Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
Viewpoints:

How CO2 inclusions form from aqueous fluids

Understanding heterogeneous fluids : why gold is not transported in CO2 fluids

Gold-quartz deposits form from aqueous heterogeneous fluids: NOT from CO2 fluids

Inclusion shapes can prove heterogeneous FI trapping

Disproportional FI trapping from heterogeneous fluids explains gas-dominant systems

A discussion of H2 analysis by mass spectrometry

A mechanism to form H2 in the MS ioniser during analyses


News:

Sangan skarn Fe deposits, Iran

New model 205 decreptiometer

Studies of 6 Pegmatite deposits

A study of the Gejiu tin mine, China


Exploration using palaeo-hydrothermal fluids

Using opaque minerals to understand ore fluids


Understanding baro-acoustic decrepitation.

An introduction to fluid inclusions and mineral exploration applications.



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AGCC expo, Adelaide, Aust. Oct. 14-18 2018

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ECROFI, June 24-26, Budapest, Hungary

AOGS, Singapore, 28 Jul-2 Aug 2019

SGA, Glasgow Scotland, Aug. 27-30 2019


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Acoustic Decrepitation as a means of rapidly determining CO2 (and other gas) contents in fluid inclusions and its use in exploration, with examples from gold mines in the Shandong and Hebei provinces, China.

Kingsley Burlinson

Acta Geologica Sinica


Abstract

The acoustic decrepitation method heats a small monomineralic sample and counts pressure impulses as the inclusions burst when they develop high internal pressures. For aqueous fluids, the decrepitation temperature is correlated with the homogenisation temperature, but gas rich fluids give a distinct and characteristic low temperature decrepitation peak which can be used to recognize gas rich fluid inclusions. This information is useful in exploration for Au deposits, which are frequently associated with CO2 rich and sometimes CH4 rich fluids.

This distinctive decrepitation occurs because the CO2 rich inclusion fluids expand according to the gas law and develop internal pressures high enough to burst the host mineral grain at temperatures well below their homogenisation temperatures. In contrast, aqueous fluids condense to a liquid and vapour phase during post-entrapment cooling. Upon subsequent heating their internal pressures do not increase significantly until after homogenisation to a single phase occurs and hence they do not decrepitate "prematurely" as gas rich inclusions do.

This behaviour is usually regarded as an annoyance in conventional microthermometric homogenisation studies, but can readily be used as an exploration aid to find mineralisation deposited from such gas rich fluids. Decrepitation results on samples from Cowra Ck, NSW, Australia, which have also been microthermometrically measured for CO2 content, show that amounts of less than 5 mole % CO2 are easily distinguished by decrepitation and amounts as low as 1 mole % CO2 may be determinable.

Examples of the use of acoustic decrepitation in the study of 6 gold mines in the Shandong and Hebei provinces of China are discussed.

Key Words: carbon dioxide, decrepitation, exploration, fluid inclusions, gold, hydrothermal fluids, Hebei, Shandong


1 Introduction


The fluid inclusion decrepitation method was first used in the 1940's as a method to measure homogenisation temperatures. The method was soon replaced by microthermometry which was recognized as being more accurate for determining these temperatures. However, it was not understood that much of this early decrepitation work was subject to interference caused by the presence of CO2 rich fluid inclusions. Consequently much of the criticism of the technique, which led to its demise for academic research purposes, is now known to be inaccurate. The technique was subsequently used in Russia and China in the 1970's in exploration programmes, in which economical and rapid analyses are more important than modest inaccuracies. However, even this work failed to realise the effects caused by the presence of CO2 rich inclusions. It was not until 1983 (Burlinson, 1984) that the effect of CO2 rich fluid inclusions upon decrepitation data was recognized. Such CO2 rich fluid inclusions develop high internal pressures at low temperatures resulting in decrepitation long before the inclusion reaches homogenisation temperature. This behaviour is frequently observed in microthermometric studies where it is a major problem when trying to measure the homogenisation temperature. However, this decrepitation gives rise to distinctive low temperature peaks on the acoustic decrepigrams which can be used to recognise the presence of CO2 rich fluid inclusions. This led to the recognition that CO2 measurements by acoustic decrepitation could be used as an exploration technique for Au deposits, which are often associated with CO2 rich hydrothermal fluids.

