Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
Newest Topics:

New model 216 decreptiometer

Exploration of the Mt. Boppy Au deposit, NSW

Forensic tests on soil samples

Viewpoints:

Do IOCG deposits form from CO2 fluids?

How CO2 inclusions form from aqueous fluids (UPDATED)

Understanding heterogeneous fluids : why gold is not transported in CO2-only fluids

Gold-quartz deposits form from aqueous - CO2 fluids: NOT from CO2-only fluids


Discussions why H2 analysis by mass spectrometry is wrong



News:

Gold at Okote, Ethiopia

Kalgoorlie Au data

Sangan skarn Fe deposits, Iran

Studies of 6 Pegmatite deposits

A study of the Gejiu tin mine, China


Exploration using palaeo-hydrothermal fluids

Using opaque minerals to understand ore fluids


Understanding baro-acoustic decrepitation.

An introduction to fluid inclusions and mineral exploration applications.



 Interesting Conferences:

-----2023-----

ECROFI Iceland
     July 2-6

AOGS Singapore
    30 Jul - 4 Aug 2023

SGA Zurich Aug 2023


Comprehensive Geology Conference Calendar


Ballarat drillhole samples

Geologically controlled interpretation

Grouped by spatial location


Only limited sample location information is available so it is not clear which samples are from the same quartz lodes. unfortunately, no attempt was made during sampling to collect several closely spaced quartz samples within a single quartz vein to check for quartz vein zonation and small-scale heterogeneity. Even when samples from adjacent 1 metre long assay intervals in the core were sampled, there is no way of knowing where the decrepitation sample came from within these intervals and so such adjacent samples could be 2 metres apart. This lack of control over the sampling seriously restricts this interpretation of the data.

The classification "mineralised "or "unmineralised" in the sample descriptions was done solely on the gold analysis value and does not reflect the spatial association of the sample with known lodes. As such, this classification is unhelpful in the interpretation of this data as the intent here is to outline broad mineralisation haloes, not to merely provide a confirmation of a gold analysis!

Samples were analysed from 5 separate drillholes over an interval of about 400m of the main lode. Drillhole suffix letters indicate wedge-offs from the same drillhole collar.  Four of the holes were drilled eastwards into the main reef, which is approximately vertical here as seen on the cross section of hole 4.   Hole 5 was drilled westwards and from its collar position it is clearly targeting a different line-of-lode to all the other holes.
 
 
 

Summary

The most noticeable difference between samples in this suite are the variation in temperature and intensity of the low temperature peaks between 200 and 300 C, these features being caused by variations in the amount of CO2 in the fluids. The reported Au analyses do not show any particular correlation with the presence of these CO2 caused peaks. Nor does there seem to be any spatial consistency within drillholes of samples rich in or lacking CO2 , although such relationships are often observed in other studies. (Mavrogenes et. al, Partamies and Poutiainen).  Clearly the fluids involved in formation of the quartz veins at Ballarat were often CO2 -rich, but varied considerably in their mole percent of CO2 , including no detectable CO2  at times. The veins were not formed from a single type of fluid in a single event, but are the result of considerably variable fluid systems, probably telescoped over time, resulting in complex zonation of the quartz, which is not readily visible in hand specimen but can be ascertained from fluid inclusion decrepitation studies. It is not clear which  of these fluid stages are related to the gold mineralisation, probably because none of the samples collected in this study are located to better than one metre accuracy, when it is likely that the quartz is zoned on a scale of centimetres or less.

Using the peaks present in the 400 to 550 C region it is apparent that several different quartz generations occur,  (see the accompanying "blind" interpretation) but there is a continuum of types rather than distinct separate types and it is not possible to reliably isolate specific quartz types using these peaks.

The temperature of the onset of massive decrepitation can be used to provide an indication of formation temperature of the quartz, and in these samples this temperature ranges from 340 to 370 C.  The closeness of these temperatures across all the samples in this study does suggest that the fluid temperature was less variable than the CO2  content.  Hence it is suggested that fluid compositional variations were of more significance in the gold transport and deposition that was fluid cooling.

