68
C. Variations of Stable Isotope Ratios in Nature
We shall discuss two types of ore deposits in more detail, the liquidmagmatic and the stratiform deposits.
The liquid-magmatic ore deposits, representing the most simple case
in which the origin of the sulfur is in the lower crust or upper mantle,
show within single deposits and between the various deposits a rather
uniform distribution around zero (THODE et ai., 1962; SMITHERINGALE
and JENSEN, 1963; SHIMA et ai., 1963). For example, THODE et ai. (1962)
found that sulfides in Stillwater sills have J34S-values around zero. For
the Insizwa sill, South Africa (THODE et ai., 1962) and for the Muskox
intrusion, Canada (SASAKI, 1969) a rather large variation range has been
reported.
Another important category of sulfide deposits of world-wide distribution is that of the "stratiform" deposits. Depending on the types of
stratified rocks, stratiform deposits can be divided into "normal" and
"volcanic" types. The normal-type deposits occur in more or less nonvolcanic marine sediments, and the volcanic-type deposits are characteristically associated with various types of volcanic rocks. While the sedimentary rocks associated with the normal type appear to have formed in
relatively shallow basins on continental basements, the volcanic rocks
containing the volcanic type seem to be deposited in mobile geosynclinal
depressions.
A majority of the stratiform deposits studied by SANGSTER (1968)
have a relatively narrow spread in o34S-values, and all ore bodies are
depleted in 34S relative to contemporaneous seawater sulfate.
SANGSTER (1968) suggested that all of the sulfur in "sedimentary"
type strata bound sulfide deposits and probably most of the sulfur in
volcanic strata bound deposits was derived from sulfate in ancient seas
and was reduced to sulfide by bacterial action.
SASAKI (1970) argued that the parallel variation trend observed between sulfate and sulfide may be an indication of isotopic exchange
reaction between seawater sulfate and ore sulfides. Since the amount of
sulfur from ocean water would normally be much larger than that from
volcanic sources, it is expected that the isotopic compositions of stratiform ore deposits are largely controlled by the 0 34 S of seawater.
2. Hydrothermal Carbonates
Carbonates, mainly calcite, are some of the most common gangue
minerals. The stability field of carbonates is large, even at very low CO2
fugacities and high temperatures.
If we assume that an isotopic equilibrium between hydrothermal
carbonates and fluids was established, then the carbon and oxygen isotopic composition ofthe carbonates associated with ore minerals may be
C. Variations of Stable Isotope Ratios in Nature
We shall discuss two types of ore deposits in more detail, the liquidmagmatic and the stratiform deposits.
The liquid-magmatic ore deposits, representing the most simple case
in which the origin of the sulfur is in the lower crust or upper mantle,
show within single deposits and between the various deposits a rather
uniform distribution around zero (THODE et ai., 1962; SMITHERINGALE
and JENSEN, 1963; SHIMA et ai., 1963). For example, THODE et ai. (1962)
found that sulfides in Stillwater sills have J34S-values around zero. For
the Insizwa sill, South Africa (THODE et ai., 1962) and for the Muskox
intrusion, Canada (SASAKI, 1969) a rather large variation range has been
reported.
Another important category of sulfide deposits of world-wide distribution is that of the "stratiform" deposits. Depending on the types of
stratified rocks, stratiform deposits can be divided into "normal" and
"volcanic" types. The normal-type deposits occur in more or less nonvolcanic marine sediments, and the volcanic-type deposits are characteristically associated with various types of volcanic rocks. While the sedimentary rocks associated with the normal type appear to have formed in
relatively shallow basins on continental basements, the volcanic rocks
containing the volcanic type seem to be deposited in mobile geosynclinal
depressions.
A majority of the stratiform deposits studied by SANGSTER (1968)
have a relatively narrow spread in o34S-values, and all ore bodies are
depleted in 34S relative to contemporaneous seawater sulfate.
SANGSTER (1968) suggested that all of the sulfur in "sedimentary"
type strata bound sulfide deposits and probably most of the sulfur in
volcanic strata bound deposits was derived from sulfate in ancient seas
and was reduced to sulfide by bacterial action.
SASAKI (1970) argued that the parallel variation trend observed between sulfate and sulfide may be an indication of isotopic exchange
reaction between seawater sulfate and ore sulfides. Since the amount of
sulfur from ocean water would normally be much larger than that from
volcanic sources, it is expected that the isotopic compositions of stratiform ore deposits are largely controlled by the 0 34 S of seawater.
2. Hydrothermal Carbonates
Carbonates, mainly calcite, are some of the most common gangue
minerals. The stability field of carbonates is large, even at very low CO2
fugacities and high temperatures.
If we assume that an isotopic equilibrium between hydrothermal
carbonates and fluids was established, then the carbon and oxygen isotopic composition ofthe carbonates associated with ore minerals may be