Confirmation of the relationship between low temperature decrepitation and CO2 rich fluids was provided in 1995 by Mavrogenes et al. (Mavrogenes, J.A. et al., 1995), who used quadrupole mass spectrometric analyses of gases released from fluid inclusions from samples from the Cowra Creek goldfield in NSW, Australia. In this work, 5 samples with prominent low temperature decrepitation peaks were shown to give rise to CO2 rich gases when the fluid inclusions were opened either by sequential heating or mechanical crushing of the same samples.

Following from this work, acoustic decrepitation is now a useful exploration technique for detecting CO2 rich fluids, or in their absence, for measuring Td (temperature of decrepitation). Although it is not a high accuracy technique it is ideal for exploration because it does not require the preparation of polished thin sections and is a rapid and cheap method which has been automated using modern computers and electronics (Burlinson, 1988). Because the method uses a relatively large sample (0.5 grams) and counts many inclusions (often more than 100,000) in each sample, it provides representative and statistically meaningful data which can be difficult to match with microthermometric analyses and this is of fundamental importance in its application as an exploration tool.


2 Decrepitation of gas-rich inclusions

2.1 Explanation of decrepitation

To understand the decrepitation behaviour of aqueous CO2 rich inclusions, it helps to realise that the unusual fluid is not actually CO2, but water. In the 50 to 600 C temperature range and 1 bar to 3 Kbar pressure range of interest for Th (temperature of homogenisation) measurements, CO2 does not change phase and behaves according to the Gas law (PV=nRT) with only minor variation due to non-ideallity. CO2 rich inclusions often have high internal pressures even at room temperature, their internal pressures increase linearly with temperature when they are heated and they will rapidly develop enough internal pressure to cause decrepitation. However, water exists as both a liquid and a vapour phase under these conditions and changes between the liquid and vapour phase depending on the pressure and temperature. Because of the existence of both liquid and vapour phases in most aqueous inclusions at room temperature, inclusion pressures are low until homogenisation to a single phase occurs at Th. Only above Th and only if the inclusion homogenises to a liquid phase, will aqueous inclusions be able to develop enough internal pressure to cause decrepitation.

This is easily seen in figure 1. Consider a pair of inclusions formed in quartz at about 380 C and 1 Kbar pressure. (Shown as the formation point in figure 1). One inclusion is comprised of water (of low salinity) with no CO2 and the other inclusion is of CO2 with no water. At these conditions the aqueous inclusion is a liquid and the CO2 inclusion is a gas, highly compressed and of high density. As the aqueous inclusion cools, it follows the water isochore down to the vapour liquid curve at the homogenisation point. It then nucleates a vapour bubble and on further cooling it follows the vapour liquid curve back to room temperature and pressure, the volume of the vapour phase increasing and the liquid phase contracting as the inclusion cools. At room temperature the internal pressure of the aqueous inclusion is near zero bars.

In contrast, as the CO2 inclusion cools it will follow a PT path along the bold line which is determined by the gas law, PV/T = k, where k is a constant. As the volume of the inclusion is constant, this is equivalent to a path of P = k' T (non-ideal behaviour is minor and can be ignored). CO2 does not condense to a liquid until below 31 C, its critical point temperature. CO2 inclusions at room temperature typically have high internal pressures. (Pressures of at least some 40 to 70 bars are required to give a liquid CO2 phase at room temperatures.)

During cooling from their formation conditions, there is a confining pressure on the quartz host mineral similar to the internal pressure of the inclusions, so the inclusions will not burst. But when observed and heated in the laboratory, the confining pressure on the quartz host mineral is only 1 bar. During heating the inclusion conditions follow the same pathways as when they cooled and soon develop internal pressures sufficient to break the quartz now that it has only 1 bar of confining pressure.

