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 But geologists routinely envisage tectonic, sedimentary and metamorphic events which are no longer active and should also envisage the palaeo-fluid systems which led directly to the economic ore bodies that are now exploration targets.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.
Topics on this page
- Basic explanation of fluid inclusions.
- What information can be deduced from fluid inclusion measurements.
- Methods of fluid inclusion study.
- Practical application of fluid inclusion data in mineral exploration.
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.
(Image from USGS)
This introduction to fluid inclusions avoids many issues and complexities to focus only on aspects relevant to mineral exploration, but there is a more thorough introduction here, by Prof. P. Brown, Madison, Wisconsin. And another brief overview of both fluid and melt inclusions by the USGS is here.
I have deliberately avoided discussion of petroleum systems which are reviewed here. Fluid inclusion data is routinely used in hydrocarbon exploration and helps determine maturity.
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.
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.
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.
CO2A 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
- The Cowra goldfield in NSW, Australia
- The Woods Point gold deposit, Victoria, Australia
- The Brusson gold mine, northern Italy
- Kalgoorlie area archaen deposits, Western Australia
TemperatureDecrepitation 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.
SalinityThe 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.
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.
Salinities of fluid inclusions are of limited use in exploration and are difficult to measure. However, they can be used to recognise intrusion related hydrothermal systems.
Many case studies using fluid inclusion data in mineral exploration are documented here on this website.
LimitationsBaro-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.