Sulfur Isotope Ratios of Ore Deposits
67
2) Sulfides probably connected with magmatism rarely vary in b 34 S_
composition by more than 5%0, even throughout a whole mining district.
These deposits are probably derived from homogeneous sources, such as
hydrothermal ore solutions, which seem to be almost uncontaminated
by surrounding rocks.
3) Sulfides collected from two ore deposits, which according to geological evidence have the same origin, may show quite different ratios.
On the other hand, certain ore provinces seem to have a typical b 34 S_
composition (for instance almost all Cordilleran hydrothermal ore deposits, e.g., porphyry coppers exhibit b 34 S-values near zero, JENSEN, 1967).
4) Systematic regional and depth variations have been found in detailed studies of particular deposits (African Copperbelt, DEC HOW and
JENSEN, 1965; Quemont Mine, RYZNAR et at., 1967; Mississippi Valley,
BROWN, 1967, and others). However, no convincing interpretations
about the spatial distribution of sulfur isotopes have been given.
5) In a given mineralization there is a tendency for the low-temperature sulfides to have a wider range of variation as compared with the
high-temperature sulfides. During progressive metamorphism there
seems to take place a homogenization of the sulfur isotopes.
6) In many suites of coexisting sulfide minerals galena is the lightest,
sphalerite is intermediate, and pyrite is the heaviest in sulfur isotope
composition. The magnitude of the differences (normally 2 to 5%0) decreases with increasing metamorphic grade of the host rock, indicating
that the minerals are in isotope equilibrium.
7) It has been tried to correlate variations of lead and sulfur isotope
ratios in galenas (e.g. BROWN, 1967; ROBERTSON and CUMMING, 1968;
LANCELOT et aI., 1971; GREIG et aI., 1971). In general, the light sulfur 32S
is associated with lead relatively enriched in the radiogenic isotopes 206,
207, and 208.
In these generalizations it is assumed that the ore minerals accurately
reflect the isotopic compositions of the ore-forming fluids. This assumption was questioned by OHMOTO (1972), who demonstrated that the
isotopic composition of sulfide minerals may be influenced by 1) isotopic
composition of total sulfur in the fluids, 2) temperature, 3) fugacity of
oxygen, and 4) pH of the fluids. At 250
0
C for example, OHMOTO (1972)
could show that within geologically important fo -pH regions an increase info -value by 1 log unit or in pH by 1 unit ~an cause a decrease
in b34S-val~es of sulfides by as much as 20%0. However, as OHMOTO
(1972) pointed out, these considerations are based on the assumptions
that both chemical and isotopic equilibria are established among
aqueous sulfur species and between aqueous sulfur species and precipitating sulfides. Both of these assumptions may be not valid in many
geologic processes.
67
2) Sulfides probably connected with magmatism rarely vary in b 34 S_
composition by more than 5%0, even throughout a whole mining district.
These deposits are probably derived from homogeneous sources, such as
hydrothermal ore solutions, which seem to be almost uncontaminated
by surrounding rocks.
3) Sulfides collected from two ore deposits, which according to geological evidence have the same origin, may show quite different ratios.
On the other hand, certain ore provinces seem to have a typical b 34 S_
composition (for instance almost all Cordilleran hydrothermal ore deposits, e.g., porphyry coppers exhibit b 34 S-values near zero, JENSEN, 1967).
4) Systematic regional and depth variations have been found in detailed studies of particular deposits (African Copperbelt, DEC HOW and
JENSEN, 1965; Quemont Mine, RYZNAR et at., 1967; Mississippi Valley,
BROWN, 1967, and others). However, no convincing interpretations
about the spatial distribution of sulfur isotopes have been given.
5) In a given mineralization there is a tendency for the low-temperature sulfides to have a wider range of variation as compared with the
high-temperature sulfides. During progressive metamorphism there
seems to take place a homogenization of the sulfur isotopes.
6) In many suites of coexisting sulfide minerals galena is the lightest,
sphalerite is intermediate, and pyrite is the heaviest in sulfur isotope
composition. The magnitude of the differences (normally 2 to 5%0) decreases with increasing metamorphic grade of the host rock, indicating
that the minerals are in isotope equilibrium.
7) It has been tried to correlate variations of lead and sulfur isotope
ratios in galenas (e.g. BROWN, 1967; ROBERTSON and CUMMING, 1968;
LANCELOT et aI., 1971; GREIG et aI., 1971). In general, the light sulfur 32S
is associated with lead relatively enriched in the radiogenic isotopes 206,
207, and 208.
In these generalizations it is assumed that the ore minerals accurately
reflect the isotopic compositions of the ore-forming fluids. This assumption was questioned by OHMOTO (1972), who demonstrated that the
isotopic composition of sulfide minerals may be influenced by 1) isotopic
composition of total sulfur in the fluids, 2) temperature, 3) fugacity of
oxygen, and 4) pH of the fluids. At 250
0
C for example, OHMOTO (1972)
could show that within geologically important fo -pH regions an increase info -value by 1 log unit or in pH by 1 unit ~an cause a decrease
in b34S-val~es of sulfides by as much as 20%0. However, as OHMOTO
(1972) pointed out, these considerations are based on the assumptions
that both chemical and isotopic equilibria are established among
aqueous sulfur species and between aqueous sulfur species and precipitating sulfides. Both of these assumptions may be not valid in many
geologic processes.