Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
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Understanding baro-acoustic decrepitation.

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Tanami area samples - interpretation with sample location details

Kingsley Burlinson

Burlinson Geochemical Services

Summary:

These samples were collected from a wide range of deposits and background locations throughout the Tanami proterozoic Au province. They were initially studied using conventional microthermometry and also laser Raman analyses, and then submitted for acoustic decrepitation analysis to see how the methods compared and how fluid inclusion data might help with Au exploration.

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.

Dead Bullock Soak area.

The Callie mine and Fumarole Pit are part of the "Dead Bullock Soak" workings. (Geology & location map).
Of the four ore-related quartz samples, only 2 show the low temperature decrepitation typical of CO2-rich fluid inclusions. Note that sample 11762 shows this low temperature decrepitation despite all the inclusions being too small for the microthermometric method to be used.
The average Th of 262 C obtained on sample 11359 is within the region of CO2-rich inclusion decrepitation. This temperature is probably too low as it reflects the inclusions which were decrepitating and the microscopist failed to notice this decrepitation!

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 mine               596300E    7729800N    Ore stage q vein;    (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


 

Other "Granites region" mine samples

The Bunkers hill pit is at "the granites" mine area, and the Oberon pit is near Mt. Ptilotus, about 30 Km. NW.
These 3 samples all have remarkably low decrepitation activity. Low temperature decrepitation occurs in all of these ore samples, but is seen only as a low intensity broad "toe" in the 2 Oberon samples, (red and green). This suggests the mole fraction of CO2 in the inclusions is relatively low and/or the deposition occurred at shallow depths. Sample 11847 (in blue) from Bunkers hill pit at "the granites" area shows significant low temperature decrepitation from CO2-rich fluids. Decrepitation can provide this information despite the microscopist's comment that inclusions in this sample were too small to work with.


 
 

H1558  11487    Oberon                 602249E     7756335N      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 between Au and CO2-rich fluid inclusions in this province. Note that the Th on sample 11740 (in blue) from White Range reflects the beginning of the main decrepitation peak at 430 C, and agrees with an estimated Th of about 350 which would be made from the decrepitation data. (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 Ptilotus            621000E   7756000N   Ore stage q vein  358 g/t Au
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


 
 

Groundrush Mine.

This area is some 150 Km. from "The Granites" and is near the Tanami gold mine. (Geology & location map).
These samples are confused by a sample mis-numbering before reaching the laboratory, but they all show low or no decrepitation due to CO2-rich fluid inclusions. Wygralak claims a depth of deposition of  5.7 to 8.3 Km for this mine and also claims the inclusions are dominated by methane. Such inclusions with a high mole fraction of methane and high internal pressures due to deep formation, should decrepitate at low temperature and the lack of decrepitation on these samples is a major discrepancy requiring explanation. The data are unreliable because of the mis-numbering, but it is also possible that the formation depth estimates are incorrect. Note that Wygralak claims the nearby Tanami mine formed at a much shallower depth of just 0.4 to 1.8 Km.

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 to untangle this mess.
 


 

Sample 11526, Groundrush area, has CH4 + lesser CO2 fluid 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.


 
 

Winnecke Granophyre

This area is 150 Km north ot the Tanami and Groundrush area. (Geology and location map).
There is no evidence of low temperature decrepitation of CO2-rich fluid inclusions in any of these samples. There are 2 different quartz types recognizable, with 11656 (red) and 11858 (green) having  a decrepitation peak at 500 C, while the remaining 3 samples have broader decrepitation from 470 to 520 C. The higher temperature decrepitation seems to be correlated with the gold bearing samples, except for sample 11862 which has lower temperature decrepitation but is thought to be associated with gold.

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. His 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 area*     0617520E   7985795N   Ore stage q vein;    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
 


 

Background Samples

Pargee Range is about 50 Km NW of the Tanami mine area. This sample of barren quartz shows no low temperature decrepitation, and unusually low decrepitation intensity overall.
Mt. Davidson is approximately 60 Km. east of "The Granites" mine. This sample shows significant low temperature decrepitation due to CO2-rich fluids, despite being from a presumed barren quartz.  It cannot be expected that all CO2-rich fluids will result in gold deposition and decrepitation is just a way of adding information in the search for mineralisation. The CO2-rich fluids indicate a deep seated fluid source for the rock under investigation and this is only an indirect, albeit frequently useful, indicator for gold.

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    Tanami 1:250K map sheet;   AMG:279233     Barren q vein
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.

H1584  11872      0601791E    7857814N     Automorphic quartz from geode in Antrim Plateau Volcanics
 
 
 
 

Sample numbering error

When the geologist concerned provided sample location information, it became clear there had been a numbering problem. He claims a sample numbered 11424 was analysed, but no such sample was received at the laboratory. The geologist claims that sample 11424 is actually sample 11524 mis-numbered. But his sample descriptions of 11524 and 11424 differ, so this cannot be correct. Because the geologist declined to provide any sample information for 13 of the samples the data set is not closed and it is impossible to deduce which sample is the one he claims as 11424 with certainty. It is likely that the undescribed sample 11425 may actually be 11424, but the geologist refuses to resolve this discrepancy.

Consequently I consider that the results for samples 11524 and 11425 and 11424 in this report and in the geological information the  geologist concerned has published elsewhere 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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