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Hidrotermal Processes and Mineral System - Franco Pirajno

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However, these definitions are somewhat simplistic, and it is often difficult to
distinguish between, say, secondary and pseudosecondary, or primary and
pseudosecondary inclusions.
Some of the abbreviations used in fluid inclusions studies are temperature of
trapping Tt, temperature of homogenisation Th, temperature of decrepitation
Td and eutectic temperature Te. Clathrate is a mixture of H2O with gas (H2S,
CO2 and CH4). Tables of empirical criteria for the identification of the genetic
types of fluid inclusions are given in Roedder (1979, 1984). A fluid inclusion
contains one or more phases:
� L = liquid (e.g. H2O, hydrocarbons)
� V = vapour (e.g. H2O, CO2, CH4)
� S = solid (e.g. NaCl, KCl, carbonates, sulphides)
A fluid inclusion is heated on heating-freezing stage mounted on a petrological
microscope until the vapour and liquid phases homogenise. Homogenisation of
the liquid and gas phases will be seen to occur at a given temperature on heating
of the inclusions, when observed under the microscope. This homogenisation
temperature is a lower limit having been obtained at atmospheric pressure, and
therefore a pressure correction for the original depth at which the fluids were
discharged is necessary. The Th is plotted on a P-T diagram to determine the
range of pressure at which the inclusion may have been trapped. Salinity is
measured in terms of NaCl wt% equivalent and is based on freezing point
depressions, determination of Te and Raman spectroscopy. The salinity of the
1.4 Hydrothermal Fluids 39
inclusion is determined by first freezing the inclusion and then raising the
temperature of the stage, and observing the first and final melt temperatures
(Tm-ice). The first melt temperature indicates the type of salt (NaCl orMgCl, for
example), while the temperature of the last melt indicates the degree of salinity,
usually measured in equivalent NaCl. Decrepitation temperatures are obtained
by crushing and bursting of inclusions by heating.
Fluid inclusions usually occur in combination, such as L + V, L + V + S,
V1 + V2, L1 + L2 etc. Co-existing L-rich and V-rich inclusions are indicative
of either boiling or mixing. The classification scheme of Shepherd et al. (1985) is
given below, and diagramatically shown in Fig. 1.8B.
Type I: Liquid (L)
Type IV: Vapour only (V)
Type II: Liquid >50% (L)
Type III: vapour >50% (V)
Type V: Liquid ±
+ vapour <50% (V)
+ liquid <50% (L)
+ solids (S) < 50%
Liquids L + L1 2
Type VI: Immiscible
two phase
two phase
Fig. 1.8 Schematic
representation of types and
classification of fluid
inclusions (A) Primary (P),
secondary (S) and
pseudosecondary (PS) fluid
inclusions in a quartz crystal
(B) Classification of fluid
inclusions as observed at
room temperature. Both
(A and B) after Sheppard et
al. (1985)
40 1 Water and Hydrothermal Fluids on Earth
1. Monophase inclusions: entirely filled with liquid (L).
2. Two-phase inclusions: filled with a liquid phase and a small vapour bubble
3. Two-phase inclusions: in which the vapour phase is dominant and occupies
more than 50% of the volume (V+L).
4. Monophase vapour inclusion (V): entirely filled with a low density vapour
phase (generally mixtures of H2O, CH4 and CO2).
5. Multiphase inclusions containing solids (S+L+/–V): contain solid crystal-
line phases known as daughter minerals. These are commonly halite (NaCl)
and sylvite (KCl), but many other minerals may occur including sulphides.
6. Immiscible liquid inclusions: contain two liquids, usually one H2O-rich and
the other CO2-rich (L1+L2=/–V).
In general, the coexistence of types 2 (L+V) and 3 (V+L) may indicate that the
fluid was boiling at the time of entrapment. In the case of boiling of a one-
component system, the gas bubble is the vapour phase of the host liquid; or, in
the case of a heterogeneous system, the gas phase exsolves by effervescence.
However, it must be cautioned that the presence of a gas bubble may also
indicate immiscibility. This is the case with CO2, which if present in the fluids,
will separate on cooling (Roedder 1979). In systems that contain volatiles such
as CO2 it is more appropriate to use the term effervescence rather than boiling.
Boiling or effervescence result in the partitioning of a vapour phase from a
residual liquid that is more saline. Phase separation and mixing of fluids are
important processes in several hydrothermal ore deposits. For example, sulphur
and metals are generally transported by separate fluids and precipitation of
sulphides occurs when these two fluid mix.
The presence of daughter minerals, on the other hand indicates that solids
nucleated from an oversaturated liquid solution. It is found that in these
hypersaline fluids, Na+, Cl–, Mg2+ and Ca2+ are the most common dissolved
ions. The concentration of the salts in the solutions ranges from less than 1 wt%
to greater than 50 wt%. The diagram in Fig. 1.9 reflects the range of salinities
(wt% equivalent) and homogenisation temperatures of a range of hydrother-
mal mineral deposits (Large et al. 1988).
The composition of inclusion fluids is becoming increasingly more accurate
using Raman spectroscopy, proton-induced X-ray emission (PIXE), synchro-
ton X-ray fluorescence (SXRF), secondary ion mass spectrometry (SIMS) and
cathodoluminiscence (CL) techniques. A discussion of these techniques is
beyond the scope of this book, but for a useful review the reader should consult
Boiron and Dubessy (1995).
The halogen content and their ratios in inclusion fluids can be used to
characterise different hydrothermal fluids (Wilkinson 2001). Ratios and plots
commonly used are Cl/Br versus Na/Br and Cl versus Br, which can provide
clues as to the origin of fluids (e.g. seawater, continental brines). Other useful
elements in the study of inclusion fluids are the noble gases, He, Ar, Kr, Xe. He
isotopes are especially useful to trace fluids that may either derive form the
1.4 Hydrothermal Fluids 41
mantle or have a link with mantle degassing. The 3He/4He ratio can discrimi-
nate whether or not fluids originate from the mantle (3He/4He > 1.4� 10–6),
from the crust (3He/4He< 1.4� 10–6) or are of atmospheric origin (3He/4He =
1.4� 10–6). However, as pointed out by Wilkinson (2001), the presence of a
mantle contribution as revealed by the 3He/4He in a hydrothermal system does
not necessarily mean that the fluids are directly derived form the mantle, but
may simply be an indication of mantle heat contribution, or degassing. This is
an important aspect and I discuss this further in Section 1.4.11 in which some
case studies are examined.
1.4.7 Dissolved Constituents and Metals Partitioning
in Hydrothermal Solutions
From the study of fluid inclusions, hot springs and fluids encountered during
drilling operations in geothermal areas and oilfields, it is apparent that the
amount of dissolved solids in hydrothermal solutions varies from approxi-
mately < 1% to > 50% by weight.
Some typical compositions are given in Tables 1.8 and 1.9. From the values
in these Tables the following two observations can be made: (1) Na, K, Cl and
0.5 2 5 10 20 50 100
Wt % NaCl equivalent
1 2
Orogenic lode Au
Massive sulphides
Porphyry Cu-Au4
Tennant Creek
Fig. 1.9 Temperature-salinity fields and mean gradient curve for a range of hydrothermal ore
systems: (1) Archaean orogenic Au; (2) Epithermal Au-Ag; (3) volcanogenic massive sul-
phides; (4) Tennant Creek Au-Cu, Australia; (5) Porphyry Cu-Au. After Large et al. (1988)