In the latter case, the geothermal waters should plot on the magmatic rectangle. Figure shows the oxygen isotopic ratios of some ore deposits. Note they plot all over the diagram. This indicates there must be a significant meteoric water component. Sulfur - important because sulfur is the dominant anion in many ore deposits and sulfur isotopes can often tell us the source of the sulfur.
Deposit is of organic origin and due to bacterial reduction.
Wild fluctuations of values between various layers coincides with extinctions. Sulfur of primary magmatic origin. Because the standard is sulfur from the Canyon Diablo meteorite. Sulfur of inorganic origin and probably the result of inorganic reduction of sulfate. Values closer to zero may be due strictly to temperature fractionation.