The data show interesting variations within the quartz, but because the sampling was compromised by the failure to allow for small scale zoning of the quartz veins, it is not possible to determine the significance of the observed variations and how they relate to the gold mineralisation. It is possible to use decrepitation within this type of gold-quartz system to observe fluid related variations. But because the quartz is strongly zoned and inhomogenous at large scales much more careful sampling is required in order to see how these variations relate to mineralisation and how to apply the decrepitation method as an exploration and development aid.

Back to Victorian Palaeozoic gold index

Back to Main Contents


The data in this survey came from 5 drillholes located near the First Chance and Last Chance shafts within the town of Ballarat. These drillholes are highlighted in the following map. All samples were in quite close proximity to Au mineralisation.
 

Location Map - Ballarat Drillholes


 

Sample descriptions by site geologist, with analyses, in sample number order

Sample descriptions by site geologist, with analyses, in drillhole-depth order

Hole BDD1



 

Run g1855, Sample NG12202, BDD1 537-538  .24 Au   unmineralised qz
      Milky wh qz, weakly fractured,    ACID WASHED
Run g1834, Sample NG12201, BDD1   542-543    17.0 Au   mineralised qz
      Milky wh qz, weakly fractured
Run g1835, Sample NG12204, BDD1 549-550   .05 Au   unmineralised qz       *****adjacent \
      Milky wh qz, moderately fractured
Run g1856, Sample NG12203, BDD1 550-551  <.02 Au   unmineralised qz      *****adjacent /
      Milky wh qz, very few fractures,    ACID WASHED
Run g1858, Sample NG12205, BDD1 555-556  7.6 Au   mineralised qz
      Milky wh qz, weakly fractured, brecciated,   ACID WASHED
Run g1836, Sample NG12206, BDD1 561-562   13.9 Au   mineralised qz
      Milky wh qz, weakly fractured
 
 

Holes BDD1A & BDD1C


Run g1867, Sample NG12221, BDD1A 467-468   28.5 Au   mineralised qz
      Milky wh qz, weakly fractured, minor vugs,    ACID WASHED
Run g1868, Sample NG12222, BDD1A 479-480   .07 Au   unmineralised qz
      Milky wh qz, very fractured, sericitic, much CO3=,    ACID WASHED
Run g1866, Sample NG12220, BDD1A 493-494   5.6 Au   mineralised qz
      Milky wh qz, not fractured,    ACID WASHED
Run g1871, Sample NG12226, BDD1C 547.5-548.6   4.5 Au   mineralised qz     ### probably a different lode
      Milky wh qz, very fractured, very sericitic, CO3=,    ACID WASHED
 
 

Holes BDD1B & BDD1C


Run g1869, Sample NG12223, BDD1B 451.5-452.5   43.0 Au   mineralised qz
      Milky wh qz, very fractured, sericitic,     ACID WASHED
Run g1844, Sample NG12224, BDD1B 454.4-455.4   3.43 Au   mineralised qz
      Milky wh qz, weakly fractured
Run g1870, Sample NG12225, BDD1B 456.8-458.0   0.22 Au   unmineralised qz
      Milky wh qz, weakly fractured,       ACID WASHED
Run g1871, Sample NG12226, BDD1C 547.5-548.6   4.5 Au   mineralised qz     ### probably a different lode
      Milky wh qz, very fractured, very sericitic, CO3=,    ACID WASHED

Holes BDD2A & BDD2C



 

