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Fluid inclusion trapping from heterogeneous
primary fluids
Contrasting types of heterogeneous hydrothermal
fluids
Aqueous boiling versus gas-aqueous fluid
immiscibility (gas is most often CO2)
Kingsley Burlinson, November 2014
The most common types of heterogeneous fluids which form
hydrothermal deposits are comprised of a gas and a liquid phase,
although fluids which carry a solid phase also occur. Oil bearing
fluids with 2 immiscible liquid phases are common in oilfields,
but are a special field of study and are not discussed here.
Most studies of heterogeneous fluids are concerned with boiling
of aqueous fluids, as these systems commonly form epithermal
deposits and often control the deposition of economically
important minerals, including gold. But this single-component
heterogeneous fluid system is just a special case where both
phases have almost the same chemical composition, H2O.
(Silica and some other dissolved components also occur in the
aqueous liquid phase and there may be traces of gas such as CO2
in the gaseous phase.)
Although less commonly recognized, multi-component heterogeneous
fluids comprising water with an immiscible gas phase such as CH4,
N2 or CO2 are important in hydrothermal
systems. There are widely held misconceptions about the appearance
of their fluid inclusion assemblages in thin sections as it is
assumed that they have the same characteristics as
single-component boiling heterogeneous fluid systems, which is not
necessarily true. They are also difficult to identify in
microthermometric studies, which has led to very few studies of
them reported in the fluid inclusion literature. These immiscible
gas type heterogeneous fluids are quite possibly very common, yet
they are poorly understood and little documented.
R. J. Bodnar, T. J. Reynolds and C. A. Kuehn provide a
comprehensive review of fluid inclusion techniques for studying
epithermal boiling fluid systems in chapter 5 of "Reviews in
Economic Geology, Volume 2 (1985): Geology and Geochemistry of
epithermal systems. Published by the society of economic
geologists" They explain that boiling systems should
trap inclusions with variable liquid - vapour ratios, representing
the end-member liquid and vapour phases as well as inclusions with
random mixtures of these phases. Plots of the liquid-vapour phase
ratios, or the observed homogenization temperatures should show a
bimodal distribution. But that discussion applies specifically to
single-component heterogeneous systems (boiling aqueous
systems)
This example of bimodal distribution of homogenization temperatures
and phase ratios in a boiling system is from:
Fluid immiscibility in natural processes: use and misuse of
fluid inclusion data.By: Claire Ramboz, Michel
Pichavant and Alain Weisbrod, Chemical Geology V37 (1982)
pp29-48.
Legend: An example of heterogeneous trapping in a
quartz-cassiterite vein from Saint-Cierge, Massif Central,
France. Distribution of the inclusions in hyaline quartz and
related microthermometric results. 1 = homogenization to the vapour (coloured red) 2 = homogenization to the liquid
(coloured green) 3 = critical homogenization 4 = inclusions remaining unhomogenized below 550 C L = expansion of the liquid phase upon heating (liquid
homogenization seen or expected) G = expansion of the vapour phase upon heating (gas
homogenization seen or expected) D = inclusions decrepitated below 550 C TmCO2
= melting temperature of CO2 TmI = melting temperature of
ice mechanism of formation of these 2 types of inclusions TmC = melting temperature of
clathrate Th = bulk homogenization
temperature DF = degree of filling of the inclusion at 20 C
Data points above the horizontal axis are for homogenization
into the vapour, below the horizontal axis are for
homogenization into the liquid
Note that to see a bimodal distribution in homogenization
temperature one must measure the homogenization of both liquid and
vapour dominant inclusions, which is usually difficult to do. Some
studies fail to measure the vapour rich inclusions and so fail to
see a bimodal distribution. But a bimodal distribution can also be
observed by measuring the degree of fill of the inclusions (DF).
Note also that this data is from a single co-genetic assemblage of
fluid inclusions.
In contrast, this data from boiling fluids at the Luis Lopez
epithermal manganese deposits, New Mexico fails to show bi-modal
histograms on boiling fluids, probably because only liquid phase
homogenization temperatures were collected.
Homogenization temperatures for the individual
deposits studied, and in the centre of the figure, the combined
data for the Luis Lopez district. Since the hydrothermal fluids
were boiling, no pressure correction is required and the
homogenization temperatures should accurately indicate the
temperatures of the mineralizing fluids.
From: Mineralization of the Luis Lopez epithermal Manganese
deposits in light of fluid inclusion and geologic studies. By:
David I Norman, Khosrow Bazrafshan and Ted L Eggleston, New Mexico
Geological Society guidebook, 34th field conference, Socorro region
II, 1983.