A very approximate estimate of the overpressure needed to break the quartz is shown by the shaded pressure band between 500 to 700 bars. Although previous work (Leroy, 1979; Bodnar et al., 1989) has suggested that internal pressures of 850 bars or even up to 2000 bars are needed for inclusions in quartz to decrepitate, these studies were concerned only with well preserved inclusions remote from and unaffected by any discontinuities in the quartz such as fractures, grain boundaries or other fluid inclusions. In actual analytical samples the quartz is structurally heterogeneous and inclusions can decrepitate at far lower internal pressures, estimated to be about 600 bars. It is likely that only those inclusions near quartz grain surfaces or fractures are able to decrepitate, and they do so at lower than theoretical internal pressures because the quartz near these inclusions is mechanically weak. Inclusion abundance counts on a sample with inclusions of maximum size 10 microns suggest that only a very small number (less than 1%) of the inclusions larger than 5 microns gave rise to detectable decrepitation events and that none of the inclusions smaller than 5 microns decrepitate.

In Fig. 1 the aqueous inclusion will generate little internal pressure during heating as it progresses up the vapour-liquid equilibrium curve to the homogenisation point. At this temperature the liquid has expanded to fill the inclusion and there is no longer a vapour bubble. Upon further heating the inclusion progresses up the water isochore (dashed line) passing through the homogenisation point, at 320 C in this case. The liquid rapidly develops a high internal pressure as it tries to expand within the fixed volume of the inclusion and by 350 C has developed enough pressure to burst the quartz host grain. The observed decrepitation temperature of 350 C is acceptably close to the original formation temperature of 380 C, and acoustic decrepitation observations on aqueous inclusions can be used as a reasonable indicator of the formation temperature.

For the CO2 inclusion, the pressure immediately starts to increase as it is heated, progressing along the PT line shown in bold and by 220 C the internal pressure is sufficient to burst the host quartz grain. The observed decrepitation temperature of 220 C is a poor indicator of the formation temperature of 380 C. However, the very low temperature decrepitation is characteristic of the presence of CO2 fluid inclusions and is easily measured using the acoustic decrepitation technique.

PT decrepitation

Fig. 1 P-T diagram for water and CO2 gas, showing decrepitation pressures. CO2 rich inclusions decrepitate at very low temperatures.

Mixtures of CO2 and water within an inclusion will give results intermediate between these 2 end-point cases (Shown here). Precise calculations based on the mole fraction of CO2 present will depend also on the solubility of CO2 in the aqueous phase, and this in turn will depend upon the salinity of the aqueous phase. However it is clear that as CO2 is added to an aqueous system the decrepitation temperature will be lowered, and perhaps substantially so. It is this lowering of the decrepitation temperature with the increase of CO2 in the system that is easily measured by the acoustic decrepitation method making it a useful exploration tool in finding interesting hydrothermal systems with significant amounts of CO2-rich fluids. Such CO2-rich fluid systems are frequently associated with gold mineralisation. (Burlinson, 1991; Mavrogenes et al., 1995; Partamies & Poutiainen, 2001, Lowenstern, 2001).

Pure CO
2 fluids are uncommon and most inclusions contain CO2 and water and perhaps some NaCl . Methane and nitrogen are less common additional constituents. The addition of other non-condensable gases such as methane and nitrogen does not change the decrepitation behaviour, as these gases also follow the gas law equation during heating and cooling. Different chemical species of gases do not have different P-T expansion properties, the minor differences due to non-ideallity have no significant effect upon the decrepitation behaviour of inclusions. The addition of solutes to the aqueous phase tends to suppress the solubility of CO2 and so increase its pressure in the gas phase, causing slightly more lowering of the decrepitation temperatures.

