SIMULTANEOUS PB-ISOTOPE AND TRACE-ELEMENT CHARACTERIZATION OF GALENA BY QUADRUPOLE-BASED LA-ICP-MS

Galena is a common ore mineral in volcanogenic massive sulfide (VMS) deposits, carbonate-hosted Mississippi Valley type (MVT), and diverse vein deposits. It remains widely used for Pb-isotope measurements to discriminate sources of ore fluids in these deposits. Trace-elements in galena analyzed by LA-ICP-MS have also shed light on processes related to episodic fluid input and redistribution of volatile- (Ag, Cd, In, Sb, Tl) and other low-melting-point chalcophile elements during metamorphism and deformation.

Whereas Pb-isotopes in galena are typically measured by TIMS or (LA)-MC-ICP-MS in order to obtain high-precision measurements on individual sample aliquots, Q-LA-ICP-MS could potentially achieve usable precision (e.g, 0.1%) on Pb-isotopes using pooled weighted averages for larger (e.g., n >20) populations. Ablation conditions for galena were optimized to produce a near-steady-state mass transfer to the Q-ICP-MS plasma: 0.35 J/cm² fluence, 2.5 Hz pulse rate, 60 µm diameter craters and 60 second ablation. The large Pb ion beams produced under these conditions allow short quadrupole dwell times to capture time-dependent fluctuations while at the same time maintaining good counting statistics. All data are externally standardized to a fragment of Broken Hill galena using Pb-isotope reference values of Stacey et al (1969) and trace-element concentrations measured in-house relative to USGS MASS-1. This approach provides near-perfect matrix-matching. A 40s background is sufficient to return to baselines and the high Pb/Hg (typically >1E6 cps) precludes any interference from ²⁰⁴Hg. Several important trace-elements (Ag, Cd, Sn, Sb, and Tl) can also be simultaneously analyzed.

Propagated errors on individual spots is typically between 0.3% to 0.6%. The Isoplot ‘Weighted Average’ routine (weighted by assigned 2σ error, and reported at 95% confidence) reduces these errors to <0.10% for ²⁰⁷Pb/²⁰⁶Pb, ~0.10% for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁶Pb and 0.12% for the ²⁰⁸Pb/²⁰⁴Pb ratios. Although these errors are still significantly larger than TIMS or MC-ICP-MS they are of sufficient precision for rapid reconnaissance studies. The validity of the method was tested by analyzing galena from a wide variety of well-known VMS and vein deposits in Canada, Germany, Ireland, and the USA.