Immiscible gas heterogeneous systems differ from boiling
systems.
In boiling fluid systems, silica deposition and fluid inclusion
trapping and sealing are very rapid, so inclusions represent the
complete range of phase mixtures as they are sealed before phases
can migrate and agglomerate. And both interference type inclusions and
crystal defect type inclusions are abundant in this rapidly
deposited quartz.
As the epithermal fluid boils, changing much of the liquid to
vapour, the volume of the liquid is drastically reduced and all the
solutes in the parent fluid, including silica, are concentrated into
this reduced liquid volume. The silica concentration exceeds
saturation, but cannot become supersaturated because of the great
turbulence in the boiling fluid and so silica is deposited rapidly
at a rate to match the rate of boiling. Under these conditions of
rapid deposition, the quartz is full of crystal defects, which
become fluid inclusions when they are sealed over by continuing
quartz growth. These "defect type" inclusions trap the parent fluid
liquid phase, as well as mixtures of the liquid and gas phases. Gas
filled inclusions are also trapped when a gas bubble attaches to a
crystal growth surface and forces the quartz to deposit around it.
These inclusions may well show a spherical
shape due to the original shape of the bubble in the liquid
and they trap only the gas phase with none or very little of the
liquid phase that was of course also present. Overall these boiling
systems are dominated by defect type inclusions which contain both
liquid and gas phases from the parent fluid. Consequently the
boiling fluid inclusion assemblage data could give bimodal data
distributions. But there is no certainty that all boiling systems
will show bimodal data distribution plots and the lack of such
bimodal plots cannot prove the absence of boiling.
However, immiscible gas fluid systems (multi-component), which did
not boil, do not deposit silica so quickly and this can give a very
different appearance to the fluid inclusion assemblages. As CO2 is
exsolved from the parent fluid, the solubility of silica in the
aqueous component fluid actually increases, which dramatically
slows or even prevents the deposition of silica and trapping of
fluid inclusions. This very slow silica deposition can lead to
preferential trapping of one of the 2 phases present because the
liquid rich inclusions occur as "defect type" inclusions and the gas
rich inclusions occur as "interference type" inclusions and the mechanism of formation of these 2
types of inclusions are almost completely independent of one
another. Deposition of silica may become diffusion limited and very
slow, which will give very few crystal defects and therefore no way
to trap the dominant liquid phase as inclusions. But gas bubbles
will be efficiently trapped by interference with the deposition of
quartz, which is forced to deposit around the bubbles which then
become pure gas filled inclusions. Consequently it is unlikely that
plots of such immiscible gas fluid inclusion assemblage data would
be bimodal, despite this being a true heterogeneous fluid system. It
is quite possible that only fluid inclusions of the minor CO2 gas
phase are trapped with no inclusions of the dominant aqueous phase.
Or the reverse may occur, giving a fluid inclusion assemblage with
only aqueous inclusions and no CO2 rich inclusions if the
fluid flow flushed the buoyant CO2 bubbles away before
they could be trapped within the slowly depositing silica.
Selective trapping is very likely to occur in CO2 rich
immiscibility type heterogeneous fluid systems.
Summary
Although boiling epithermal aqueous fluids and immiscible gas
bearing (CO2) aqueous fluids are both heterogeneous fluid
systems, the inclusion assemblages trapped in these systems differ
markedly because the inclusion assemblages trapped are greatly
influenced by the rate of silica deposition as the inclusion
cavities are sealed up. The rapid silica deposition in turbulent
boiling systems may give inclusion assemblages that contain a full
range of mixtures of the phases present in the original system. But
the slow silica deposition typical of mesothermal quartz vein
systems which contain enough CO2
and low enough temperatures for immiscibility to occur, will
preferentially trap individual phases rather than both phases or a
range of mixtures. One important case of this selective trapping
gives assemblages of pure CO2 inclusions without any
associated aqueous inclusions, even though water was the dominant
phase in the parent fluid. This can easily lead to misguided
interpretations of the nature of the source fluids, as seen in the incorrect assertion of a non-aqueous fluid as
the source for the quartz vein and gold in Ghana.
Interpretative methods based upon single component (H2O)
heterogeneous fluid systems (boiling) cannot be used to understand
the very different multi-component, immiscible gas (CO2
and H2O) heterogeneous systems
(non-boiling). Doing so completely ignores the complex effects of
variable silica deposition rates, selective trapping and
immiscibility in the fluid.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