Real fluid inclusions vary greatly in their size and morphology. This has a major effect on the decrepitation behaviour as large or angular inclusions can more easily break the host quartz grain. While aqueous inclusions are often 5 to 20 microns across and somewhat rounded, it is common for CO2-rich inclusions to be quite large (50 or more microns) and extremely irregular with sharp points. The stress concentration at these points on the inclusions makes it much easier to break the host quartz grain. This accentuates the lowering of decrepitation temperature by CO2-rich fluids as they are able to decrepitate at even lower pressures.

If the inclusions are formed at low pressure (epithermal environments), they may not decrepitate at all. And low density water inclusions ("steam") may homogenize to a vapour phase instead of a liquid and these would never develop enough internal pressure to decrepitate. This agrees with decrepitation measurements on high level systems in which it has been found that decrepitation is usually very weak or absent.

To demonstrate the similarity of decrepitation behaviour for different non-condensable gases, the equation of state calculations for CO2, N2 and CH4 are shown in figure 2. This was calculated using the Redlich-Kwong equation of state (Redlich and Kwong, 1949) as modified by de Santis (de Santis et al., 1974) using a computer program by Holloway (Holloway, 1981). From this data it can be seen that inclusions of any of these gases formed at the same PT conditions (and thus having the same molar volume) will behave similarly in respect of their decrepitation when heated. The small differences due to non-ideallity of the gases do not result in a significant alteration to the decrepitation behaviour of the inclusions. Consequently the decrepitation method is not compromised by the presence of N2 or CH4 mixed with CO2 and will respond equally to any mixture of these gases.

PT comparison different gases

Fig. 2 P-T diagram for CO2, CH4 and N2 using the modified Redlich & Kwong equation of state. Neither the different gases nor variations from ideal gas behaviour affect the decrepitation behaviour.


2.2 Interpretation of decrepigrams

Some typical decrepigrams of quartz samples are shown in figure 3.

typical results

Fig. 3 Typical decrepitation results from quartz showing features relevant to the interpretation of decrepigrams.


Quartz B in this example begins to decrepitate at 350 C and shows only a single decrepitation peak with a maximum at 420 C. Quartz A begins to decrepitate at 390 C and has 2 peaks, with maxima at 490 C and 580 C. The beginning of decrepitation can be used in these examples as an estimate of the homogenisation temperature of the quartz and these two quartz samples formed at different temperatures. The second peak in quartz A, at 580 C, is related to the alpha-beta phase transition in quartz. The crystallographic transition itself does not cause the observed counts, or we would also observe this peak in Quartz B, which is not the case. At this transition temperature the Young's modulus of quartz decreases markedly, which facilitates the decrepitation of inclusions already close to bursting. Because of the acoustic configuration of the instrument, it is impossible for the very small crystal volume change of the quartz as it changes from alpha to beta phase to produce a detectable pressure pulse. At temperatures above 600 C there is no decrepitation in quartz as all inclusions capable of decrepitating did so at the transition temperature when the quartz was physically weak. Neither quartz A nor quartz B show any significant decrepitation at low temperatures from 100 to 300 C, although this is the range in which secondary inclusions would be opening, leaking or decrepitating. It is thought that secondary inclusions do not give a response because they either leak rather than burst suddenly, or that by opening at low temperatures they do not result in the “steam explosion” of superheated water which is necessary to produce a detectable pressure pulse in the instrument. (Hladky, 1983) The analysis of “re-used quartz” was done on a sample which had already been heated above 600 C, cooled and then re-analysed. On the second analysis, no decrepitation counts were observed, which confirms that the counts being measured are catastrophic, irreversible events like the bursting of fluid inclusions, rather than any crystallographic or thermal expansion events.

In contrast to the quartz A and quartz B samples, the quartz containing CO2 rich fluid inclusions (as verified by observation in thin section) shows significant decrepitation at low temperatures, below 300 C. This low temperature peak is characteristic of CO2 (and other gas rich inclusions) and is a primary means of using the decrepitation method as an exploration technique.


