Timing of porphyry (Cu-Mo) and base metal (Zn-Pb-Ag-Cu)
mineralisation in a magmatic-hydrothermal system – Morococha
district, Peru
Appendix 1
Corresponding author:
Honza Catchpole1,#
#
Corresponding author:
Current address: Vale Exploration Canada Inc., 2060 Flavelle Boulevard, Mississauga, Ontario,
Canada L5K 1Z9
email: catchpole@gmx.de
phone: +1 416 357 2231
Authors:
Kalin Kouzmanov1, Aldo Bendezú1 , Lluís Fontboté1, Maria Ovtcharova1, Richard
Spikings1, Holly Stein2,3 , Lluís Fontboté1
1
Department of Earth and Environmental Sciences, University of Geneva, Rue des Maraîchers
13, 1205 Geneva, Switzerland
Kalin.Kouzmanov@unige.ch
Aldobendezujuarez@yahoo.com
Maria.Ovtcharova@unige.ch
Richard.Spikings@unige.ch
Lluis.Fontbote@unige.ch
2
AIRIE Program, Department of Geosciences, Colorado State University, Fort Collins, CO
80523-1482, USA
3
Geological Survey of Norway, 7491 Trondheim
Holly.Stein@colostate.edu
Analytical procedures
Sample preparation
Phlogopite and muscovite samples were picked from vugs within the quartz-rich
veins, and hand picked in pure ethanol under a 50x binocular magnification to obtain an
inclusion free and pure grain sample. For all other minerals, including monazite, titanite,
adularia, and muscovite samples, a bulk vein / manto / rock crushing and separation protocol
was used, after diamond saw separation of the host rock.
Monazite and titanite separation: the crushed and sieved monazite-bearing fraction
was separated from the sulphide dominated >3.3 g/cm3 fraction with the heavy liquid
methylene-iodide (MEI). A concentrate of the paramagnetic monazite and residual pyrite with
alien sulphide inclusions was consequently separated using a Frantz Isodynamic Magnetic
Separator (University of Geneva) with parameter settings for monazite as outlined in
Rosenblum and Brownfield (1996). Monazites (80 – 100 µm) were handpicked from a pyrite
dominated separate under a 50x binocular magnification and separated from attached fine
grained and partly intergrown muscovite in an ultrasonic bath. The same procedure was
applied for titanite including parameter settings for magnetic separation of titanite as outlined
in Rosenblum and Brownfield (1996).
Molybdenite and pyrite separation: a small hand-held drill was used to extract
molybdenite as a powder from the quartz vein sample; although the effort was made, it was
impossible to avoid small quantities of finely-intergrown pyrite when creating the
molybdenite separate. To try dating of polymetallic base-metal mineralization, two pyrite
samples were acquired – one from a massive pyrite body at Sulfurosa and one from the
polymetallic San Andrès vein (Manuelita). For the San Andrès, a vial with pyrite mineral
separate was provided to the AIRIE Program. For the Sulfurosa, a sawed rectangle, ca. 2 cm
on a side, of massive granular pyrite was provided and a separate was drilled from a single
region of the cube for the Re-Os analysis.
Adularia separation: a crushed and sieved quartz-adularia-bearing non-magnetic
concentrate was obtained after magnetic separation. A heavy liquid separation MEI was used
to separate the >3.3 g/cm3 fraction, and consequently repeated steps of diluted sodium
polytungstate (SPT) at 2.6 g/cm3 using a centrifuge to achieve a clean separation of adularia
from quartz. Small sulphide inclusions impeded a pure separate in some cases, so all samples
were handpicked in pure ethanol under a 50x binocular magnification to obtain an inclusion
free and pure grain separate. The samples were finally cleaned for several minutes in
deionised water in an ultrasonic bath.
Final separates were tested for purity by powder X-ray diffraction (Philips X’Pert
APD, University of Geneva) of a crushed aliquot of each grain separate. Additional aliquots
were used for thin section grain mounts of ~20 – 40 grains of each sample and checked for
alteration textures and compositional inhomogeneities using scanning electron microscope
backscatter
electron
imaging
(SEM-BSE;
CamScan4,
University
of
Lausanne).
Representative grain compositions were subsequently quantified by electron microprobe
analysis (Table 2).
