Rapid fluid inclusion data
for exploration (decrepitation)
There is an approximate, though imperfect, relationship between Au mineralisation and the presence of CO2-rich fluid inclusions, with methane-rich fluid inclusions present in a few areas. The decrepitation results are a good, quick indicator of the presence of CO2-rich fluid inclusions, without the pain of preparing double polished thin sections and undertaking tedious microthermometry. And in at least one case, decrepitation detected the presence of CO2-rich fluids in a sample in which the microthermometric method could not be used because the fluid inclusions were too small.
In general, low temperature decrepitation below 300 C indicates CO2-rich fluids and is a favourable sign for gold mineralisation. The exceptions might not be a failure of the method as much as difficulties with collection of samples and classification into ore and non-ore suites. The variations in decrepitation response suggest that the quartz is zoned or banded, but no samples were collected to check this. Such banding, if unrecognized during sample collection, would seriously interfere with sample classification. Additional work should be done to investigate such banding and multiple stage formation of the quartz.
The decrepitation data indicates that in some cases the microthermometry was applied to secondary instead of primary inclusions, resulting in incorrect temperature estimates, and possibly also incorrect formation depth estimates. If microthermometry is being used in an exploration programme, decrepitation data should also be collected to ensure integrity of the data.
One or more samples were reported to contain predominantly methane fluid, and to have formed at a depth of 6 to 8 Km where the pressure was 1500 to 2000 bars. But these samples showed no low temperature decrepitation. This is inconsistent with the phase behaviour of methane. In this case the decrepitation data shows there is a serious problem with either the chemistry or pressure calculations in the microthermometry. These samples, from the Groundrush deposit, should be viewed with extreme suspicion until additional work to resolve this discrepancy is undertaken.
Sample 11359 has significant CO2 and average Th 262
C, with visible Au.
Sample 11762 has "clusters of small gas rich/gas only inclusions, no microthermometry performed", with visible Au.
H1565 11359 Callie
596300E 7729800N Ore stage q
(no assay results)
H1586 11756 Callie mine 596300E 7729800N Ore stage q vein; visible Au
H1587 11757 Callie mine 596300E 7729800N Ore stage q vein; visible Au
H1578 11762 Callie mine 596300E 7729800N Ore stage q vein; visible Au
Of 3 presumed non-ore quartz samples, one did show low temperature decrepitation due to CO2-rich fluid inclusions. It is possible that sample 11426 (shown in red, below) from the fumarole pit is more closely related to mineralisation than expected.
The correlation between low temperature decrepitation of CO2-rich fluid inclusions and ore is inconsistent in this set of samples. This may be due to complex growth zoning in the samples or to difficulty in identifying the different types of quartz in outcrop. A more detailed study of these deposits, together with more careful microthermometry is needed to confirm the suspected relationship between Au mineralisation and CO2-rich fluid inclusions.
H1551 11426 Fumarole pit (DBS goldfield) 597000E 7729800N Post ore stage buck q vein
H1576 11448 Callie mine 596300E 7729800N Post ore stage q vein
H1577 11451 Callie mine 596300E 7729800N Post ore stage q vein
Ore stage q vein with visible Au; 11.6 g/t Au
H1559 11490 Oberon 602249E 7756335N Ore stage q vein with visible Au; 6.5 g/t Au
H1594 11847 Bunkers Hill pit 641500E 7725700N Ore stage q vein; ~2 g/t Au
Other information reported by Wygralak states that "Sample 11847 has gas-only inclusions which are too small for microthermometry, 1.7 g/t Au." It is not clear why his 2 statements about this one sample have slightly differing gold grades. Is this another sample mix-up?
The sample from East Ptilotus shows no low temperature decrepitation but had very high gold content. The nearby Oberon samples (see above) showed weak low temperature decrepitation in ore samples. It is not clear if this is a sampling problem, perhaps due to zoned quartz, or an inconsistency in the Au - CO2 association.