3 Gas-rich inclusions in exploration for Au.

It is commonly observed that CO2 rich fluid inclusions occur in gold deposits of many different types. (Groves et al., 1988; Goldfarb et al., 1988; Mavrogenes et al., 1995; Partamies et al., 2001; Garba & Arkande, 1992) Because there is no known complexing between gold and CO2, it has been suggested that the relationship is due to the CO2 buffering the fluid pH in a range which enhances stability of the gold – bi sulphide complex, thus facilitating Au transport. (Phillips & Evans, 2004; Naden & Shepherd, 1989) An extensive microthermometric study of fluid inclusion CO2 contents of ore and non-ore quartz was carried out at the Hollinger-McIntyre Au deposit in Timmins, Ontario, Canada (Smith & Kesler, 1985) and it was shown that samples with greater than 4 mole % CO2 were associated with auriferous quartz, while fluid inclusions from sub-ore quartz contained less than 4 mole % CO2.

A comprehensive study of 5 auriferous samples from the Cowra Creek goldfield, NSW, Australia measured the gas contents of the fluid inclusions by quadrupole mass spectrometry (QMS) and compared the results with acoustic decrepitation data on the same samples. (Mavrogenes et al., 1995) The gases were released from 4 of the samples by mechanical crushing, and from the fifth sample by sequential thermal decrepitation. The gases released were comprised of CO2, CH4 and N2. The total mole % of gases released is shown in Fig. 4 along with the decrepitation results for each of these samples. This shows an approximate correlation between the total decrepitation counts up to 300 C and the mole % gas results. The main exception is sample Q4, which had less decrepitation than expected for a sample with 40 mole % gas. This may be because this sample had an unusually high CH4 content, which may have reacted with CO2 to produce C and H20, thereby reducing the internal pressure of these inclusions and reducing the decrepitation. The decrepitation method easily detected the gas in the sample with just 5 mole % total gas, and may be sensitive enough to detect gas contents as low as 1 mole % based on this data.


cowra

Fig. 4 Decrepitation results of 5 auriferous samples from the Cowra Ck. gold district, NSW, Australia. Mole % total gas measured by quadrupole mass spectrometry are compared with the amount of low temperature decrepitation.



Although there are likely to be cases where the CO2 content of inclusions is at low pressure and would not therefore be detectable by decrepitation, many Au deposit styles are amenable to the use of decrepitation as a means of quickly and easily providing a representative and reproducible value for CO2 content which can then be used as an exploration guide. Although not completely quantitative, this data can be applied in the same manner as geochemical soil analyses in order to define prospective areas of interest. The Au deposits from the Archaean greenstone deposits in Western Australia, the Abitibi province, eastern Canada, the proterozoic slate belt hosted deposits from Victoria, Australia and Nova Scotia, Canada all contain abundant CO2 rich fluid inclusions which are readily identified in the decrepitation results. Typical examples of decrepitation results from these deposits are shown in Fig. 5, showing intense low temperature decrepitation due to the high CO2 content of these hydrothermal systems.

typical co2 rich

Fig. 5 Typical decrepitation of gold mineralised samples from greenstone and slate-hosted deposits in Australia and Canada.


4 Decrepitation data from Shandong & Hebei mines

A total of 66 samples were collected from 6 mines in the Shandong and Hebei provinces in China in August 2005. These samples came from the Canshang, Jiaojia and Sanshandao mines in Shandong, and the Dongping, Hougou and Huangtuliang mines in Hebei. Most of the samples were collected from active ore faces in the mines or from ore stockpiles to ensure they represented the auriferous quartz. Samples of approximately 100 to 200 grams were crushed and sieved to obtain 0.5 gm of grains for analysis. These samples were analysed by acoustic decrepitation using the BGS model 105 decrepitometer and a selection of representative results are shown in figures 6, 7 and 8.

