the Abitibi. However, as pointed out by DIVEX, a drastic reduction in the discovery rate in these terrains has recently forced the mineral industry to adapt by diversifying its exploration into lesser known environments such as the uncharted frontier high-grade metamorphic terrains of the Canadian Shield. In these terrains, the traditional mapping exploration tools developed to find ore deposits in greenstone belts are not adequate to recognize the amphibolite to granulite facies equivalents and need to be refined.
Large hydrothermal alteration halos are commonly developed around hydrothermal ore deposit types (VHMS, epithermal and lode gold deposits, ...). High grade metamorphism of such altered rocks results in the formation of new « diagnostic » rocks (such as tourmalinite, gahnite-rich rocks, coticule, sillimanite-rich gneiss, cordierite-orthopyroxene gneiss...), which are generally atypical in appearance, mineral assemblage and mode. With the coarsening of grain size, they become relatively easy to recognize in the field, and constitute a key element in exploration for hydrothermal ore deposits in high-grade metamorphic terrains.
Compositional phase diagrams are another useful exploration tool which is used to visualize stable mineral assemblages in metamorphosed hydrothermally-altered rocks. One of the major advantages of using these diagrams is that they are used to establish potential vectors for assessing alteration type. Modal proportions of altered rock types can be easily determined from field observations, and using such diagrams they can be compared with what is expected from the average unaltered equivalent. Thus, if a specific bulk composition departs significantly from the expected modal proportion, then it can be considered as a signal to trigger further investigation into the type of alteration, which may be associated with an ore deposit.
The proposed tools allow for the recognition of metamorphosed hydrothermal systems in high-grade metamorphic terrains, with a reasonable degree of confidence. As hydrothermal ore deposits should be potentially associated with such hydrothermal systems, the proposed tools may be helpful in finding new base metals and gold deposits in the unexplored high-grade metamorphic terrains of the Canadian Shield.
hydrothermalism, hydrothermal ore deposit, high-grade metamorphism, Exploration, Hydrothermal alteration