Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
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Baro-acoustic decrepitation of Hall Mo samples, Nevada

Using skew-gaussian curve fitting to determine inclusion population parameters


By fitting gaussian component populations to the observed baro-acoustic decrepitation data it is possible to derive reproducible and precise temperatures for the individual populations. Using the mode temperature of these component populations it is possible to compare and contrast different samples to assist mineral exploration and drill targeting.

These samples from the Hall deposit show quite complex combinations of fluid inclusion populations, often with 5 components required to give a good fit to the data. Most samples show a component with a temperature mode near 450 C as well as about 550 C. Note that the peak at 580 to 590 C is related to the alpha - beta quartz transition and is not used in interpretation (explanation here). A significant number of samples also have a distinct component with a low mode temperature near or below 400 C. This separate low temperature component can be used to classify the quartz into at least 2 types. Only sample 1188A had visible molybdenite, which was abundant, but most if not all of these samples are from potentially molybdenum bearing phases according to Shaver's classification scheme.

This plot summarises the individual fit results showing the mode temperature of each component, with the size of the peak (logarithmic area) represented by the circle diameter at each plotted point. Note the presence of 350-400 C peaks on the 4 leftmost samples, and the lack of this temperature on the rightmost samples.  

summary of fit temperatures
                (Sample SN_91 on this plot is actually SN_91-617)

This limited study of only 7 samples, together with the lack of location information for some samples makes it impossible to actually determine what decrepitation features might be used to directly pinpoint mineralized quartz veins. However, the great variations observed do suggest that fluid inclusion decrepitation data within porphyry systems has the potential to identify and outline features of importance which could facilitate exploration.

Individual sample fitting results

In these plots the green dots are the raw data points. The red curve is the best fit sum of the component skew-gaussian curves, each shown in blue. The "residuals" value indicates the goodness of fit, with low values being a better fit to the raw data.
The curve fitting was done with the program fityk and the summary plot was prepared with the program grace. (program information here)

Sample 1188A, run G1644  5 peak fit, residuals=23. Sample has low temperature components.
KQM1 zone, aplitic quartz monzonite, north stock, with abundant molybdenite mineralization.

fit with LT peak

Sample 1193A run G1658 5 peak fit, residuals=54. Sample has a very large and very low temperature component.
KC1 zone, chill zone of north stock. A molybdenite bearing phase, but barren at the sample point.
NOTE: This is a subsample at the same location as G1661 below.

fit with lt peak

Sample 1192A run G1655 4 peak fit, residuals=28. Sample has a small but distinct low temperature component.
Upper mine bench, sheeted quartz, north stock.

fit with lt peak

Sample 1193C run G1661 4 peak fit, residuals=54. Sample has only a very small low temperature component.
KC1 zone, Chill zone of north stock. A molybdenite bearing phase, but barren at the sample point.
NOTE: this is a subsample at the same location as G1658 above.

fit with weak lt peak

Sample 91-617 run G1544 4 peak fit, residuals=84. Sample has no low temperature component.
Sample location and zone are unknown, sample was from Shaver's museum collection.

fit with no lt peak

Sample 1189A run G1646 5 peak fit, residuals=35. Sample has no low temperature component.
South stock, upper mine bench, sheeted zone perimeter

fit with no lt peak

Sample 1195B run G1668 3 peak fit, residuals=63. Sample has no low temperature component.
KAP2 zone, aplitic porphyry, sheeted quartz zone of south stock

fit with no lt peak

This plot shows the decrepitation results of all the above fitted samples superimposed for comparison. The low temperature component is visible on samples 1188a, 1193a and 1192a but it is not obvious on 1193c, although the low temperature component was very small on 1193c. The other 3 samples had no low temperature component, but without the gaussian fitting they cannot be easily distinguished on this plot of unprocessed data.Samples 1193A (green) and 1193C (magenta) are from the same location, but a few metres apart. They give quite different decrepitation curves although they both have a low temperature component population. This indicates that local variability can be significant, perhaps because of overlap of successive fluid events.
combined plot for the 7 fitted samples

Sample information (opens in a new tab)

Summary (at top)

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