Newest
Topics:
For the latest news, see the NEWEST TOPICS page.
Google is too dumb to let me put the list of news in this column and falsely claims that all my pages are self-duplicates.
Google-NONSENSE
Google's so-called "Artificial Intelligence" is an abuse of the concept of intelligence!
Fluids are the source of
hydrothermal ore deposits.
Information about these fluids is an
invaluable aid in mineral
exploration.
You do not need to use complex
microscope / laboratory methods to analyse fluid inclusions
For many years now, geologists have studied the fluids which
transport and deposit ore minerals as a way of understanding just
how economic mineral concentrations were formed. The fluids of
interest are trapped as fluid inclusions in the ore and gangue
minerals and there are many methods to examine these fluid
inclusions, despite their small size (typically 10 microns).
But most mineral exploration
programs fail to take advantage of this knowledge in the search
for additional resources. This is a major oversight.
In mineral exploration, fluid inclusions are considered to be merely
an academic exercise, perhaps because one must visualize the large
fluid systems, long since terminated, that once deposited the
ore body. But geologists routinely envisage tectonic, sedimentary
and metamorphic events which are no longer active and should also
envisage the palaeo-fluid systems which led directly to the economic
ore bodies that are now exploration targets.
Typical studies of fluid inclusions are myopic and too often fail to
see beyond the minute details of individual fluid events and do not
scale to be usable as exploration methods. University tuition
concentrates so much on forensic analysis of a few inclusions in a
few samples from a hydrothermal system that it fails to show how to
use fluid information in mineral exploration where we need to
examine a large spatial array of samples to determine a favourable
mineralised target location. Academic fluid inclusions studies
typically only use microthermometric methods, which are
inappropriately slow and tedious for use in routine mineral
exploration.
There is a simple resolution to
these conflicting requirements. In mineral exploration, we can use
the decrepitation method to overcome the limitations of the
academic methods and to quickly and economically obtain
representative, objective and exploration relevant data on a large
suite of samples. Many examples and case studies are documented here.
Easy fluid inclusion introduction for mineral
exploration
Fluid inclusions are "bubbles" of fluid trapped within the host
mineral during its deposition from its parent hydrothermal fluid.
They are tiny remnants of the exact fluid from which the host
mineral and its associated ore minerals deposited and they provide
direct information about the fluid composition, temperature and
pressure at which the hydrothermal deposit formed. They are usually
very small, from 1 to 20 microns across so you need a microscope at
high magnification to study them. They are surprisingly
abundant and milky quartz typically contains some 1 billion fluid inclusions per
gram! Most fluid inclusion studies are carried out on
quartz, simply because it is transparent, but inclusions occur
in all hydrothermal minerals.
In this image of quartz at low magnification (*40), most of the
numerous spots are fluid inclusions.
During the active deposition of the hydrothermal minerals, the
parent fluid is usually a single phase, most likely a supercritical
aqueous fluid with salt and perhaps CO2 at high
temperature and pressure. But when the fluid cools it will probably
condense to liquid water with a vapour bubble, CO2 may
form a separate phase if it is abundant and salt may crystallise as
halite if it exceeds the saturation level of 23% NaCl. Observations
of the complex phase relations are used to deduce the original
deposition temperature, pressure and salinity in genetic studies of
ore deposits.
The most common fluid inclusions contain simply water, now condensed
to liquid with a bubble of water vapour. From these, the original
deposition temperature can be deduced, but that is not particularly
useful in mineral exploration unless you have a large array of
samples with which to map out palaeo-thermal anomalies. These images
are at a much higher microscope magnification of about *400.
(image from University of Lille)
Many fluid inclusions contain substantial amounts of CO2.
This is very useful in mineral exploration as mineral deposits, such
as gold, are frequently associated with such CO2 rich
hydrothermal fluids. If the internal pressure of the inclusion is
high enough (above about 50 bars), CO2 can be present as
a liquid phase at room temperatures (below 31 C), as seen in this
image.
(image from University of Lille)
And if the hydrothermal fluid is highly saline, above 23% NaCl, then
halite can crystallise upon cooling as seen here, with other
additional solid crystals also present. Such fluids usually occur in
porphyry copper deposits or the core of igneous intrusion derived
fluid systems. Although such saline inclusions are less common, they
can be very useful in mineral exploration.
Information provided by fluid inclusion
measurements
Temperature of the fluid system.
This is the most common measurement
available from fluid inclusion studies as it requires the least
equipment to measure. It is also very tedious work so it is
frequently assigned as a student task. The aim is to determine the
temperature of deposition of the mineral system, but this depends
upon the pressure, so some external geological understanding of
the depth of emplacement is usually required. Without estimates of
the depth of emplacement, the temperatures are the minimum at
which the deposition could have occurred. A serious complication
is the great variability of hydrothermal systems both spatially
and temporally. They do not form as a single homogeneous,
instantaneous event and the fluid inclusions record numerous
temperatures from the many events during emplacement and
subsequent collapse of the hydrothermal system. Measurements of
many fluid inclusions, from many different zones within the host
mineral should be made and these are usually reported as
histograms of temperature versus number of observations.
