Appendix 
For XRD analysis, random powder samples were analysed from 3 to 65º 2θ, speed 2º per minute, time constant of 1 s, and 1º mm slit-width using a copper radiation source in a Phillips 1130/90 diffractometer. Pure alunite and datable Mn-oxides detected by XRD analysis, were selected for 40Ar/39Ar analyses, and five to ten grains from each sample were also prepared for SEM (SE and BSE modes) and EMPA analysis. Although hollandite-group minerals are extremely hard and commonly well-crystallized, the samples analysed here represent loose aggregates of crystals usually up to 30 mm in size. These aggregates were thus difficult to polish without impregnation. Due to the very small quantity of material available (5 or 10 grains up to 200 mm in size) for XRD, microprobe and 40Ar/39Ar analysis, impregnation and polishing of samples was not a viable option. Unpolished Mn-oxide crystal fragments and polished thin sections containing alunite were analysed using a CAMECA SU-30 SEM-probe (Department of Geology, University of Chile). Analytical conditions were: 10 nA beam current, 15 kV accelerating voltage, and 2 to 20 mm beam diameter; natural minerals were used as standards. 

K-Ar procedures.
Monomineralic crushed samples (0.5-1gr), 180-250 mm grain size, were washed in distilled water and pure ethanol. Using a sample splitter, two representative fractions were obtained: one for K analysis by absorption spectrometry (AAS), and the other for Ar analysis by dilution mass-spectrometry. The mean of three aliquots analysed separately was used for age calculations. For Ar analysis, samples were fused in molybdenum crucibles within a Pyrex vacuum line (initial vacuum in the range 10-8 mmHg), using an induction furnace. In order to limit the release of sulphur-bearing gases, the samples were packaged in molybdenum foil together with degassed Cu-filings and heated to 800ºC. The complete degassing of alunite and jarosite at 800ºC was tested by heating samples to this temperature, collecting the Ar released, and then reheating to fusion and collecting a second aliquot of gas. Analyses of the two fractions showed that negligible gas was released above 800ºC, confirming complete degassing at this temperature. These results confirm those obtained by Itaya et al., (1996). Prior to heating, an 38Ar spike, whose volume had been determined and calibrated using international standards and inter-laboratory analysis, was added. The evolved gas, once purified, was concentrated in a collector containing activated carbon, which was then sealed, separated from the extraction line, and connected to the mass spectrometer line. Two aliquots of each sample were analysed using a partially automated MS10S mass spectrometer. Six analytical cycles were performed on each aliquot, and the mean ratios of 40Ar/38Ar and 38Ar/36Ar were obtained. The radiogenic, atmospheric and spike argon contents were calculated by isotope dilution. Sample ages were calculated using K decay constants published by Steiger & Jäger (1977).
Some K-Ar results were reproduced, within error, by 40Ar/39Ar analyses of the same samples; and corresponding 40Ar/39Ar data are reported. Because the jarosite samples contain Fe sulphate minerals (e.g. coquimbite and copiapite), which contaminate the very sensitive 40Ar/39Ar extraction line, jarosite-bearing samples were analysed exclusively by the K-Ar method.