4.1 Shandong province

The sample shown from the Canshang mine in Fig. 6 begins to decrepitate at about 480 C, while the 2 samples from the Jiaojia mine begin to decrepitate at about 440 C. This temperature of the start of decrepitation can be used as an estimation of the homogenisation temperature of the inclusions and the temperature difference indicates that the Canshang deposit formed from slightly hotter fluids than did the Jiaojia deposit. However the most notable feature of these results is the lack of any decrepitation below 300 C in the samples from both mines, from which it is inferred that the fluids lacked significant content of CO2. This is in contrast with the background (unmineralized) sample collected from a thin quartz vein within the Guojialing granodiorite several kilometres away from the Jiaojia mine. Although CO2 rich fluids do occur in the Shandong region, they are not associated with the mineralised quartz, but with the barren regional quartz veins. This lack of CO2 is in contrast to claims that the Au deposits in this province were formed from CO2-rich fluids. (Qiu et al., 2002)

Shandong Au

Fig. 6 Decrepitation results from the Canshang and Jiaojia mines, Shandong province, China showing the absence of low temperature decrepitation and inferring the absence of CO2 rich fluids.



The differing opinions on the CO2 content of the mineralising fluids may be caused either by the failure of the acoustic decrepitation method to detect low pressure CO2 containing fluid inclusions, or by lack of representative sampling used in the microthermometric studies. It is common for petrologists to focus their studies on interesting inclusions or large inclusions, as small inclusions cannot easily be studied. This bias during observational analysis can lead to incorrect assumptions about exactly which fluid phase is the host fluid for the all important Au mineralisation. Almost all quartz is strongly zoned and formed from many different fluid pulses over a substantial geologic time interval and it is very difficult when studying tiny samples to be certain of the exact significance of a microscopic sample to the megascopic mineralising system. All too often the interesting CO2 rich inclusions are studied extensively when they represent only a minor event in a mineralising system which is dominated by boring, but possibly more relevant, aqueous inclusions. Although the present acoustic decrepitation data does not conclusively prove that these fluid systems were CO2 deficient, there is serious concern that CO2 is in fact not a significant component of the fluids in the Shandong gold deposits.

4.2 Hebei province

All of the 20 samples collected from the Dongping mine, (Fig. 7) lack low temperature decrepitation and are inferred to have no CO2 in the fluid inclusions. However detailed microthermometric fluid inclusion studies (Mao et al., 2003; Feng-Jun Nie, 1998) have reported CO2 rich fluid inclusions which have from 4 to 95 mole % CO2 and trapping pressures of 0.6 to 1.2 Kbars. Such inclusions would give an obvious low temperature decrepitation peak because of their high internal pressures. The absence of any low temperature decrepitation indicates that the CO2 rich phases that have been reported are not actually a significant component of the mineralising fluid system.

Samples from the Hougou mine (Fig. 8) show low levels of decrepitation below 300 C and are inferred to contain low levels of CO2 in their fluids. Of the 66 samples collected in Shandong and Hebei, only these samples from the Hougou mine in Hebei showed any low temperature decrepitation caused by CO2 rich fluids. Microscope observation of grains of one of these samples showed that the largest inclusions were less than 15 microns across and most inclusions were equant and less than 10 microns across, with some rare elongated inclusions with maximum dimensions of 15 by 5 microns. Almost all inclusions were 2 phase and most had about 25% vapour, but there was a small quantity of inclusions with a larger vapour bubbles close to 50% by volume. Extremely rare 3 phase inclusions with a liquid CO2 phase were observed even though the specimen temperature was 24 C. These microscope observations confirm the existence of CO2 rich fluid inclusions and also confirm that they are of very low abundance, as expected from the decrepitation data.

Hebei - Dongping

Fig. 7 Decrepitation results from the Dongping mine, Hebei province, China showing the absence of low temperature decrepitation and inferring the absence of CO2 rich fluids.