Pressure of the fluid system.
Although it is necessary to know the
pressure during deposition to accurately estimate the temperature,
it is rarely possible to determine the pressure based on fluid
inclusion measurements alone and external geological inferences
are usually necessary.
Salinity of the aqueous fluid.
Salinity can be measured by
observing the depression of the freezing point of the aqueous
fluids in the inclusion. This is used to express the salinity as
NaCl equivalent as the presence of other ions, such as Ca++,
cannot be determined and will greatly influence the salinity
estimates. Salinity measurements are more complex as you need to
cool the sample during observation, usually with a liquid nitrogen
system. It is also possible, but very difficult, to measure the
concentrations of the separate ionic species of complex saline
fluids and it is rarely carried out and not really useful in
mineral exploration. The presence of halite daughter crystals in
an inclusions provides a quick and easy way to recognize highly
saline inclusions with NaCl > 23%.
Gas composition of the fluid.
Gases are a common and very
important constituent of fluid inclusions. CO2 is the
most common gas, but CH4 is also found in many fluids.
Nitrogen and inert gases may also be present. Microthermometry is
useful in measuring the presence and quantity of CO2
using freezing temperature measurements on the CO2-clathrate.
Methane can be detected by its very low liquefication temperature
below -56 C. But most measurements of the individual gas species
contents of the fluids require the use of laser Raman or
other expensive and complex instrumentation.
Methods used to analyse fluid inclusions
Microthermometry
Most fluid inclusion studies are
done using microthermometry.
This requires the preparation of a doubly polished thin section of
the sample and its examination on a microscope fitted with a
heating and cooling stage, together with a supply of liquid
nitrogen for the freezing temperature measurements. The
measurements are slow and only one or a few inclusions can be
measured at a time and numerous heating and cooling runs are
necessary to acquire enough information to be representative of
the overall hydrothermal system. In fact many studies probably do
not measure enough inclusions on enough samples and are at great
risk of measuring only a few of the stages in the hydrothermal
system, with possibly misleading conclusions. Measurements on
fluids of complex composition rely on complicated thermodynamic
models to determine the concentrations of the various components
of the fluid. Although precise results can be obtained, it is
usually very slow work and prone to various biases from the
operator's choice of just which few of the millions of fluid
inclusions to measure. There is a strong tendency to measure large
inclusions as it is very difficult to work on small inclusions,
but the large inclusions may not be representative of the entire
hydrothermal system or of the mineralising episode.
Laser Raman microprobe
To determine gas contents properly,
it is necessary to use a laser
raman microprobe or other expensive and specialised
equipment. Such analyses are informative, but generally
impractical for routine mineral exploration use. The petroleum
exploration industry does use this equipment and it is very useful
in petroleum exploration.
Manual manipulation under the
microscope
Inclusions immersed in oil may also
be individually broken open
with a sharp instrument while being observed under a microscope
and the size of the bubble expansion into the oil used to estimate
the gas content of the inclusion. But this method is slow,
demanding and very tedious and is rarely used. Observation of a
halite daughter mineral in inclusions indicates that the salinity
is > 26% NaCl.
Laser ablation with mass
spectrometry
The inclusions can also be opened,
usually using laser ablation,
and the contents analysed by mass spectrometer, but this technique
is also slow and uses expensive instrumentation.
Baro-acoustic decrepitation
The decrepitation method was used in the early days of
fluid inclusion research. The method heats a sample of crushed
mineral grains and "listens" to "explosions" as the pressure
builds up within the inclusions and bursts the inclusions open.
The result is a histogram of decrepitation counts versus
temperature. The data is not as accurate as that from
microthermometry, and there were many serious misunderstandings
about the method which led to its disuse in western countries in
the 1950's. However, it was widely used in Russia in the 1970's
and 1980's as a mineral exploration method. With the benefit of
our much improved understanding of the thermodynamics of complex
gaseous fluids it is now clear that the decrepitation method does
provide useful data, and it does so quickly without needing
complicated sample preparation and it also measures a large number
of inclusions in the sample, giving a representative and objective
measurement of the sample. These characteristics of the
method make it ideal for mineral exploration usage. Decrepitation
instruments are currently in use in China
and a modern computer controlled digital instrument is used by Burlinson Geochemical Services, described
here.
