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Rapid fluid inclusion data for exploration (decrepitation)

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Decrepitation data of gold deposits in the Kalgoorlie area, Western Australia

In 1994 an extensive suite of samples was collected from several gold deposits in and near to Kalgoorlie

Summary

It is well known that Au mineralization is frequently associated with CO₂ rich fluids. Such fluids produce distinctive decrepitation graphs with significant decrepitation at low temperatures, below 300 C, particularly for epithermal type deposits, such as those in the Kalgoorlie region of West Australia. This extensive decrepitation study aimed to document the distribution of such CO₂ rich fluid systems and their relationship to gold mineralization..

Many of the samples in the study show evidence of CO₂ rich fluids, particularly at the Mt Pleasant and Kalgoorlie superpit areas. In fact samples from these two areas (and the Victory mine at Kambalda) had the most intense low temperature decrepitation of all the samples in my entire database of more then 5000 samples. However there are also areas in which CO₂ rich fluids are absence despite known mineralization, such as the Bayley's mine at Coolgardie.

It is also notable that close groups of samples ( those with the same sample number with alphabetic suffixes, collected within a radius of a few metres maximum) can show great variation in low-temperature decrepitation intensity and thus CO₂ content. It is clear that quartz veins are quite inhomogeneous, which is not apparent in hand specimen and can only be determined by study of their fluid inclusion populations.

Although low-temperature decrepitation, indicating the presence of CO₂ rich fluids,  can be used as a guide to gold mineralization , each deposit and region needs to be assessed individually to confirm the relationship and the degree of local heterogeneity within the quartz veins.

This data is grouped  into seperate areas

Discussion of the Kalgoorlie and Coolgardie areas follows below.

        Brief sample descriptions of the Kalgoorlie area samples are here.

        Brief sample descriptions of the Coolgardie area samples are here.

Discussion of  the Victory mine, near Kambalda (point 3 on this map) is here

Discussion of  the Southern Cross area is here

Discussion of  the Londonderry pegmatite (non-gold) is  here

In all cases same-numbered samples with alphabetic suffixes were collected from the same location within a few metres and show the local variability of the data.

The maps herein are from the Geological Society of Australia (WA division) Excursion guidebook 8 by W.K. Witt, D.I. Groves & S.E. Ho  1994. This document contains extensive geological details of the region and mines visited.

Regional location map

Samples were collected from locations   1,2,3,4 & 5 (numbers inside black circles)  on the following map of the kalgoorlie region

Kanowna Belle

At Kanowna Belle low temperature decrepitation occurs in the ore zone, but some 20m away  (sample 1504) there is only weak low-temperature decrepitation, perhaps reflecting distance from mineralization.

Sample 1504 is some distance from the main ore zone and shows only weak low-temperature decrepitation

Mt Pleasant

The 3 pink shaded deposits were sampled in this study.

Sample 1505 has some of the most intense low temperature decrepitation ever observed. However this quartz was waste rock on the coreshed floor and it is uncertain if it was gold mineralized.

Sample 1508 of breccia contained feldspar which caused the high counts above 600 C. Quartz does not decrepitate above the alpha beta transition temperature of 573 C.

Kalgoorlie Superpit

Samples from the Kalgoorlie superpit ore zone often show very intense low-temperature decrepitation due to CO₂ rich fluid inclusions.

Coolgardie

The 3 mines shaded pink were sampled in this study. The Londonderry pegmatite was also sampled and is reported upon separately here.

Unmineralized granite hosted quartz veins

Note the lack of low-temperature decrepitation on these unmineralized quartz samples.

Bayley's mine

Only a few of the samples from Bayley's mine show low-temperature decrepitation.  Such decrepitation cannot be assumed to be necessary for gold mineralization on a regional scale.

King's Cross mine

Samples from the King's Cross mine show no low temperature decrepitation.

Three Mile Hill mine

The three mile hill mine is only about 3 Km from Bayley's and King's Cross mines, but has very different fluid inclusion character with intense low-temperature decrepitation.

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