Giant quartz vein systems in accretionary orogenic belts: the evidence for a metamorphic fluid origin from δ15N and δ13C studies

Nitrogen isotope compositions of hydrothermal micas in giant structurally hosted quartz vein systems place constraints on the origin of the hydrothermal fluids. The vein systems are from Neoarchean terranes in the Superior Province of Canada, and the Norseman terrane, Western Australia. The four quartz vein systems studied in different terranes formed at metamorphic grades ranging from lower greenschist to lower amphibolite facies, coevally with metamorphism of the host terrane. Nitrogen resides predominantly as NH4+ in hydrothermal mica. For lower to upper greenschist facies quartz veins, N contents and δ15N values of micas are between 40 and 200 ppm and 15‰ and 21‰, respectively, whereas in quartz veins formed at the greenschist to amphibolite transition and lower amphibolite facies, micas have N contents of 20–70 ppm and δ15N of 11–24‰. In contrast, micas and K-feldspars from granitoids in the Neoarchean Abitibi and Red Lake greenstone belts are characterized by systematically lower δ15N of −5‰ to 5‰, and generally lower N contents of 20–50 ppm, comparable to other granitoids. Carbon and oxygen isotope compositions of hydrothermal ferroan dolomite and calcite show systematic depletions with increasing metamorphic grade. The mean values range from −2.2‰ to −3.7‰ for δ13C and 13.8‰ to 12.3‰ for δ18O in veins formed at lower to upper greenschist facies, but from −5.8‰ to −7.1‰ for δ13C and 11.4‰ to 9.8‰ for δ18O at higher metamorphic grades. Nitrogen isotope compositions rule out mantle (δ15Nmean=−5‰), magmatic (δ15N=−5‰ to <10‰), or meteoric fluids (δ15Nmean=4.4±2.0‰) for the quartz veins. Accordingly, the results are consistent with fluids derived from metamorphic dehydration of oceanic crust and sediments within accretionary orogenic belts.