Practical application of fluid inclusion
data in mineral exploration
Although much information on temperatures, salinities and gas
contents of fluid inclusions can be obtained by careful microscopy
and laser techniques, acquiring this data is very slow, tedious and
expensive. And much of this information is too general to be of use
in mineral exploration. There is little point in knowing that the
quartz in a gold deposit formed at a particular temperature or
salinity, as 90 % of all gold deposits and much barren quartz all
forms at the same temperature and salinity. We need to measure a
spatial array of samples and a parameter that changes as we near the
ore zone so we can use it to provide a vector to potentially
mineralised zones. For exploration, we need to trim down the study
of fluid inclusions to focus on aspects that will help define new
targets without year long delays while detailed academic studies are
done. This requires a rapid analytical method and the analysis of
hundreds of samples, just as in geochemistry or geophysics where we
measure some property, chemical or electrical, and look for spatial
changes in that property as a means of locating anomalous areas
which may be mineralised. Individual data points are of very limited
value in exploration - we need a spatial array of data points, and
the myopic academic focus on explaining the genesis of ore-forming
fluids fails to address the need for spatial data.
CO2
A particularly useful fluid parameter to know is the CO2
content, because there are numerous case studies in the literature
which clearly document a strong correlation between CO2
content and gold mineralisation at many deposits. An explanation of
this relationship has been presented by Phillips
& Evans, 2004. A comprehensive study of the CO2
and gold relationship was carried out at the Hollinger-McIntyre mine area,
Ontario, Canada by Smith and Kesler, who contoured the
CO2 in the district and noted that the gold mines most
likely occur within the 4 mole% CO2 contour.
This is the contour map from their work.
The baro-acoustic decrepitation
method is the easiest way to obtain CO2
information. It can be used on large numbers of
samples to quickly provide a spatial array of data which is
necessary for exploration applications. If fluid inclusions contain
a significant gas content, CO2 or CH4, then
the decrepitation histogram has a characteristic low temperature
peak below 300 to 350 C. This is because these gases do not condense
to a liquid phase, as water does, and will rapidly generate high
pressures when the sample is heated, leading to premature
decrepitation. A thorough thermodynamic explanation of this is here. This effect is well known from
microthermometric studies which often note the premature
decrepitation of inclusions before homogenization. But the
exploration significance of this effect has been completely ignored
by academic studies which only regard this behaviour as a nuisance.
There are numerous case studies on this website showing the
application of and utility of this detection of CO2 rich
fluids as an exploration guide. Several examples are
Confirmation that the low temperature decrepitation peaks are caused
by decrepitation of CO2-rich fluid inclusions has been provided by
performing mass spectrometry of the gases released during thermal
decrepitation of samples from Cowra, NSW
and also Pine Creek, NT, Australia.
Temperature
Decrepitation can also be used to estimate the fluid temperatures
from which samples formed, although the results are more difficult
to interpret and are best used only to define relative temperature
variations across a spatial suite of samples. The temperatures are
however highly reproducible and representative of the entire sample
and of numerous fluid inclusions, overcoming some serious
limitations of the microthermometric determination of fluid
temperatures.
An example of the use of temperature variations in the decrepigrams
is at the Malanjkhand copper mine,
India, where lower decrepitation temperatures were observed on
the main Cu ore zones, and higher temperatures in less mineralized
or molybdenite bearing zones.
Salinity
The decrepitation method does not provide salinity information. It
is necessary to use complex cooling stage microthermometric studies
to measure salinities. Salinities are not often useful in mineral
exploration because they do not vary much except for a few deposit
styles, such as intrusion related systems, particularly including
including porphyry Cu deposits. But Terry Mernagh has used salinity
variations to discriminate between orogenic gold deposits, intrusion
related gold deposits and potentially tin-tungsten mineralized areas
in the Tanami
region,
NT, Australia. Simple microscope
observations can be used to identify highly saline fluids in
exploration for intrusion related systems including porphyry Cu
systems.
Conclusions
Conventional academic methods of analysing fluid inclusions are too
slow and tedious to be of practical application in typical mineral
exploration activities.
However, the academic data from numerous studies does show that CO2
is an exceptionally important indicator when exploring for most
types of gold deposit.
Because the baro-acoustic
decrepitation method is a rapid and reliable method to measure CO2
contents in fluids, it can be used to study a spatial array of
data and it is an invaluable and practical exploration method.
Measurements of temperatures of fluid
inclusions does not usually help in mineral exploration as
hydrothermal minerals deposit over a wide temperature range and
there is no specific temperature which is indicative of
mineralisation. However, if temperatures are available on a large
spatial array of samples, then temperature trends may be a useful
exploration method to find the hottest part of the system, which is
presumably the location of the best economic mineralisation. Baro-acoustic decrepitation is the most
practical method to determine temperatures of the large numbers of
samples required.
Baro-acoustic decrepitation works best with dense fluids. Epithermal
fluids, formed at near surface conditions are usually of low density
and give weak decrepitation responses. They are difficult, but not
impossible, to work with and some successful studies have been done,
such as at the Favona mine, NZ.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