U-Pb analytical methods
Sample clean laboratory preparation, mass-spectrometry and blank
Before dissolution fractions of 20-30 monazites and titanites of each sample were
placed into 3ml screw-top Savillex vials and were fluxed for about 1h in 3N HNO3 on a hot
plate at ca. 80C. The acid solution was removed and the fractions were rinsed several times
in ultra-pure water and acetone in an ultrasonic bath. Single to multi-grain fractions of
monazites and titanites were selected, weighed and loaded for dissolution into pre-cleaned
3ml screw-top Savillex vials and miniaturised Teflon vessels, respectively. After adding a
mixed
205
Pb-233U-235U spike (EARTHTIME, spike calibration described by Condon et al.
2010) and 120 l 6N HCL the Savillex vials with monazites were arranged into a Teflon
ParrTM vessel with 2 ml 6N HCl, and placed in an oven at 206C for 24 hours. The same
205
Pb-233U-235U spike was used for the titanites which were dissolved in 63 l concentrated
HF with a trace of 7N HNO3 at 206C for 2 days. After evaporation and overnight
redissolution in 36 l 6N HCl at 206C the titanite samples were brought up in 36 l 1N HBr.
For monazite Pb and U were separated by anion exchange chromatography (Krogh 1973) in
40 l micro-columns, using minimal amounts of ultra-pure HCl, and finally dried down with
3 l of 0.05M H3PO4. Titanite Pb and U separation was performed in two steps HBr
chemistry modified after Krogh, (1973). Uranium and Pb were collected in two different
beakers and dried with a drop of 0.05 M H3PO4.
The isotopic analyses were performed in University of Geneva on TRITON mass
spectrometer equipped with a linear MasCom electron multiplier. The SEM dead-time
(23.5ns) was determined and monitored periodically by measurements of NBS 982 for up to
1.3 Mcps. The linearity of the MasCom multiplier was calibrated using U500, Sr SRM987,
and Pb SRM982 and SRM983 solutions. The mass fractionation of Pb was controlled by
repeated SRM981 measurements (0.13 ± 0.02 %/amu 1-sigma). Uranium mass fractionation
is calculated in real-time using a 233U-235U double spike. Both U and Pb were loaded together
(for monazites) or separate (for titanites) with 1 μl of silica gel–phosphoric acid mixture
(Gerstenberger and Haase 1997) on outgassed single Re-filaments. Lead isotopes were
measured on the electron multiplier, while U (as UO2) isotopic measurements were made in
static Faraday mode or, in case of very low-U samples, on the electron multiplier. Total
procedural lab blank was determined to be 1.8 ± 0.5 pg common Pb for monazite and 5 ± 1 pg
for titanite, which was used in the reduction of the data and corrected with the following
isotopic composition:
206
Pb/204Pb: 18.30 ± 0.71%;
207
Pb/204Pb: 15.47 ± 1.03%;
208
Pb/204Pb:
37.60 ± 0.78% (all 1 sigma). Uranium blanks are <0.1 pg and do not influence the degree of
discordance at the age range of the studied samples. Therefore, a value of 0.05 pg +/- 50%
was used in all data reduction.
Data reduction, age reporting, and errors
Age calculations are based on the decay constants of Jaffey et al. (1971). The initial
statistics was done using the TRIPOLI program (Bowring et al. 2011), followed by data
reduction and age calculation using the YourLab xls program (Schmitz and Schoene 2007),
EARTHTIME project - http://www.earth-time.org/u-pb.html -, and applying the algorithm of
Ludwig (1980). Generation of concordia plots and averages are base on Isoplot/Ex v.3
(Ludwig 2005). All uncertainties are reported at 2 sigma level. All data are reported in Table
4 with internal errors only (including counting statistics, uncertainties in correcting for mass
discrimination, and the uncertainty in the common Pb composition). Weighted mean age and
the mean square of the weighted deviates (MSWD) are calculated according to York (1966;
1967), and reported at 95% confidence level. Concordia diagrams for all samples are shown
in Figure 9. The accuracy of the data was assessed by repeated analysis of the international
R33 standard zircon (Black et al. 2004), which was pre-treated by annealing–leaching, and
measured at an average
206
Pb/238U age of 419.08 ± 0.19 Ma (N = 27; MSWD = 0.70). In
addition, a synthetic NIGL solution (Condon et al., in prep) was measured and yielded an
external reproducibility better than 0.1% in 206Pb/238U date.
