PACROFI VI - Electronic Program

Fluid Inclusion Decrepitation - an overview of exploration applications

Kingsley Burlinson

P.O. Box 37134, Winnellie NT, 0821, Australia

Fluid inclusion data is rarely used in exploration due to the expense and time involved in collecting microthermometric or microanalytical data. The decrepitation technique is a rapid and cheap means of obtaining data for exploration purposes, where the limitations of the method are of secondary importance to the need for low cost data. A computer controlled instrument is used to collect the data as a histogram of audible inclusion bursts versus temperature. This method can be applied to opaque minerals and to materials which contain inclusions too small for microthermometric analysis as no optical observation is involved.

The method is useful as a qualitative means of discriminating between samples of similar appearance or mineralogy and can assist in identifying different stages of quartz vein formation in a deposit. It also provides information on the CO2 content of mineral forming fluids as CO2 rich fluid inclusions decrepitate at unusually low temperatures near 200oC. Both of these methods can assist in gold exploration where CO2 rich fluids and multiple overlapping quartz vein sets commonly occur. Examples of exploration studies where decrepitation data can be usefully employed are displayed in this poster.

Quartz and Silica: Almost all quartz vein samples contain abundant fluid inclusions which give intense decrepitation responses. Even in zones of pervasive silicification, where the inclusions are usually very small, good decrepitation histograms can usually be obtained. In contrast, chert and barren Jasperoid samples give negligible decrepitation. Gold mineralized Jasperoid from the small Cranovitch deposit in the Robinson district, Nevada, shows significant decrepitation whereas numerous barren Jasperoids in Eastern Nevada showed no decrepitation at all.

Haematite and magnetite: At Tennant Creek, NT, Australia, gold and copper mineralization is associated with massive haematite/magnetite/chlorite lodes. Both the haematite and magnetite show intense decrepitation responses up to 800oC. Similar but unmineralized iron formation samples from the Alice Springs region some 500 Km away have negligible decrepitation, leading to the conclusion that the Tennant Creek deposits are of epigenetic origin, contrary to some proposed models for their formation. The decrepitation of the haematite indicates that it is of primary origin rather than being an oxidation product of the magnetite. Magnetite from a similar gold-magnetite deposit at Upper Beaver, Ont., Canada again shows intense decrepitation. Skarn magnetite from Lyon, Nevada also has intense decrepitation, although the decrepitation tends to occur over a narrower temperature interval than in the other deposits.

Carbon dioxide: Quartz samples from gold deposits frequently show the characteristic low temperature decrepitation peak caused by the presence of CO2 rich fluid inclusions. Economic gold mineralization in the Archean greenstone belts of Western Australia and Ontario show a close relationship with such CO2 rich fluids. At the Victory mine, Kalgoorlie, WA., several quartz vein generations have been identified and gold is preferentially associated with flat lying veins which also have intense low temperature decrepitation due to CO2 rich fluids. The barren vertically oriented veins in the same pit lack this low temperature decrepitation peak. Proterozoic basin type gold deposits such as Victoria and Pine Creek, NT., Australia also show this close association between economic mineralization and CO2 rich fluids. In contrast, the higher level, more recent deposits typical of Nevada rarely show the low temperature decrepitation peak and lack CO2.

In exploration it is frequently useful to distinguish between numerous similar looking quartz samples. By using simple mathematical transforms of the decrepitation data it is possible to identify groups of similar samples as an aid in the selection of exploration or drilling target areas. An example of such an exercise using quartz samples from Victoria, Australia shows this technique.