Wall Rock Alteration
69
used to obtain more detailed information about 1) the temperature of
these carbonates as compared, for instance, with fluid inclusion studies,
2) the isotopic composition of the hydrothermal fluids involved as an
indicator of their origin, and 3) the origin of the CO2 or HC0 3 that
determined the carbon composition of these carbonates.
OHMOTO (1972) has demonstrated that the carbon isotope composition of hydrothermal carbonates should be also affected by the physicochemical conditions of hydrothermal fluids. For instance an increase in
fo by 1 log unit or in pH by 2 units can cause a decrease of about 30%0
in ' £5 13 C-values of carbon-bearing minerals.
Hydrothermal carbonates have been analyzed by ENGEL et al. (1958),
FRIEDMAN and HALL (1963), GARLICK and EpSTEIN (1966), RYE (1966),
HALL and FRIEDMAN (1969), FRITZ (1969), and PINCKNEY and RYE
(1972). The observations made by these authors may be interpreted as
the result of exchange of carbonate rocks with hydrothermal fluids. Isotopic exchange of carbonates with water at elevated temperatures leads
to a decrease in 18 0 of the carbonates, the decrease being greater the
higher the temperatures.
There are some indications that the £5180-value of carbonates of host
rocks of ore deposits seem to decrease nearer to the ore deposit. In some
cases, it was suggested that this relationship might be a useful guide to
ore. However, in most cases, the analyzed £5180-values (and to a smaller
extent the £5 13 C-values) do not follow this simple relationship (see also
PINCKNEY and RYE, 1972).
High-temperature hydrothermal carbonates tend to have lower £5 13 C_
values than low-temperature carbonates. A £5 13 C-value of - 7%0 has been
obtained by RYE and O'NEIL (1968) during their investigation of the
Providencia (Mexico) mine for the CO2 in inclusions in sphalerite which
is consistent with a primary magmatic origin.
3. Wall Rock Alteration
The environment of ore deposition can be interpreted in part from
the assemblages of alteration minerals. Wall rock alteration includes
those mineralogical and chemical changes brought about by circulating
solutions within the host rock of ore bodies.
Applying stable isotope techniques, the following problems are especially interesting: 1) the delineation of hypogene and supergene alteration products ("hypogene" refers to ore deposits or associated gangue
and alteration products that formed in the presence of generally ascending hydrothermal solutions; "supergene" processes refer to the formation of minerals and ores near the surface by low-temperature, descending waters); 2) the relative importance of meteoric H 20 versus "mag-
69
used to obtain more detailed information about 1) the temperature of
these carbonates as compared, for instance, with fluid inclusion studies,
2) the isotopic composition of the hydrothermal fluids involved as an
indicator of their origin, and 3) the origin of the CO2 or HC0 3 that
determined the carbon composition of these carbonates.
OHMOTO (1972) has demonstrated that the carbon isotope composition of hydrothermal carbonates should be also affected by the physicochemical conditions of hydrothermal fluids. For instance an increase in
fo by 1 log unit or in pH by 2 units can cause a decrease of about 30%0
in ' £5 13 C-values of carbon-bearing minerals.
Hydrothermal carbonates have been analyzed by ENGEL et al. (1958),
FRIEDMAN and HALL (1963), GARLICK and EpSTEIN (1966), RYE (1966),
HALL and FRIEDMAN (1969), FRITZ (1969), and PINCKNEY and RYE
(1972). The observations made by these authors may be interpreted as
the result of exchange of carbonate rocks with hydrothermal fluids. Isotopic exchange of carbonates with water at elevated temperatures leads
to a decrease in 18 0 of the carbonates, the decrease being greater the
higher the temperatures.
There are some indications that the £5180-value of carbonates of host
rocks of ore deposits seem to decrease nearer to the ore deposit. In some
cases, it was suggested that this relationship might be a useful guide to
ore. However, in most cases, the analyzed £5180-values (and to a smaller
extent the £5 13 C-values) do not follow this simple relationship (see also
PINCKNEY and RYE, 1972).
High-temperature hydrothermal carbonates tend to have lower £5 13 C_
values than low-temperature carbonates. A £5 13 C-value of - 7%0 has been
obtained by RYE and O'NEIL (1968) during their investigation of the
Providencia (Mexico) mine for the CO2 in inclusions in sphalerite which
is consistent with a primary magmatic origin.
3. Wall Rock Alteration
The environment of ore deposition can be interpreted in part from
the assemblages of alteration minerals. Wall rock alteration includes
those mineralogical and chemical changes brought about by circulating
solutions within the host rock of ore bodies.
Applying stable isotope techniques, the following problems are especially interesting: 1) the delineation of hypogene and supergene alteration products ("hypogene" refers to ore deposits or associated gangue
and alteration products that formed in the presence of generally ascending hydrothermal solutions; "supergene" processes refer to the formation of minerals and ores near the surface by low-temperature, descending waters); 2) the relative importance of meteoric H 20 versus "mag-