Chloride from fluid inclusions in hydrothermal minerals is found to have variable and distinctive δ37Cl values spanning the range -1.1‰ to +0.8‰. In Mississippi Valley-type deposits of Tennessee, brines of high (>0‰) and low (near -1‰) δ37Cl are present. High-δ37Cl brines may be saline formation waters, but low-δ37Cl brines remain unexplained. In porphyry copper deposits, both high-δ37Cl (0.8‰, 0.3‰) and low-δ37Cl(-1.1‰, -0.7‰) hypersaline brines of probable magmatic origin occur. High-salinity magmatic brines with low δ37Cl values contrast isotopically with high-δ37Cl, less concentrated brines responsible for quartz-sericite-pyrite assemblages.