Exploring for Au using fluid inclusion decrepitation

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Exploration for gold using Fluid Inclusions – the acoustic decrepitation technique.

With examples from Hebei, China and El Penon, Chile.

Keywords: fluid inclusions, decrepitation, epithermal, carbon dioxide, temperature

Abstract

The use of fluid inclusions in exploration is frequently overlooked because conventional microthermometric measurements are slow, tedious and rarely definitive. However by using acoustic decrepitation, useful exploration data can be acquired quickly and reproducibly. The results can readily identify CO₂ rich inclusion fluids, in addition to providing relative temperatures of aqueous fluids to outline palaeo-thermal temperature gradients in hydrothermal quartz systems.

Samples from several gold mines in the Hebei province of China show that CO₂ rich fluids are quite rare in this region, in contrast with the common occurrence of very CO₂ rich fluids in typical archaean gold deposits. Because the microscopic studies on these deposits concentrated on the rare but prominent CO₂ rich inclusions, the deposits have been wrongly classified as being derived from CO₂ rich fluids, wheras acoustic decrepitation shows that those CO₂ rich fluids were merely minor phases of the overall hydrothermal system.

At the El Penon epithermal gold deposit in Chile, there are some CO₂ rich fluids present, indicating a deeper and higher pressure origin than is usual for epithermal deposits. Variations in the aqueous fluid temperatures of about 50 C suggest that mapping of palaeo-thermal gradients can be done, which may help to outline favourable mineralised zones within the quartz system.

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 inappropriate. 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 recognize the importance of the effects caused by the presence of CO₂ rich inclusions. It was not until 1983 (Burlinson, K., 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 recognize 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 as they are often associated with CO₂ rich hydrothermal fluids.

Confirmation of the relationship between low temperature decrepitation and CO₂ rich fluids was provided by Mavrogenes et al. (1995), who performed 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 acoustic 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.

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, whereas water undergoes a phase change.

Figure 1 shows the behaviour of 2 hypothetical fluid inclusions of different compositions, formed at the “formation point” conditions. The pure CO₂ inclusion follows the gas law isochore, whereas the pure water inclusion follows the isochore for water, eventually reaching the liquidus curve where it nucleates a vapour phase. During cooling and exhumation the confining lithostatic pressure prevents decrepitation of the inclusions. However, when heated in the laboratory there is only one bar of confining pressure and when the inclusions generate approximately 600 bars of overpressure they can fracture the host quartz and decrepitate. During heating the CO₂ inclusion will generate high internal pressures and decrepitate at a low temperature unrelated to its formation temperature (approximately 220 C in Fig. 1), while the water inclusion will remain intact until about 350 C, close to its formation temperature.

Many workers have noted a common association between gold mineralisation and CO₂ rich forming fluids, although this relationship is indirect and due to the buffering of the fluid pH in a range which enhances the solubility of complexes of gold with reduced sulphur (Phillips, G.N. & Evans, K.A., 2004). CO₂ rich fluids are common in many gold deposit styles, particularly the archaean deposits in the Canadian Abitibi province and the West Australian Yilgarn province, as well as the turbidite hosted gold deposits in Victoria, Australia, and Nova Scotia, Canada. A detailed study of the Timmins, Ontario region (Smith, T.J. & Kessler, S.E., 1985) has documented the relationship between CO₂ levels and the potential for Au mineralisation in that region, showing its use as a regional exploration indicator. Figure 2 shows typical decrepitation results from these types of deposits and the CO₂ rich fluids are evident from the prominent acoustic decrepitation peaks below 300 C.

In deposits formed at shallow depth, such as epithermal deposits, most of the CO₂ is exsolved from the fluid before inclusion trapping occurs. Such inclusions have low internal pressures and lack a low temperature acoustic decrepitation peak. The decrepitation observed is from dominantly aqueous inclusions and reflects the temperature of the deposition from these fluids. Variations in this temperature can be used to map out temperature gradients within the hydrothermal system as a means of identifying zones of quartz of economic interest.

39 samples were collected from 3 mines in the Hebei province. These are from intrusion related gold deposits in structurally controlled zones, but despite the involvement of deep-seated fluids, only trace amounts of low temperature decrepitation due to CO₂ rich fluids were found in the Hougou mine, and no such CO₂ rich fluids were identified in the Dongping or Huangtuliang mines. Although other studies, using microthermometry, have suggested the formational fluids at these mines were CO₂ rich, the acoustic decrepitation data show that such CO₂ rich fluids were volumetrically insignificant and they may not be the source of the gold mineralisation at all.

20 samples were collected from the ore and waste dumps at this mine, as underground access was not possible. Of these, 12 samples showed a low temperature decrepitation peak at about 350 C, which indicates the presence of CO₂ rich inclusions. The higher than usual decrepitation temperature indicates that the inclusions had lower internal pressures than found in typical archaean gold deposits. Although this deposit is stated to be epithermal in origin, the presence of CO₂ rich inclusions indicates a formation depth much greater than usual.

The decrepitation temperature of the main peak in these samples varies from 430 C to 460 C showing that there are small temperature differences within the quartz, from which it may be possible to map out palaeo-thermal zones within the quartz, given proper spatially located samples.

Acoustic decrepitation is able to detect the presence of CO₂ rich fluid inclusions, due to their high internal pressures, which results in premature decrepitation at low temperatures when heated. As CO₂ rich fluids have frequently been shown to be associated with gold mineralisation in many districts, this means we can use decrepitation to quickly scan large numbers of samples as an exploration method to look for potentially mineralised quartz and to discriminate between quartz samples which are visually indistinguishable.

In addition, temperature variations of primary fluid inclusions can be used to generate palaeo-thermal maps, which can be used to identify potentially mineralised zones within quartz veins and vein systems.

Fluid inclusion information is potentially very useful in exploration, but its use has been very limited to date because collection of the information is tedious, inconvenient and expensive. The acoustic decrepitation method overcomes these limitations and provides data which is very useful in exploration, despite its accuracy being less than that from traditional microthermometric methods.

Burlinson, K., 1984. Exploration for gold at Pine creek and Tennant Creek, N.T. and at Halls Creek, W.A. using the fluid inclusion decrepitation technique. Australasian Institute of Mining and Metallurgy, Conference, Darwin, NT. August 1984. (abstract only)

Burlinson, K., 1988. An instrument for fluid inclusion decrepitometry and examples of its application. Bulletin de Mineralogie, 111: 267-278.

Mavrogenes, J.A., Bodnar, R.J., Graney, J.R., McQueen, K.G. and Burlinson, Kingsley, 1995. Comparison of decrepitation, microthermometric and compositional characteristics of fluid inclusions in barren and auriferous mesothermal quartz veins of the Cowra Creek gold district, New South Wales, Australia. Journal of Geochemical Exploration, 54: 167-175.

Phillips, G.N. & Evans, K.A., 2004. Role of CO₂ in the formation of gold deposits. Nature 429: 860-863.

Smith, Ted J. & Kesler, Stephen E., 1985. Relation of fluid inclusion geochemistry to wallrock alteration and lithogeochemical zonation at the Hollinger-McIntyre gold deposit, Timmins, Ontario, Canada. Canadian Institute of Mining Bulletin 78: 35-46.

With examples from Hebei, China and El Penon, Chile.

Abstract