Whole-rock δ2H and δ18O footprint of the Canadian Malartic gold deposit, Pontiac Subprovince, Québec, Canada

Abstract

The Canadian Malartic gold deposit was produced by a hydrothermal system that altered the host metasedimentary rocks of the Pontiac Group, metavolcanic rocks of the Piche Group, and associated porphyritic intrusions and mafic dykes. The isotopic footprint in Pontiac metasedimentary rocks is marked by an increase in δ2H values from − 91‰ near mineralization to background values of ca. − 59‰, whereas δ18O values remain relatively constant near 10‰. The isotopic footprint in mafic dykes is marked by an increase in δ2H values from − 84‰ near mineralization to background values of ca. − 73‰, and a decrease in δ18O values from 9.8‰ near mineralization to background values of ca. 8.3‰. Kriging of oxygen and hydrogen isotopic compositions around the Canadian Malartic deposit delineates the isotopic footprint of the deposit. In greywackes, the − 59‰ δ2H isopleth encircles the mineralized domain and is elongated towards the northwest and southeast. In mafic dykes, the − 73‰ δ2H isopleth encircles the mineralized domain as well as a large area to the south and southeast, whereas the 8.3‰ δ18O isopleth encircles the center of the mineralized system and extends over an area towards the southeast. Host rock oxygen and hydrogen isotope compositions therefore define a cryptic alteration footprint up to 2.5 km outside of the Canadian Malartic deposit. The isotopic composition of the metamorphic fluid in equilibrium with the average least altered greywacke at upper greenschist to amphibolite conditions of ca. 550 °C is approximately δ18O = 8.6‰ and δ2H = − 19‰. This is close to the composition of the mineralizing fluid at the Canadian Malartic deposit reported by previous studies. Fluid/rock oxygen and hydrogen isotope exchange modeling indicate that the isotopic footprint formed at relatively low fluid/rock ratios up to 0.5 at temperatures near 350–400 °C.