Using sulphur isotopes in igneous petrogenesis

In this section three examples are given which illustrate different ways in which sulphur isotope variations may be explained in igneous rocks. 

4 Using sulphur isotopes to understand the genesis of hydrothermai ore deposits 

Sulphur isotope studies of hydrothermal ore depodts offer the opportunity to (1) determine the origin of the sulphur present in the orebody as sulphides and sulphates (2) determine the temperature of formation of the sulphides and of the ore-forming fluids (3) determine the water/rock ratio effective during the mineralization (4) determine the degree of equilibrium attained and (5) thus constrain the mechanism of ore deposition. 

Modem hydro- High-temperature hydrothermal vents currently active at mid-ocean ridges offer a thermal mineral- unique opportunity to study a hydrothermal mineral deposit in the process of ization at formation. The current working model assumes that cold seawater sulphate is mid-ocean ridges drawn down into sea-floor basalts, where it is heated in the vicinity of a magma chamber. Some sulphate is precipitated as anhydrite whilst the remainder is reduced to sulphide by reaction with the basalt. The fluid is vented back onto the seafloor at about 350 C laden with sulphides. On mixing with seawater these are precipitated onto the sea floor as a fine sulphide sediment whilst at the vent site itself the sulphides are built into a chimney a metre or so in height. 

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