Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 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
Field sample descriptions and sample locations
Tanami region geology (A. Wygralak, NTGS)