124 3 Variations of Stable Isotope Ratios in Nature
have convincingly demonstrated that ore formation has taken place in the Earth’s
near-surface environment by recycling processes of fluids, metals, sulfur, and car-
bon. Reviews of the application of stable isotopes to the genesis of ore deposits have
been given by Ohmoto (1986), Taylor (1987) and Taylor (1997).
In as much as water is the dominant constituent of ore-forming fluids, knowledge
of its origin is fundamental to any theory of ore genesis. There are two ways for
determining δD- and δ
18
O-values of ore fluids:
1. By direct measurement of fluid inclusions contained within hydrothermal
minerals
2. By analysis of hydroxyl-bearing minerals and calculation of the isotopic compo-
sition of fluids from known temperature-dependent mineral-water fractionations,
assuming that minerals were precipitated from solutions under conditions of iso-
tope equilibrium.
A. There are two different methods through which fluids and gases may be ex-
tracted from rocks: (1) thermal decrepitation by heating in vacuum and (2) crush-
ing and grinding in vacuum. Serious analytical difficulties may be associated with
both techniques. The major disadvantage of the thermal decrepitation technique is
that, although the amount of gas liberated is higher than by crushing, compounds
present in the inclusions may exchange isotopically with each other and with the
host mineral at the high temperatures necessary for decrepitation. Crushing in vac-
uum largely avoids isotope exchange processes. However, during crushing large new
surfaces are created which easily adsorb some of the liberated gases and that, in turn,
might be associated with fractionation effects. Both techniques preclude separating
the different generations of inclusions in a sample and, therefore, the results ob-
tained represent an average isotopic composition of all generations of inclusions.
Numerous studies have used the δD-value of the extracted water to deduce the
origin of the hydrothermal fluid. However, without knowledge of the internal dis-
tribution of hydrogen in quartz, such a deduction can be misleading (Simon 2001).
Hydrogen in quartz mainly occurs in two reservoirs: (1) in trapped fluid inclusions
and (2) in small clusters of structurally bound molecular water. Because of hydro-
gen isotope fractionation between the hydrothermal fluid and the structurally bound
water, the total hydrogen extracted from quartz does not necessarily reflect the orig-
inal hydrogen isotope composition. This finding may explain why δD-values from
fluid inclusions often tend to be lower than δD-values from associated minerals
(Simon 2001).
B. Oxygen-bearing minerals crystallize during all stages of mineralization,
whereas the occurrence of hydrogen-bearing minerals is restricted in most ore de-
posits. Examples of hydroxyl-bearing minerals include biotite and amphibole at
high temperatures (in porphyry copper deposits), chlorite and sericite at tempera-
tures around 300
◦
C , and kaolinite at around 200
◦
C.
The mineral alunite, and its iron equivalent jarosite, is a special case. Alunite
(KAl
2
(SO
2
)
2
(OH)
2
) contains four sites where elements containing stable isotopes
are found and both the sulfate and hydroxyl anionic groups may provide information
on fluid source and condition of formation.