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Hidrotermal Processes and Mineral System - Franco Pirajno

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57.00
86Kr 30.524�0.025 17.398
124Xe 2.337�0.008 0.0951
126Xe 2.180�0.011 0.0887
128Xe 47.15�0.07 1.919
129Xe 649.6�0.9 26.44
130Xe ”100 4.070
131Xe 521.3�0.8 21.22
132Xe 660.7�0.5 26.89
134Xe 256.3�0.4 10.430
136Xe 217.6�0.3 8.857
58 1 Water and Hydrothermal Fluids on Earth
general mantle volatiles have 7–9 RA. Mantle derived Ar, dominated by
40Ar,
has 40Ar/36Ar greater than 40 000; crustal values of 3He/4He are 
 0.1 RA and
40Ar/36Ar ratios are	 45 000 (Burnard et al. 1999). Noble gas isotope chemistry
can be represented as plots of 3He/4He and 40Ar/36Ar ratios, as shown in the
example of Fig. 1.13. Thus, by comparing the He-Ar isotopic composition of a
sample with the He-Ar composition of the possible sources of He and Ar,
outlined above, the origin of these gases in the hydrothermal fluid can be
determined. He from mantle fluids is obtained from the following:
HeM ¼ ½ðR�RCÞ=ðRM �RCÞ�
Where RC, RM are the
3He/4He ratios in the crust and mantle respectively, and
R is the measured 3He/4He ratio.
Noble gases isotope chemistry can be examined in ancient hydrothermal
fluids that are released from fluid inclusions providing, as already mentioned,
that these have not been subsequently modified. These post-entrapment mod-
ifications relate to noble gases released from fluid inclusion trapped in mineral
grains and may include (Burnard et al. 1999): (1) He and Ar produced by
radiogenic decay from U and/or K that may be present in the fluid inclusion;
(2) diffusion of He into the inclusion, from radiogenic decay of U, Th or K from
A
B
1.6
0.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0 2000 4000 6000 8000 10000 12000
40Ar/36Ar
3 H
e/
4 H
e,
 
R
a
Fig. 1.13 Example of 3He/4He vs 40Ar/36Ar diagram, showing mixing of a magmatic fluid
with a fluid dominated by atmospheric 40Ar/36Ar. This example is from the Ailaoshan Au
deposits, Yunnan Province, China, reported by Burnard et al. (1999). 3He-rich is magmatic,
36Ar-rich is atmospheric or modified ASW (air saturated water). Line A is for R= 1400; line
B is for r = 140, where R = (4He/36Ar)magmatic/(4He/36Ar)modified ASW
1.4 Hydrothermal Fluids 59
outside the inclusion; (3) release of lattice trapped gases, again from the decay of
U, Th or K, during extraction of the fluids; (4) post-crystallisation production
of 3He by the action of cosmic rays. These post-entrapment effects can be
effectively minimised by crushing the mineral grains rather than use step heat-
ing or fusion techniques for the release of the gases and by collecting samples
from well below the surface, since the action of cosmic rays is confined to the
first 1–15 m of the Earth’s surface.
From the numerous examples provided in the recent literature, the following
case studies of He and Ar isotope systematics are described here. Stuart et al.
(1995) in their work on hydrothermal fluids from the Dae Hwa W-Mo vein
deposit in South Korea, pointed out that the conservative behaviour of the He,
Ar andNe gases in the crust andmantle are invaluable in constraining the origin
of the fluids and their mixing. Stuart et al. (1995) used correlations between He
and Ar isotopes in fluid inclusions, in conjunction with d18O data, to identify
mantle and crustal end member components. The noble gases were extracted
from the crushing of primary fluid inclusions in scheelite (CaWO4) crystals.