Both of the other mineralised samples are from distant deposits, but
the prominent low temperature decrepitation confirms the relationship
Au and CO2-rich fluid inclusions in this province. Note that
the Th on sample 11740 (in blue) from White Range reflects
beginning of the main decrepitation peak at 430 C, and agrees with an
Th of about 350 which would be made from the decrepitation
(Td - 80 C).
( White Range is shown on the geology map, and Twin Bonanza is approx. 130 Km. WNW of "The Granites".)
Sample 11740 has CO2-rich inclusions, average Th 322 C , with visible Au.
H1568 11498 East
621000E 7756000N Ore stage q vein 358 g/t
H1591 11514 Twin Bonanza* 0520258E 7780530N Ore stage q vein 39.3 g/t Au
H1573 11740 White Range (Arltunga goldfield) 470600E 7407700N Ore stage q vein; visible Au
The Th of 261 C reported for sample 11526 is too low and is probably a measurement of secondary inclusions. The Th based on the decrepitation data should be approximately 390 C.
There are major problems with the fluid inclusion data and sampling
at this mine and a complete repeat of this work needs to be performed
untangle this mess.
Sample 11526, Groundrush area, has CH4 + lesser CO2
inclusions, Th 261 C and visible gold.
Samples 11425 and 11524 are the subjects of a sample numbering mixup and the microthermometric data on these samples is unreliable and/or wrong.
H1557 11425 ????????????????
- Possibly Groundrush ??????? No location information provided.
H1570 11524 Groundrush (Tanami 1:250K map sheet; AMG:F604200) Ore stage q vein; 3.6 g/t Au
H1571 11526 Groundrush (Tanami 1:250K map sheet; AMG:F604200) Ore stage q vein; 9.2 g/t Au
Sample numbering error: The geologist claims 11524 is actually 11424. But this cannot be correct and perhaps 11425 is actually 11424.
Wygralak reports Th of 200-220
C on these samples and this is probably data from secondary inclusions
and an estimate of Th from decrepitation is 390 to 410 C.
low formation depth of 0.4 to 0.5 Km may also be incorrect as it seems
he was observing secondary inclusions.
H1572 11656 Winnecke Granophyre
0617520E 7985795N Ore stage q
2.0 g/t Au
H1580 11858 Winnecke Granophyre area 0617527E 7985792N Ore stage q vein (no assay results)
H1581 11859 Winnecke Granophyre area 0617401E 7985810N Post ore stage q vein
H1582 11862 Winnecke Granophyre area 0616832E 7985855N Ore stage q vein (no assay results)
H1583 11869 Winnecke Granophyre area 0621103E 7976024N Post ore stage q vein
The reported Th of 204 C on the Mt Davidson sample (11221) is a measurement within the CO2 peak and is not a reliable indicator of the homogenization temperature, which is probably about 370 C based on the decrepitation data.
Sample 11221 has CO2-rich inclusions, average Th 204 C.
H1554 11161 Pargee Range
1:250K map sheet; AMG:279233 Barren
H1555 11221 Mt Davidson Mt Solitaire 1:250K map sheet Quartzite hosted q vein
The Antrim Plateau is outside of and far to the north of the Tanami province and is a deliberately remote background sample. This quartz has almost no decrepitation either from a lack of or only very tiny fluid inclusions. It has a response not unlike surficial jasperoid samples. This is clearly not a hydrothermal sample and would not be considered prospective in the Tanami geological province.
7857814N Automorphic quartz from geode in
Consequently I consider that the results for
11524 and 11425 and 11424 in this report and in the geological
the geologist concerned has published
must be considered as unreliable and incorrect.
Complete run-sample cross reference in RUN order
Complete run-sample cross reference in SAMPLE order
Hand Sample descriptions
Field sample descriptions and sample locations
Tanami region geology (A. Wygralak, NTGS)
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