Fiddlers Ck gold decrepitation

Decrepitation analyses from the Fiddler's Creek mine, Victoria, Australia

A Project with Melbourne University by James Long and Reid Keays, 2023

Summary of conclusions:

Decrepitation analyses show that the fluids which produced the Fiddler's Creek auriferous lode contained very high levels of CO₂, consistent with many other gold producing regions in Victoria. Samples from nearby reefs and regional extensions show that these auriferous fluids are of much greater extent than the Fiddler's Creek workings and this region has high potential for extensions and nearby additional gold deposits.

Fluid inclusion decrepitation is a useful exploration method to find potentially auriferous quartz formed from CO₂ rich fluids.

The project:

The Fiddler's Creek gold deposit was found in the 1850's during the gold rush in Victoria. This area contains several old mines with shafts to about 60m depth and has produced some 700,000 oz of gold, largely from alluvial workings. It is presently being explored by Core Prospecting Pty. Ltd. Information on Fiddler's Creek is on their website at https://coreprospecting.com.au/ The location is west of Ballarat and Avoca.

The gold mineralisation occurs in fault related quartz reefs within host shale, siltstone and sandstones.

As part of James Long's university project, he collected a suite of quartz samples for decrepitation analyses and these were analysed by Burlinson Geochemical Services on the model 216 decrepitometer at the laboratory in Darwin. (Described here)

Many of the samples gave extremely intense decrepitation at low temperatures, between 200 and 300C. Such decrepitation is caused by CO₂ rich fluid inclusions, as discussed elsewhere on this site. Such inclusions are commonly associated with gold mineralisation as they indicate significant deep-seated fluids which can transport gold from depth. There are also a number of mid temperature peaks from 350 C to 550C caused by multiple other fluid events which cannot be readily interpreted, but show that there were complex, multiple fluid events at this deposit.

The complete set of analyses are shown in these 4 plots which list the analytical run number and field sample identifier.
group1
group2
group 3
group 4

Sample description and location details (where known) are here.

Several samples were observed microscopically as grain mounts in RI oil to try and understand the extremely intense decrepitation at low temperature.

Samples were classified based on the height of the decrepitation peak between 200 C and 300 C, which is an estimation of CO₂ content. These values are colour coded onto the sample location maps here.
Regional sample location map: The small purple dots are sample locations. The large coloured circles encode the low-temperature decrepitation counts as per the legend at top left..

Both the Poverty fault (J266, J270) and "west of england" fault zones (J267, J268, J271) show very high low-temperature decrepitation from CO₂ rich fluids indicating that these 2 zones may be as prospective as the Fiddler's reef which also has very high low-temperature decrepitation from CO₂ rich fluids - seen in the detail map following.The distant surface samples J296, J298 and J299 are particularly interesting as these align with the "west of england" zone indicating considerable potential for more than 2Km along this zone.

The detail map of the Fiddler's mine (same decrepitation legend as above map) :

The Fiddler's reef has consistently very high low-temperature decrepitation counts caused by CO₂ rich fluids, shown as the many yellow and green decrepitation count results. There is a good correlation between past gold production of the lode and the presence of high decrepitation counts caused by CO₂ rich fluids.

The FIDD samples are from diamond drill holes at unknown declination and cannot be extrapolated correctly onto the lode positions and the lower (blue) decrepitation counts do not detract from the overall values in the lode which are much higher (yellow and green). Note that J276 and J277 have the same location coordinates with very different decrepitation results (J276=yellow, J277=purple). Sample J277 (FIDD041) was from a visibly different quartz generation and not part of the productive lode quartz and gave low decrepitation while J276 was from the lode and had high decrepitation counts. Decrepitation analyses provide a means of readily identifying different generations of quartz even when this may not be visually recognisable in hand specimen.

De-convolution of decrepitation results to component skew-gaussian fluid-inclusion populations.

Decrepitation analysis provides a complete composite result of all the fluid-inclusions in a sample, which frequently includes fluids of multiple different origins, times and compositions. In many cases the simple presence of intense decrepitation between 200 C and 300C is adequate to guide exploration by focusing on fluids rich in CO₂, which would have had a deep origin and could potentially have carried economic minerals (gold). But it may be possible to further distinguish between fluids and origins by looking at the entire decrepitation curve, which is best done by de-convoluting the total curve to its component populations. Individual fluid populations depend on the fluid temperatures and compositions, but also on physical aspects of the inclusions including their size and morphology. Consequently populations show a gaussian distribution, and often there is a skew component to the distribution.

The program fityk is used to de-convolve decrepigrams and these next 2 examples from Fiddler's Creek show the complexity and multiple inclusion populations of these samples. In these graphs the black dots are the original data observations and the yellow curve is the calculated sum of the component distributions. (A discussion on de-convolution of decrepitation data is on this website here.)

J276 ( FIDD044, main lode) has intense low-temperature decrepitation at 200 C which dominates the result, but also peaks centred at 430 C and 530 C. (The peak at 580 C is related to the quartz alpa-beta inversion and is not used in interpretation).

J276 4 peak fit

J286 ( FQ11, main lode, south end) has less intense but still very high low-temperature decrepitation at 250 C, but multiple other overlapping fluid-inclusion populations at temperatures of 320 C, 360 C, 430 C and 550 C. Although it is unclear what fluid events caused these populations of inclusions, it may be possible to use these peaks to discriminate between quartz samples within a region and prioritise exploration targets.

J286 peak fit

Decrepitation analyses from the Fiddler's Creek mine, Victoria, Australia

A Project with Melbourne University by James Long and Reid Keays, 2023

Summary of conclusions:

The project:

De-convolution of decrepitation results to component skew-gaussian fluid-inclusion populations.

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