Run g1843, Sample NG12219, BDD2A 232.81-233.3   <.02 Au   unmineralised qz
      Milky wh qz, not fractured, clean white
Run g1865, Sample NG12218, BDD2A 243.6-244.6   120.0 Au   mineralised qz
      Milky wh qz, weakly fractured,    ACID WASHED
Run g1841, Sample NG12217, BDD2A 255.1-256.1   <.02 Au   unmineralised qz
      Milky wh qz, not fractured
Run g1840, Sample NG12216, BDD2A 263.3-264.3   .14 Au   unmineralised qz
      Milky wh qz, moderately fractured
Run g1845, Sample NG12227, BDD2A 650-651.2   0.15 Au   unmineralised qz   ### probably a different lode
      Milky wh qz, not fractured
Run g1872, Sample NG12228, BDD2C 534-535   <0.02 Au   unmineralised qz    ### probably a different lode
      Milky wh qz, not fractured, much  CO3=,    ACID WASHED
 
 

Holes BDD2D & BDD2E & BDD2C



 

Run g1873, Sample NG12229, BDD2D 245.0-245.7   2.48 Au   mineralised qz           *****adjacent \
      Milky wh qz, weakly fractured,     ACID WASHED
Run g1874, Sample NG12230, BDD2D 245.7-246.1   0.02 Au   unmineralised qz       *****adjacent /
      Milky wh qz, not fractured,     ACID WASHED
Run g1876, Sample NG12232, BDD2E   238.2-238.9    <0.02  Au   unmineralised qz   *****adjacent \
      Milky wh qz, weakly fractured, minor CO3=,     ACID WASHED
Run g1875, Sample NG12231, BDD2E 238.9-240.1   11.3 Au   mineralised qz               *****adjacent /
      Milky wh qz, very fractured, much CO3=,    ACID WASHED
Run g1859, Sample NG12207, BDD2D   464.8-465.8    0.44  Au   unmineralised qz   ### probably a different lode
      Milky wh qz, weakly fractured,    ACID WASHED
Run g1872, Sample NG12228, BDD2C 534-535   <0.02 Au   unmineralised qz           ### probably a different lode
      Milky wh qz, not fractured, much  CO3=,    ACID WASHED
 

Hole BDD3A


Run g1846, Sample NG12233, BDD3A 461.4-461.9  .05 Au   unmineralised qz       *****adjacent \
      Milky wh qz, weakly fractured
Run g1847, Sample NG12234, BDD3A 461.9-462.8  7.8 Au   mineralised qz            *****adjacent /
      Milky wh qz, rarely fractured
Run g1849, Sample NG12236, BDD3A 472.8-473.9  .04 Au   unmineralised qz      *****adjacent \
      Milky wh qz, intensely fractured
Run g1848, Sample NG12235, BDD3A 473.9-474.9  1.76 Au   mineralised qz          *****adjacent /
      Milky wh qz, weakly fractured, trace S=
 
 

Hole BDD3B


Run g1877, Sample NG12237, BDD3B   474.2-475.2      1.85  Au   mineralised qz
      Milky wh qz, weakly fractured, much CO3=,     ACID WASHED
Run g1850, Sample NG12238, BDD3B 477.1-478.1  .05 Au   unmineralised qz
      Milky wh qz, weakly fractured, clean quartz
Run g1839, Sample NG12213, BDD3B   647-648    0.06 Au   unmineralised qz           ### probably a different lode
      Milky wh qz, not fractured
Run g1862, Sample NG12211, BDD3B 649.3-650.4   .03 Au   unmineralised qz          ### probably a different lode
      Milky wh qz, very weakly fractured,    ACID WASHED
Run g1860, Sample NG12208, BDD3B 653.5-654.6   .02 Au   unmineralised qz  **adjacent \  & diff. lode
      Milky wh qz, very weakly fractured,    ACID WASHED
Run g1838, Sample NG12212, BDD3B 654.7-655.8   .06 Au   unmineralised qz  **adjacent / & diff. lode
      Milky wh qz, very weakly fractured
 

Sample descriptions by site geologist, with analyses, in sample number order

Sample descriptions by site geologist, with analyses, in drillhole-depth order
 
 


 

Drillhole section BDD4 and wedge-offs

Between Britannia and First Chance shafts.