Re-Os analytical methods
The use of molybdenite as a single-mineral chronometer is based on its extraordinary
187
Re/188Os ratios (Stein et al. 2001; Stein et al. 2003). In some cases, other sulphides (e.g.
pyrite) may have molybdenite-like Os isotopic compositions, but Re and Os levels are one or
more orders of magnitude lower than molybdenite (Stein et al. 2000). For this study, Re-Os
analyses were performed on two pyrite samples (530 mg) and one molybdenite sample (29
mg). The Re-Os data for the molybdenite were acquired using a Carius tube dissolution and a
mixed Re-double Os spike. The double Os spike is especially useful for working with young
and/or low Re molybdenites (Markey et al. 2003) or sulphides with molybdenite-like isotopic
compositions (Stein et al. 2000). The double spike allows determination and correction for
common Os as well as a mass fractionation correction for measured Os isotope ratios. Mass
fractionation for Re was overcome with analysis by total evaporation. Similarly, the Re-Os
data for the two analysed pyrites were acquired via Carius tube dissolution and sample-spike
equilibration, but using single
185
Re and
190
Os spikes. Further analytical details are found in
Markey et al. (2003). The Henderson Reference Material (RM8599), available through NIST,
is run routinely (Markey et al. 2007). Isotopic ratios were measured by negative thermal ion
mass spectrometry (NTIMS) on a Triton machine housed within the AIRIE Program at
Colorado State University. Blank values associated with the molybdenite analysis are Re =
3.35  0.06 pg, total Os = 2.00  0.02 pg, and
187
Os/188Os = 0.230  0.001. Blank values
associated with the pyrite analyses are Re = 15.64  0.39 pg, total Os = 0.721  0.004 pg, and
187
Os/188Os = 0.182  0.002.
Infrared (CO2) laser 40Ar/ 39Ar analysis
Samples for
40
Ar/
39
Ar analysis were loaded into Cu foil packages, mounted inside a
silica glass tube. Fish Canyon Tuff sanidine was added as a flux monitor assuming a standard
age of 28.02 ± 0.28 Ma (Renne et al. 1998). The tube was irradiated for 3 hours in the
cadmium-lined in-core irradiation tube (CLICIT) facility of the TRIGA reactor at the Oregon
State University Radiation Center, USA. The reactor irradiation correction values (J-values)
for each sample were calculated by interpolation of sample position in the silica glass tube.
Samples were analysed via incremental heating using a MIR10 IR 30W CO2 laser and
a stainless steel extraction line coupled with a multi-collector Argus mass spectrometer (GV
Instruments), housed at the University of Geneva, and equipped with four high-gain (1012 Ω
resistivity) Faraday cups for the measurement of
36
Ar, 37Ar, 38Ar, and 39Ar, and a single 1011
Ω-resistivity Faraday cup for 40Ar measurements. The samples were placed in small pits in a
Cu planchette and step-heated by laser rastering the pit area to ensure even-heating of the
grains. Usual degassing consisted of 9 - 15 temperature steps. Blanks for all argon isotopes
were measured at the beginning of a sample measurement and after every second step
incremental heating experiment and comprise the following ranges: 36Ar: 0.7–5.2 x 10-17 mol;
37
Ar: 0.25–1.1 x 10-16 mol; 38Ar: 0.7–7.4 x 10-17 mol; 39Ar: 0.17–2.0 x 10-16 mol; 40Ar: 0.54–
1.4 x 10-14 mol. The released gas fraction was purified for six minutes using two hot getters
(0.45A and 1.45A). Purified gas was then allowed to diffuse for 50 seconds into the staticvacuum multi-collector Argus mass spectrometer. The high stability of Faraday baseline
measurements rendered it unnecessary to record baselines for each analysis. Isotopic
mass/charge ratios were collected using 12 cycles, which were then exponentially regressed to
determine the abundance of individual argon isotopes prior to fractionation in the source unit.
Age plateaus were determined using the criteria of Dalrymple and Lanphere (1971), which
specify the presence of at least three contiguous incremental heating steps with concordant
ages that constitute >50% of the total
39
Ar released during the step-heating experiment. All
ages were calculated using decay constants and isotopic compositions of Steiger and Jäger
(1977). Inverse isochron diagrams (McDougall and Harrison 1999) allow to test the
assumption made for the plateau ages where any trapped non-radiogenic argon has an
atmospheric composition (40Ar/36Ar = 295.5). Raw data was reduced and the ages calculated
using the ArArCALC Excel macro application (Koppers 2002). The mass discrimination
factor was calculated from analysed clean air-standards in double-air pipette mode (mass
discrimination factor: 0.989 ± 0.001 per atomic mass unit), including correction for peak
heights and background, the radioactive decay of
37
Ar and
39
Ar, as well as for nucleogenic
interference from Ca-, K-, and Cl-derived isotopes. Propagated systematic errors comprise
decay constant, mass discrimination, and J value uncertainties. All errors are reported at 2σ
level.
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