Proceso de defuzzyficacion o concreción

The multi-phase inclusions used in these experiments contained liquid + vapour + one or more solid phases. A single solid phase was probably halite, but multiple solid phases may have included halite, sylvite, anhydrite, and other, unidentified, minerals. Final homogenisation of these inclusions was by both halite- and bubble-disappearance, but predominantly by halite disappearance (≈ 60 % of hypersaline fluid inclusions), with an average of 462°C (1σ = 54°C). (Table 7-1, Fig. 7-20). The bubble disappearance data has a very wide range, and rather irregular distribution, with an average of 385°C (79°C, 1σ).

Table 7-1. Summary of homogenisation temperatures of fluid inclusions from sample 9915. The maximum recommended operating temperature for the Linkam THMS 600 stage is 600°C; a small number of inclusions were noted that retained a small amount of halite at 600°C, and homogenisation is presumed to be by halite disappearance at 600-650°C.

Using the tables in Bodnar and Vityk (1994), the final Th of 461°C implies an

average salinity of 56 wt% NaClequiv and a range of 48 - 61 wt% NaClequiv (at 1σ). Given

the good repeatability for the homogenisation experiments, and a general

correspondence between Th and the percentage of crystals seen in the inclusion, the wide

spread in the data seems to be real. A slight discrepancy between the data for

Multi-phase inclusions 2-phase inclusions

Homogenisation by: Halite disappearance Bubble disappearance Bubble disappearance

Th max. 600°C 600°C 360°C Th mean 462°C 385°C 292°C 1 stdev 54°C 79°C 22°C Th min. 340°C 165°C 260°C Th mean + 1 stdev 516°C 464°C 314°C Th mean - 1 stdev 408°C 306°C 270°C

Multi-phase inclusions 2-phase inclusions

Homogenisation by: Halite disappearance Bubble disappearance Bubble disappearance

Th max. 600°C 600°C 360°C Th mean 462°C 385°C 292°C 1 stdev 54°C 79°C 22°C Th min. 340°C 165°C 260°C Th mean + 1 stdev 516°C 464°C 314°C Th mean - 1 stdev 408°C 306°C 270°C

homogenisation by halite disappearance and homogenisation by bubble disappearance is apparent. Although there is extensive overlap, the average is higher for halite

disappearance, with a tighter spread (Table 7-1, Fig. 7-20). It is possible that there are two separate, but overlapping populations of hypersaline fluid inclusions, one higher temperature, homogenising by halite disappearance, the other slightly lower in

temperature, homogenising mainly by bubble disappearance. However, both Vargas et al. (1999), Skewes et al. (2003) report both types of homogenisation in the same fluid inclusion population, and the apparent discrepancy is not large.

Homogenisation temperatures can also provide some estimate of trapping temperatures, if the fluid inclusions represent trapping in a 2-phase assemblage (ie boiling). In that case the homogenisation temperatures approximate the trapping temperatures. However, the petrographic evidence for a boiling assemblage in the hypersaline fluid inclusions from the Don Luis Porphyry is equivocal. The strict test for a boiling assemblage is the occurrence of liquid- and vapour-rich fluid inclusions randomly arranged in the same trapping plane (Roedder, 1984). This is difficult to satisfy in the samples used, as the trapping planes are commonly several hundred µms thick and cross and anastomose irregularly.

Fig. 7-20 Histograms of the distribution of homogenisation temperatures by halite dissolution and by bubble disappearance for hypersaline fluid inclusions from sample 9915

Another approach to this question is to ask whether the fluid could have been sub-critical at the time of trapping, on the basis of estimated depth of formation. A

Halite Disappearence 0 5 10 15 20 25 200 240 280 320 360 400 440 480 520 560 600 Halite Disappearence Temperature °C

F requen cy Bubble Disappearence 0 2 4 6 8 10 12 200 240 280 320 360 400 440 480 520 560 600 Bubble Disappearence temperature °C

F

requenc

paleodepth of < 2km is estimated from fluid inclusions in mineralised breccias at Río Blanco (Holmgren et al., 1988; Vargas et al., 1999), in units adjacent to, but slightly older than the La Copa Rhyolite. More recently, Skewes et al. (2003) published fluid inclusion data from the Donoso breccia, which is also slightly older than the Don Luis Porphyry, and located ≈ 1km to the east, in the Los Bronces section of the property. Here crystallisation pressures were estimated at ≥ 1 kbar. Although insufficient

geological evidence exists to give a precise paleodepth of crystallisation of quartz in the Don Luis Porphyry samples, it seems reasonable to estimate a trapping pressure of 0.5 Kbar, with an upper limit of ≈ 1 kbar.

Experimental work by Knight and Bodnar (1989) has shown that 30 wt% NaCl solutions have a critical pressure of 1.574 kbars at 820°C. Modelling by Shinohara (1994), based in part on the work of Knight and Bodnar (1989), has suggested that rhyolitic magmas exsolving fluids at depths less than 5.2 km. (1.3 kbars lithostatic pressure) must do so in the subcritical region (L+V, L+V+Halite in a high-salinity system). On the pressure correction tables for 40 wt% NaCl, from Bodnar and Vityk (1994), the critical temperature/pressure curve for homogenisation temperatures greater than 500°C is always greater than 0.5 kbars. While acknowledging the risk in using pressure corrections with salinities above those on which the tables were calculated (50- 60 wt% NaClequiv in the Don Luis Porphyry), the absence of experimentally verified

tables leaves little choice.

Additional evidence for sub-critical conditions is provided by the occurrence of fluid inclusions containing > 95 wt% halite (Fig. 7-17). Roedder (1984) and Campbell et al. (2001) noted that such inclusions have been observed, but are only possible if the solution was oversaturated with respect to NaCl at the time of trapping, ie. the halite had already crystallised when trapped.

Thus, the evidence suggests that the Don Luis Porphyry hypersaline fluid inclusions were trapped under sub-critical conditions, and the Th for these inclusions is a

reasonable approximation to Ttrapping.