Hole BDD4 and wedge-offs is the only one for which a cross section is available. From this it can be seen that all the samples are from the main reef lode. They should all be classified as mineralised, despite the Au analysis in one sample being low.
 


 
 

Holes BDD4 & BDD4A & BDD4C


Run g1861, Sample NG12210, BDD4 646.5-647.5   13.3 Au   mineralised qz
      Milky wh qz, very weakly fractured,    ACID WASHED
Run g1837, Sample NG12209, BDD4 655.3-656.2   220 Au   mineralised qz
      Milky wh qz, weakly fractured
Run g1863, Sample NG12214, BDD4A 644.0-644.4   21.6 Au   mineralised qz
      Milky wh qz, not fractured,    ACID WASHED
Run g1864, Sample NG12215, BDD4A 648.5-649.4   275.0 Au   mineralised qz
      Milky wh qz, not fractured,    ACID WASHED
Run g1879, Sample NG12241, BDD4C 476.6-477.3      0.13 Au   unmineralised qz
      Milky wh qz, weakly fractured,    ACID WASHED
 
 

Holes BDD4B & BDD4A & BDD4C


Run g1878, Sample NG12239, BDD4B 567.8-568.8   16.5 Au   mineralised qz
      Milky wh qz, very fractured,    ACID WASHED
Run g1852, Sample NG12240, BDD4B 573.7-574.2  16.8 Au   mineralised qz
      Milky wh qz, moderately fractured, very white
Run g1863, Sample NG12214, BDD4A 644.0-644.4   21.6 Au   mineralised qz
      Milky wh qz, not fractured,    ACID WASHED
Run g1864, Sample NG12215, BDD4A 648.5-649.4   275.0 Au   mineralised qz
      Milky wh qz, not fractured,    ACID WASHED
Run g1879, Sample NG12241, BDD4C 476.6-477.3      0.13 Au   unmineralised qz
      Milky wh qz, weakly fractured,    ACID WASHED
 
 
 
 
 


 

Holes BDD5 & BDD5A & BDD5B


Run g1880, Sample NG12242, BDD5 251.9-252.9    <0.02 Au   unmineralised qz
      Semi translucent qz,    ACID WASHED
Run g1881, Sample NG12243, BDD5 261.0-261.6    0.03 Au   unmineralised qz
      Milky white qz, weakly fractured,    ACID WASHED
Run g1882, Sample NG12244, BDD5A 240.2-240.75     6.2 Au   mineralised qz
      Milky white qz, moderately fractured,    ACID WASHED
Run g1883, Sample NG12245, BDD5A 245.5-245.95     0.02 Au   unmineralised qz
      Milky white qz, moderately fractured,    ACID WASHED
Run g1853, Sample NG12246, BDD5A   253.6-254.3    <0.02  Au   unmineralised qz
      Milky wh qz, not fractured
Run g1854, Sample NG12248, BDD5B   258.9-259.9    30.0  Au   mineralised qz
      Milky wh qz, weakly fractured
 
 

Holes BDD5B & BDD5A


Run g1854, Sample NG12248, BDD5B   258.9-259.9    30.0  Au   mineralised qz
      Milky wh qz, weakly fractured
Run g1851, Sample NG12247, BDD5B   266.4-267.4    0.04 Au   unmineralised qz
      Milky wh qz, weakly fractured
Run g1882, Sample NG12244, BDD5A 240.2-240.75     6.2 Au   mineralised qz
      Milky white qz, moderately fractured,    ACID WASHED
Run g1883, Sample NG12245, BDD5A 245.5-245.95     0.02 Au   unmineralised qz
      Milky white qz, moderately fractured,    ACID WASHED
Run g1853, Sample NG12246, BDD5A   253.6-254.3    <0.02  Au   unmineralised qz
      Milky wh qz, not fractured
 

Sample descriptions by site geologist, with analyses, in sample number order

Sample descriptions by site geologist, with analyses, in drillhole-depth order
 
 

Back to top

Back to Victorian Palaeozoic gold index

Back to Main Contents