40Ar/39Ar procedures and Irradiation.
Selected minerals are placed in a high purity aluminium disk (18.5x4.8 mm, diameter x high) which holds up to twenty-one different samples in small wells (3x3 mm, diameter x depth). In each well, one or two monitor grains of Fish Canyon Sanidine (FCS, 28.03±0.1 Ma., Renne et al., 1994) are placed together with the sample grains. Then the disk is sealed with an aluminium plate and sent for irradiation in the 5 MW Herald-type pool reactor (Comisión Chilena de Energía Nuclear, La Reina, Santiago, Chile). The samples, surrounded by a cadmium shield to minimize thermal neutrons, are irradiated for a period of 12 to 24 hours in a stable position inside the reactor (Position A-09). Upon return from the reactor, FCS monitors from each orifice are analysed by the total fusion 40Ar/39Ar method, and individual J factors are calculated for each grain, which represents an individual position in the disk. Due to the large neutron flux variation observed across the sample discs (10-12%), some J factor variability is observed due to random positioning of the FCs grains at the margins of the orifices. In order to reduce and account for within-orifice J factor variability, a “J-surface” is calculated across the disc by applying a 2-dimensional polynomial fit to the FCs analyses, using the x-y positions of the centres of the orifices. The average percentage difference between calculated J factors and individual factors measured on FCs grains is used as the percentage error for the disc and is applied to the calculated J factor for each orifice. Percentage errors of 0.4 to 0.7% are typically obtained. For studied samples, representative J-factor values are 0.00053340.0000032 (12 hrs irradiation) and 0.00127090.0000066 (24 hrs irradiation).
The samples are introduced into a Cu-disk with capacity for ten different samples, covered with a laser transparent potassium bromide disk, and introduced into a sample chamber, which is connected to an ultra high vacuum line. The sample chamber is covered by a zinc selenide window, which is transparent to the CO2 laser. The samples are analysed by incremental heating using an integrative lens which allows even heating of a plane of 6x6 mm (each sample hole has a diameter of 5mm). A line blank is analysed after each sequence of three heating steps. The noble gases are separated from active gases by a cold trap operated at -133 ºC and a ST101 getter operated at 2.2 A. Once purified, the noble gases are introduced into a high resolution MAP 215-50 mass spectrometer equipped with an electron multiplier. The isotopes 36Ar, 37Ar, 38Ar, 39Ar and 40Ar were analysed in 10 cycles, and the 36Ar/40Ar, 37Ar/40Ar, 38Ar/40Ar and 39Ar/40Ar ratios were calculated for time zero (moment of introduction of the gas into the spectrometer). The baseline is analysed at the beginning and end of the analysis, for each step, and subtracted from the peak heights.
Isotope fractionation and other kinetic effects are corrected using periodic analyses of air samples, from which a correction factor (discrimination factor, D) is calculated. A representative value of discrimination factor was 1.00400±0.00020. Correction factors for interfering nuclear reactions are determined using fluorite, synthetic kalsilite (potassium aluminium silicate) and Ca-S glasses and correspond to (40Ar/39Ar)K=0.0014057±0.0003348; (36Ar/37Ar)Ca=0.000333±0.0000012; and (39Ar/37Ar)Ca=0.000778±0.0000033, for the period during which these experiments were performed. 40Ar blanks tend to rise during the analysis of K-rich samples, but never exceed double the initial value. Typical blanks obtained during the analysis were as follows: 40Ar=3.06x10–16, 39Ar=1.56x10–18, 38Ar=2.88x10–19, 37Ar=9.04x10–19,  36Ar=9.55x10–19 mol.
Plateaus are defined using the approach of Fleck et al. (1977) and must comprise three or more sequential steps containing at least 50% of the total liberated 39ArK, concordant at the 2 error level. Inverse isochrons are plotted using individual steps, and eliminating those considered suspect on the basis of visual inspection, MSWD values (<2), and unreasonable 36Ar/40Ar axis intercepts. These steps generally correspond to low-temperature steps considered to represent mixed gas evolved from minor contaminant phases or late alteration of the primary minerals.

Cited references
Fleck, R. J., Sutter, J.F. & Elliot, D.H. 1977. Interpretation of discordant 40Ar/39Ar age-spectra of Mesozoic tholeiites from Antartica. Geochimica et Cosmochimica Acta, 41, 15-32.
Itaya, T., Arribas, A. Jr. & Okada, Y. 1996. Argon release systematics of hypogene and supergene alunite based on progressive heating experiments from 100 to 1000ºC. Geochimica et Cosmochimica Acta, 60, 4525-4535.
Renne, P.R., Deino, A.L., Walter, R.C., Turrin, B.D., Swisher, C.C., Becker, T.A., Curtis, G.H., Sharp, W.D. & Jaouni, A. 1994. Intercalibration of astronomical and radioisotopic time. Geology, 22, 783-786.
Steiger, R.H. & Jäger, E. 1977. Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36, 359-362.