Gold contents of sulfide minerals in granitoids from southwestern New Brunswick,Canada

The abundance of gold and selected trace elements in magmatic sulfide and rock-forming minerals from Silurian–Devonian granitoids in southwestern New Brunswick were quantitatively analyzed by laser-ablation inductively coupled plasma mass-spectrometry. Gold is mainly hosted in sulfide minerals (i.e., chalcopyrite, pyrrhotite, and pyrite), in some cases perhaps as submicron inclusions (nanonuggets). Gold is below detection (<0.02 ppm) in major rock-forming minerals (i.e., plagioclase, K-feldspar, biotite, hornblende, and muscovite) and oxides (i.e., magnetite, and ilmenite). Gold distribution coefficients between sulfide and granitoid melt are calculated empirically as: . This result suggests that gold behavior in the granitoid systems is controlled by the conditions of sulfur saturation during magmatic evolution; the threshold of physiochemical conditions for sulfur saturation in the melts is a key factor affecting gold activity. Gold behaves incompatibly prior to the formation of sulfide liquids or minerals, but it becomes compatible at their appearance. Gold would be enriched in sulfur-undersaturated granitoid magmas during fractionation, partitioning into evolved magmatic fluids and favoring the formation of intrusion-related gold deposits. However, gold becomes depleted in residual melts if these melts become sulfur-saturated during differentiation, leading to gold precipitation in the early sulfide phases of a granitoid suite. Late-stage Cl-bearing magmatic–hydrothermal fluids with low pH and relatively high oxidation state derived from either progressively cooling magmas at depth or convective circulation of meteoric water buffered by reduced carbon-bearing sediments, may scavenge gold from early sulfide minerals. If a significant amount of gold produced in this manner is concentrated in a suitable geological environment (e.g., shear zones or fracture systems), intrusion-related gold deposits may also be generated. Exploration for intrusion-related gold systems should focus on the areas around evolved phases of granitoid suites that remained sulfur-undersaturated. For sulfur-saturated granitoid suites, the less differentiated phase and associated structures are the most prospective targets.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.