Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
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Brief decrepitation summary of samples from the 20 IGES field trip F1

Escondida and Chuquicamata copper mines, Chile


Samples were collected at these mines during the field trip associated with the  Geochemical Exploration Symposium.  This data is a selection of relevant results.


 Sample descriptions, Escondida

The Escondida deposit is unusual for a porphyry copper in that it has an abundance of quartz veins - at least in the area of the pit we visited. (Quartz is the preferred sample medium for  fluid inclusion decrepitation.) However, the quartz veins contained abundant sulphides and the decrepitation results are strongly influenced by inclusions within the sulphides and perhaps by sounds accompanying oxidation of the sulphides during the analysis. All of the response above 600 C is due to sulphides, and some peaks below 600 C, particularly the narrow ones, are due to sulphides. The decrepitation begins just below 400 C and from this I infer that these quartz veins formed at a little below 400 C. I am unable, in these samples, to see the 3 different hydrothermal stages described by Padilla Garza, Titley and Pimental (paper handed out during the visit) due to the interference from sulphides. Microscope observation of these samples shows that separating the quartz from the sulphide would be impossible, even by chemical attack, as much of the sulphide is very fine grained and encased within quartz grains of about 300 microns across (the analytical size fraction). However, we can see there is no decrepitation in the temperature range 200 to 350 C, where we would see evidence of CO were it present. Porphyry copper deposits would not be expected to contain CO2 rich fluids and there are none at Escondida. We did not visit enough separate localities within the pit for me to see if it is possible to discern temperature gradations within the ore body.


 Sample descriptions, Chuquicamata

Chuquicamata is a more typical porphyry Cu deposit, with only rare quartz veins present. Samples h1459 (red) and h1465 (magenta) are of quartz and they do not decrepitate above 600 C, except for some minor included sulphides. Samples 1451 (green) and 1453 (blue) were of porphyry and these continue decrepitating up to 800 C due to fluid inclusions in the feldspars.  Decrepitation begins at about 400 C, so it seems that these veins may have formed at a temperature slightly higher than the veins in Escondida. Again we see no low temperature decrepitation and deduce that there were no CO2 rich fluids involved in this system. Although not shown here, two samples of chrysocolla from the exotic Mina Sur pit gave no decrepitation at all, which is to be expected from a deposit of supergene origin with fluids of only atmospheric temperatures and pressures.

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