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Rapid fluid inclusion data for exploration (decrepitation)

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Baro-acoustic decrepitation of samples from the Favona vein, Waihi, NZ

A study of epithermal Au and Ag mineralised quartz from this area on the North Island of NZ was undertaken with Penelope Small and Prof. Andrew Rankin of Kingston University, Surrey and the cooperation of Newmont.

Ten samples were collected from drill core in an attempt to determine spatial temperature and fluid composition variations with depth and horizontal position within the vein.

Interpretation

The samples show low decrepitation intensities, as expected for dominantly vapour filled, high level epithermal fluid inclusions in quartz. However it is still possible to discern important differences between the samples which seem to correlate with Au mineralisation. In particular, there is evidence for the presence of CO₂ rich fluids correlated with the best mineralised samples. Despite the low decrepitation intensities, this data clearly shows a strong correlation between known gold grades and the presence of low temperature gas-rich decrepitation.

Additional discussion of the Favona results is here.

There are also variations of the decrepitation temperatures of the main peak near 400 C which suggest the technique is capable of mapping out thermal gradients within the quartz system, and these variations are likely to also correlate with mineralisation potential.

Incomplete sample location information and sparse sampling limit the complete interpretation of this data at present.


These initial plots have been grouped  based on the Au contents of the samples.




The above samples with high Au contents, about  80 g/t, show distinct decrepitation at low temperature near 300 - 350 C which is typical of samples which contain CO₂ rich fluid inclusions. These samples are from  an unspecified location, thought to be UW67 (not UW167)  at about 950m RL in the Favona vein.

Samples with moderate Au contents, about 40 g/t, lack this low temperature decrepitation. These samples also have very low intensity of decrepitation which is typical of high level epithermal inclusions, which are often vapour rich and do not decrepitate well. These samples are also from about 950m RL in the Favona vein.

This sample, with 40 g/t Au shows only background decrepitation. This is interpreted to be a vapour rich area of the vein system. This sample is also from about 950m RL in the Favona vein.

This sample with only low level Au of about 1 g/t  shows weak, but distinct decrepitation  and perhaps a hint of CO₂ rich inclusions. This sample is located close to the main ore zone and the fluid inclusions indicate that good mineralisation is nearby despite the low chemical analysis result. It is from about 900m RL in the Favona vein.

These samples have < 0.1 g/t Au and are distant from the active workings. The decrepitation results show a lack of CO₂ and suggest low mineralisation potential. Note that there are significant differences in the temperature of the main peak and with sufficient  sample coverage these variations might be mapped to show thermal gradients within the quartz vein system. Samples 1878 and 1880 are from the Moonlight area, while sample 1879 is from Deep in the Favona vein, about  700m RL.

These samples have only <0.2 g/t Au and lack low temperature decrepitation and CO₂. The more intense decrepitation and better shaped peaks suggest this quartz contains more liquid rich inclusions, probably formed at greater depth and pressure than other samples in this study, despite being collected shallower than other samples.
The green plot is the same sample as the red, done at higher instrumental sensitivity.  The  blue, magenta and green plots are 3 splits of the same sample to check for homogeneity. This sample is from the Favona South shoot at about 1000m RL. 

Sample locations

The  deposit comprises a swarm of quartz veins, shown here on a surface geology map

The  locations within the mine are shown on this  N-S pseudo section. Samples 1884 and 1885 were wrongly described by the student and are shown here at their probable location.

The thick grey lines are the mine workings and entrance adit.

The sample locations and gold analyses are in this table.

Sample ID	Location	Sample #	Run #	Drill Hole	From (m)	To (m)	Box No.	Ag (g/t)	Au (g/t)
UW140A	Fav, 950 RL	1881	H2195	UW140	220.00	219.86	84	128.00	41.67
UW140B	Fav, 950 RL	1882	H2196, H2197	UW140	219.80	219.77	84	128.00	41.67
UW140C	Fav, 950 RL	1883	H2198	UW140	219.42	219.30	84	128.00	41.67
UW167A	Fav 950 RL?	1884	H2199	UW167	296.43	296.25	105	56.80	78.70
UW167B	Fav 950 RL?	1885	H2200	UW167	295.75	295.63	105	56.80	78.70
UW100A	Moon 800 RL	1878	H2189, H2190	UW100	355.15	355.00	118	2.90	0.12
UW100B	Moon 800 RL	1880	H2192, H2194	UW100	350.57	350.43	119	2.90	0.12
UW283a	Favona South 1000 RL	1886	H2185, H2186, H2201, H2202	UW283	122.75	122.64	85	1.90	0.23
UW127c	Fav, 650 RL	1879	H2193	UW127	537.82	538.00	104	10.80 to 0.14	5.60 to 0.06
UW89d	Fav, 900 RL	1877	H2188	UW89	241.65	241.50	83	15.79 to 1.82	3.00 to 0.86

Additional discussion of the Favona results is here

Sample number - Run number cross index

Sample descriptions

Fitting of gaussian populations to the overall decrepitation results

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