Hebei - Hougou

Fig. 8 Decrepitation results from the Hougou mine, Hebei province, China showing minor decrepitation at temperatures below 300 C, confirming the low abundance of CO2 rich fluids.


No separate microthermometric study of the Hougou mine seems to have been reported (in the english literature) and it is generally assumed that the Hougou and Dongping mines are of similar genesis. However the decrepitation data indicates that there is a significant difference in the CO2 content of the hydrothermal fluids at these 2 mines and a separate study of the Hougou mine is needed as there are major differences from the nearby Dongping mine.


5 Summary

The acoustic decrepitation technique has mostly been used in the past to determine homogenisation temperatures of fluid inclusions. However, an understanding of the behaviour of CO2, CH4 and N2 in fluid inclusions shows that the presence of these gases gives rise to a discrete and characteristic low temperature decrepitation event. The failure to recognise this effect in previous decrepitation studies has led to incorrect estimates of homogenisation temperatures and discreditation of the method. But by understanding why these gas rich inclusions decrepitate at low temperature it is clear that some of the past criticisms of the method were unwarranted and that we can actually take advantage of this low temperature decrepitation as an exploration method to map out areas of gas-rich fluids in hydrothermal systems without having to resort to more tedious microscopic examination.

The mines in the Shandong and Hebei provinces have no low temperature decrepitation (except Hougou) which is in contrast with orogenic deposits in Australia and Canada (Fig. 5). It is inferred that these deposits either formed from fluids which lacked CO2 or under low pressure conditions (less than 600 bars), as low pressure inclusions may not be detected by decrepitation. Existing work in Hebei does not indicate that the depositional environment was of low pressure (Mao et al., 2003) and consequently these deposits must have formed from fluids which were predominantly devoid of CO2. The CO2 rich fluids found in previous research are probably a minor phase of the overall hydrothermal systems and their relevance to the Au mineralisation events needs to be re-considered.

Because the decrepitation method uses 0.5 grams of crushed sieved grains derived from a specimen of about 50-200 grams of quartz and each analysis is the result of the decrepitation of up to hundreds of thousands of individual primary inclusions, the result is highly representative of the overall hydrothermal system. In addition, it is very easy to analyse many samples to further improve the representation of the system. In contrast, microthermometric examinations must concentrate on a few tens of individual inclusions from a few thin sections and can easily suffer from unintentional bias in sample selection and fail to be representative of the overall hydrothermal system, despite the great accuracy of the few measurements obtained.

Orogenic Au

Fig. 9 Decrepitation results from other orogenic Au deposits at Muruntau, Uzbekistan and the Motherlode, California, USA. These deposits also lack low temperature decrepitation and CO2 rich fluids.


It is of interest that decrepitation analyses data from the Muruntau, Uzbekistan and California motherlode, USA deposits (Fig. 9) also lack a low temperature decrepitation peak, from which it is inferred that these systems were also formed from CO2 deficient fluids. This suggests the need to sub-divide the orogenic gold deposits classification into those formed from CO2 rich and CO2 deficient fluids. There is an important difference between these deposits and the so-called orogenic deposits in greenstones in Western Australia and Canada (which formed from CO2 rich fluids) which should not be ignored.


6 Conclusions

The acoustic decrepitation method is a useful technique to rapidly determine the presence of gas-rich fluids in inclusions. This is very useful as an exploration procedure as many Au deposits are known to be associated with CO2 rich fluids. Although decrepitation has limitations in its accuracy, it has a major advantage in its better representation of a hydrothermal system. The complexity of microthermometric studies of fluid inclusions together with the lack of specificity of the data provided has prevented the application of fluid inclusions as an exploration technique. Acoustic decrepitation overcomes these problems and despite its lesser accuracy, provides a means of applying fluid inclusions as an exploration method to find new deposits rather than limiting their use to research on ore genesis after the deposit has already been found.



References

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About the Author

Kingsley Burlinson has worked in the exploration industry for 35 years and is a consultant in Darwin, Australia.



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