Sulfur Isotope Composition of Ore Deposits

A huge amount of literature exists about the sulfur isotope composition in hydrothermal ore deposits. Some of this information has been discussed in earlier editions and, therefore, is not repeated here. Out of the numerous papers on the subject, the reader is referred to comprehensive reviews by Rye and Ohmoto (1974), Ohmoto and Rye (1979), Ohmoto (1986), Taylor (1987) and Ohmoto and Goldhaber (1997). The basic principles to be followed in the interpretation of -values in sul-fidic ores were elucidated by Sakai (1968), and subsequently, were extended by Ohmoto (1972). 

The isotopic composition of a hydrothermal sulfide is determined by a number of factors such as (1) isotopic composition of the hydrothermal fluid from which the mineral is deposited, (2) temperature of deposition, (3) chemical composition of the dissolved element species including pH and f02 at the time of mineralization, and (4) relative amount of the mineral deposited from the fluid. The first parameter is characteristic of the source of sulfur, the three others relate to the conditions of deposition. 

consider the effect of pH-increase due to the reaction of an acidic fluid with a carbonate-bearing host rocks. At pH = 5, practically all of the dissolved sulfur is undissociated H2S, whereas at pH = 9 the dissolved sulfide is almost entirely dissociated. Since H2S concentrates S relative to dissolved sulfide ion, an increase in pH leads directly to an increase in the 5 S of precipitated sulfides. 

In summary, interpretation of the distribution of 8 S-values relies on information about the source of sulfur and on a knowledge of the mineral parageneses that constrain the ambient temperature, EH and pH. If the oxidation state of the fluid is below the sulfate/H2S boundary, then the S/ S ratios of sulfides will be insensitive to redox shifts. 

In the following section different classes of ore deposits are discussed. 

---