Resolución sobre la Declaración de inicio del proceso de independencia

The average geochemical compositions of the least-altered rocks and various alteration zones associated with the basaltic-andesite, basaltic volcaniclastic, porphyry andesite, hornblende-phyric andesite, quartz-phyric dacite and breccias are given in Appendix 1.

For the mass balance calculations only two major alteration zones: the advanced argillic zone, which may be subdivided into (i) kaolinitic and (ii) alunitic; and the siliceous alteration zone are considered. Mass balance calculations for the siliceous zone were mainly based on samples from polymic-monomict breccia and porphyritic andesite, whereas samples of the basaltic andesite, basaltic volcaniclastic and porphyritic andesite rocks were used for the advanced argillic zones. These alteration zones were chosen because they also represented the main ore host rocks. The other reason is, it was almost impossible to find ‘least altered’ example of polymict-monomict breccia because the clast and/or matrix were altered to clay and siliceous alteration. Thus, it was decided to use advanced argillic altered breccia as ‘the least-altered’ rocks to calculate a relative mass balance of siliceous altered breccia relative to advanced argillic.

The method of MacLean and Barrett (1993) was employed to calculate the mass balances. The selection of immobile elements is the most important aspect of the mass balance analysis. The potentially immobile elements (i.e. Al, Ti, Nb, Y and Zr) from

variably altered and unaltered samples are calculated, whereas the gains and losses of major element concentration are illustrated graphically (Figures 6.29, 6.30 and 6.31). The

correlation coefficient, r, provides an estimate of the relative immobility of one element with respect to another element with relation to any change in mass or volume in a sample suite (MacLean and Kranidiotis, 1987). The existence of highly correlated (i.e., r >0.85) trends that pass through the origin enables selection of the optimal element to be used as the immobile monitor in the mass change calculations. The graphical process also

highlights any samples from different precursors, which should be eliminated or treated separately. Nb and Zr have been shown to the relatively immobile during hydrothermal alteration although the ‘r’ was not highly correlated.

The single-precursor mass transfer method proceeds by calculating the ratios of the proportions of an immobile element in the altered and unaltered samples. Each of the mobile element proportions is then multiplied by that ratio to obtain a reconstructed composition (Figures 6.29, 6.30 and 6.31). The mass change of each element is found by subtracting its percent proportion in the precursor from that in the reconstructed composition. The steps and the flow chart outlining the procedure for estimating mass changes in single-precursor system are described in Gifkins et al. (2005).

The absolute mass composition change (∆a_{) is defined as the concentration change}

of each oxide/element referenced to its original concentration, and can be visualized in the graph, calculated using the following equation:

∆a _{= [Z}o_/Z a _{* C}a_{] – C}o

Where ∆a _{is absolute mass exchange expressed in g/100 g}

Ca_{= wt% proportion of component in altered rock}

Co_{= wt% proportion of component in precursor}

Za_{= proportion of immobile element in altered rock}

Zo_{= proportion of immobile element in precursor.}

The mass changes may be calculated from compositions of individual altered samples or from average compositions of sample groups representing certain mineral assemblages or alteration zones (Gifkins et al., 2005).

6.6.2.1 Basaltic volcaniclastic rocks

For the volcaniclastic rocks, the mass change of the major elements is calculated from the weakly altered kaolinitic altered zone to the strongly siliceous zone. The result can be obtained by reading the plotted histogram bar (Figure 6.29). From the weakly altered to the strongly altered zone, SiO2 and S were gained, whereas Al2O3, Fe2O3, MgO, CaO, and

Na2O were lost. Moreover, the trend of mass transfer for several component induced by

the progressive alteration can also be readily interpreted. With an increase in the alteration intensity, Al2O3 shows a progressive loss. After having been gained from ‘weak altered

quartz-dickite zone’, SiO2 was progressively lost towards the vuggy quartz zone. Fe2O3

underwent loss in the weakly altered quartz-dickite zone, but then was progressively gained toward to more intense of strongly altered advanced argillic and/or siliceous zones.

Figure 6.29. Bar graph showing estimated absolute mass changes of major elements in four samples of advanced argillic alteration from weakly altered quartz-dickite rocks to strongly/partly siliceous altered rocks in volcaniclastic. Sample numbers in legend box.

Large positive and negative net mass changes are mainly due to SiO2 and S gains,

or loss of Al2O3 in the samples (Figure 6.29). However, Al2O3 are lower in concentration in

siliceous altered rocks when compared to quartz-dickite altered rocks. Increasing S and decreasing Fe2O3 indicate a trend towards increasing concentration of a sulphide mineral.

The plagioclase breakdown may lead to the depletion of Al and K. These are consistent with the observed destruction of plagioclase in intensely siliceous altered rocks.

6.6.2.2 Basaltic- andesite and porphyritic andesite

Similar to absolute mass changes in the volcaniclastic breccia (Figure 6.30), net mass changes of basaltic- to porphyritic andesite involve SiO2 and S gains and a loss of

Al2O3. SiO2 underwent an initial loss in weakly kaolin altered zone, but then was

progressively gained toward massive quartz. Sulfur was also gained in similar order with S gained in volcanic breccia. Al2O3 initially underwent little loss in the weakly altered zone,

but then was progressively decreased towards siliceous rocks through advanced argillic zone. CaO2 exhibits the strongest depletion among the alkalis. This may reflect the

destruction of Ca-rich plagioclase at a rate greater than the depletion of K2O and Na2O,

which are incorporated into alunite and natroalunite, respectively. From the weakly altered zone to the siliceous alteration, Fe2O3, MgO and K2O were consistently and almost

constantly lost. -20 -10 0 10 20 30 40 50 60 70

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 BaO S NET 135_180 153_89 163_180 163_178 M ass ch an g e ( g /100 g r) Sample No

Figure 6.30. Bar graphs showing estimated absolute mass changes of major elements in ten samples of advanced argillic alteration from weakly kaolinitic altered to strongly/partly siliceous altered andesite. Sample numbers in legend box.

6.6.2.3 Polymict-monomict breccia

In comparison to the alteration related to basaltic volcaniclastic and basaltic- andesite and porphyritic andesite, alteration of the polymict-monomictic breccia displays more significant increases in mass. The mass increase would be even greater in siliceous- altered breccia, if it is normalized to ‘’the least-altered’’ rocks due to the complete

destruction of primary mafic minerals and plagioclase. The altered rocks have an increase in all major oxides, with an exception of Al2O3 and K2O which decrease (Figure 6.31). The

addition of Fe3O2 may correspond to the high abundance of hematite (after pyrite). Sulfur

in the rocks shows a high enrichment compare to the advanced argillic altered samples. This is consistent with the high abundance of pyrite and other sulfides. The changes in concentrations of the trace elements involve a moderate enrichment of Ba, which may be in barite that occurs as vug fill in siliceous rocks, and a slight increase in P2O5, possibly

reflecting the presence of phosphate minerals. The S decrease in weakly altered samples and is strongest in massive quartz corresponding to the presence changes from oxidation zone to more sulfidic alteration.

-30 -20 -10 0 10 20 30 40 50

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 BaO S NET 153_34 135_57 135_76 164_96 164_112 164_119 164_156 164_180 198_51 198_53 Sample No M ass ch an g e ( g /100 g r)

Figure 6.31. Bar graphs showing estimated absolute mass changes of major elements of breccia from least-altered to strongly/partly siliceous alteration. Sample numbers in legend box.

6.7 Discussion