These inclusions are of two types: Type I characterised by aqueous liquid with a
vapour bubble and gas-rich type II. Homogenisation temperatures, together
with data from dD and d18O systematic, show that the mineralising fluids had a
temperature range from about 4008C (early) to 2308C (late). The scheelite
crystals examined exhibit up to nine colour zones, which reveal a systematic
decrease of homogenisation temperature and salinity from the core to the edge
of the crystals. Similarly, the d18O of the aqueous fluid (denoted as d18OH2O)
decreases from +3.2% to –2.6% from core to edge. The measured 3He/4He
ratios (R) of the released gases range from 1.5 to 0.1 RA, with
3He/36Ar ratios
from 0.003 to 0.012 being higher than air (2.4�10–7) thereby eliminating the
possibility of atmospheric contamination. Bearing in mind that the R can be
influenced by cosmogenic nuclear activities (RC = �0.1) in the upper 1–1.5 m
of the Earth’s surface, the authors ruled out this possibility because the samples
examined were collected from a mine depth of about 100 m. The d18OH2O trend
reflects dilution of a magmatic-dominated fluid (d18OH2O>+8%, T=
400–5008C) to meteoric-dominated fluids (d18OH2O> –10%) with lower tem-
peratures in the hydrothermal system. The d18OH2O trend is matched by a
progressive decrease in the 3He/4He, 3He/36Ar and 40Ar/36Ar ratios, which
also record a dilution from fluids with more primordial and radiogenic He
and Ar to meteoric fluids enriched in radiogenic He and atmospheric Ar. Stuart
et al. (1995) calculated an undiluted magmatic fluid 3He/4He ratio of between
0.9 and 2 RA and
3He/36Ar > 0.01 and 40Ar/36Ar > 1000 for the Dae Hwa
W-Mo hydrothermal system. The authors concluded that the above values are
below typical mantle values and therefore they are likely amixture of radiogenic
and mantle values.
Zhang et al. (2002) studied the noble gas isotopes of the Denggezhuang,
Jiaojia, Pengjiakuang, and Fayunkuang orogenic gold deposits. Their results
show that the 3He/4He ratios of inclusion fluids in pyrite are 0.40–2.36 Ra.
More specifically, the 3He/4He ratios of Denggezhuang (quartz vein-type) and
60 1 Water and Hydrothermal Fluids on Earth
Jiaojia (fracture-type) are 1.64–2.36 RA, and 0.43–0.79 RA for the breccia type
in the Pengjiakuang-Fayunkuang deposits. The helium isotope systematics
suggests that mantle-derived fluids were involved in the metallogenic process.
Accordingly, in a model of two-member mixed system of mantle and crust,
20–30% of ore-forming fluids are from the mantle in the quartz vein-type
and fracture-type gold deposits. Similarly, a mantle fluid component for the
Mesozoic Au deposit (Dongping) on the northern margin of the North China
Craton, was detected on the basis of 3He/4He ratios (2.1–5.2 RA) in inclusion
fluids from ore zone pyrite (Mao et al. 2003). These results suggest that the ore
forming fluids had a strong mantle component, thereby reinforcing the idea of
mantle upwelling during the Mesozoic along the eastern regions of China to
account for the widespread magmatism and hydrothermal ore deposits of that
age (Mao et al. 2007). Helium isotopic analyses of inclusion fluids of ore veins
from the Tongkeng-Changpo Sn deposit in the Dachang ore field in southern
China showed 3He/4He ratios ranging from 1.2 to 2.9 RA, which combined with
a positive relationship with 40Ar/36Ar data are suggest that the ore fluids
represent a mixture of both mantle and crustal components (Cai et al. 2007).
However, the usefulness of 3He/4He ratios as tracers of mantle sources was
thrown into doubt by Stroncik et al. (2007). These workers studied He and Ne
isotopes from volcanic glasses of lavas of off-axis seamounts of the the Mid-
Atlantic Ridge and suggested that He isotopes are susceptible to decoupling
from elements such as Ne. The combined He, Ne and Ar isotopic data from
these lavas showed that there was a preferential loss of He compared to Ne and
Ar, and that only 22Ne preserved evidence of a primitive mantle component.
1.4.11.3 Sulphur Isotopic System
There are four stable isotopes of S (with natural abundances in parenthesis): 32S
(95.02%), 33S (0.75%), 34S (4.2%) and 36S (0.017%). One radiogenic isotope,
35S, has a short half life of 88 days. The isotopic ratio that is most frequently
used in the study of ore systems is 34S/32S, which is measured in d34S permil (%),
according to:
�34S ¼ 34 S=32SðsampleÞ � 34 S=32SðstandardÞ=34S=32SðstandardÞ � 1000
where the international standard is an iron meteorite,