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Ore Deposits and Hydrothermal Mineralization

The relative contribution of fluids derived from the crust vs. the mantle is an important question in the study of ore deposits. Helium isotope studies of porphyry copper deposits indicate that mantle 3He/4He ratios can be preserved in their fluid inclusions. Most moderate to high temperature (300° to 700°C) hydrothermal ore deposits derive their metals and volatiles from hydrous silicic magmas. However, mesothermal gold–quartz veins in metamorphic terranes may be an exception because the origin of fluids and metals remains cryptic; it may involve substantial contributions via metamorphic devolatilization reactions. Isotope studies of elements such as H, O, N and S commonly resemble the wall-rocks enclosing the veins, suggesting local equilibrium at low water to rock ratios. The He isotopic compositions of fluid inclusions contained in quartz and sulfide from metamorphic gold–quartz veins can provide useful insight into these processes and address questions about the ultimate source of the fluids.

The evolution of submarine hydrothermal systems may be traced using noble gas isotopes. Sulfide minerals from extinct hydrothermal vent chimneys appear to have trapped pristine samples of the original hydrothermal fluid. It has also been shown that elevated 3He/heat ratios in submarine hydrothermal fluids are an indicator of recent perturbations by magmatic input. By combining the known formation temperatures of sulfide phases of different age with measurements of helium and other noble gases, it should be possible to determine the temporal variation of 3He/heat ratios, thereby tracking the magmatic cycling of spreading ridge systems through time.