Electronic Supplementary Material
ESM 6. Analytical method
Zircon U-Pb dating
Dolerite of sample DC-13 is medium-grained and consists mainly of plagioclase
and clinopyroxene with minor olivine and Fe-oxide (Fig. 6c). Rocks of the dyke have
SiO2 (45.4-46.5 wt%), MgO (5.87-7.69 wt%), Al2O3 (14.5-16.0 wt%), Fe2O3
(12.9-15.9 wt%) and CaO (5.44-10.6 wt%) with high Na2O (2.72-3.27 wt%) and K2O
(0.85-1.5 wt%) (Zhao et al. 2010). They have chondrite-normalized REE patterns
enriched in LREE relative to HREE, and show no anomalies of Nb and Ta on the
primitive mantle normalized spiderdiagram, a pattern similar to basalts of within-plate
tectonic setting
(Zhao et al. 2010).
Zircons were separated using standard gravity and magnetic techniques and
screened under a binocular microscopeand mounted in epoxy mounts together with
standards TEMORA 2 and 91500. They were polished to approximately half their
thickness, photographed under transmitted and reflected light microscopy and imaged
using the cathodoluminescence (CL) technique to reveal their internal structures.
Measurements of U, Th and Pb isotopes were conducted using a Cameca IMS 1280
SIMS at the Institute of Geology and Geophysics, Chinese Academy of Sciences
(CAS) in Beijing. Detailed analytical procedures have been described by Li et al.
(2010). The O2− primary ion beam was accelerated at 13 kV, with an intensity of ca. 8
nA. The ellipsoidal spot is about 20×30 μm in size. Each measurement consists of 7
cycles, and the total analytical time is ca. 12 min. Pb/U calibration was performed
relative to the 1065 Ma standard zircon 91500 with Th and U concentrations of ca.29
and 81 ppm, respectively (Wiedenbeck 1995). Correction of common lead was made
by precisely measuring 204Pb.
Zircons from sample YN11-05 were analyzed using a Geolas 193 nm ArF
Excimer laser ablation (LA) system equipped with an Agilent 7700x inductively
coupled plasma mass spectrometer (ICP-MS), at the Institute of Geochemistry,
Chinese Academy of Sciences, Guiyang, China. Ablation time was about 45 s for 225
pulses of each measurement, with a 5 Hz repetition rate on a stationary spot of ~32
μm. Helium was applied as a carrier gas. Harvard zircon 91500 and NIST 610 glass
were used as the external standards to normalize isotopic discrimination and element
concentrations, respectively. Standard zircons GJ-1 and PL were tested for quality
control. Data reduction was performed off-line by ICPMSDataCal (Liu et al. 2010)
and then processed using the ISOPLOT program (Ludwig 2003).
Sulfide Re–Os dating
Chalcopyrite and bornite separates from the stratiform ores were used for Re–Os
dating. All the samples were collected as 3-5 kg grab ores at different levels of the
underground workings of the Yinmin mine. Samples were examined on both
hand-specimen and polished thin sections before separation of chalcopyrite and
bornite, and samples free of supergene effect were used for isotopic analysis.
Chalcopyrite and bornite were separated using conventional method by heavy liquid
and handpicked under a binocular microscope. Re–Os isotopic analyses were
performed in the State Key Lab of Ore Deposit Geochemistry, Institute of
Geochemistry, Chinese Academy of Sciences, Guiyang.
The analytical procedures of Re–Os isotopic analysis were described in detail by
Qi et al. (2010), and briefly summarized here. 0.5-1g chalcopyrite or bornite separates
were accurately weighted and placed in a 120 ml Carius tube, which was then
connected to a distillation system. HNO3 was slowly injected and vigorously reacted
with sulfides. The speed of the reaction was controlled by the injection speed of
HNO3. The released gas (containing Os) was liberated through the PTFE tube and
then trapped by 2 ml 10 mol L-1 HCl in a glass tube cooled by an ice-water bath. After
the sulfides were totally reacted with HNO3, the HCl solution containing all the
released Os was transferred back to the Carius tube and appropriate volumes of
weighed rhenium and osmium spikes were added. The sample was further digested in
aqua regia at 200ºC for 10 hours. Os was separated from the matrix by distillation
again. The residual solution was transferred to a 125 ml Savillex Teflon beaker and
evaporated to dryness twice with 5 ml of concentrated HCl to remove HNO3. The
resultant solution was then dissolved with 10 ml 2 mol L-1 HCl for separating Re from
the matrix using anion exchange resin. Re and Os analyses were conducted on an
ELAN DRC-e ICP-MS, and total blanks of Re and Os were 6.4 pg and 2.0 pg,
respectively. Iridium was added to Re and Os-bearing solution for mass
discrimination correction according to Schoenberg et al. (2000). Absolute
uncertainties are reported at the 2-sigma level. All uncertainties are determined by
error propagation of uncertainties in Re and Os mass spectrometer measurements,
blank abundances and isotopic compositions, spike calibrations. Molybdenite standard,
HLP (Du et al. 2004), was used as reference material to monitor analytical
procedures.
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