NATURAL VARIATION OF MERCURY ISOTOPES IN HYDROTHERMAL ORE DEPOSITS DETERMINED BY MULTIPLE COLLECTOR INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY

We present data on the Hg isotopic system determined with a Nu Plasma MC-ICP-MS using cold vapor generation (Sn (II) reduction) and mass bias correction by Tl addition using a desolvating nebulizer. The cold vapor sample introduction has a >99% efficiency and generates a signal of 550 – 700 V/ppm at a sample consumption rate of 0.75 mL/min. This translates into a total transmission efficiency of ~0.1% even with the plasma ionization efficiency of only 20% to 30%. The internal precision (1 s) at 40 ppb sample concentration and ca. 10 min analysis time is 6 to 8 ppm for ²⁰²Hg/²⁰⁰Hg. The maximum external deviation using a standard-sample-standard bracketing regime is less than 20 ppm.  

Data are presented as ²⁰² Hg/¹⁹⁸Hg ratios relative to our in-house standard of cinnabar from Almaden, Spain. Cinnabar from California Coast Range silica-carbonate deposits and sphalerite from one MVT deposit were analyzed to test the hypothesis that variation in isotopic composition between individual deposits could possibly be due to different source rocks.  The compositions ranged from +0.37‰ to +0.59‰ and from –0.59‰ to +0.72‰, indicating that this might be a possible mechanism of fractionation in systems where boiling did not occur. To test the hypothesis that boiling can fractionate Hg isotopes within a single deposit, samples were collected from veins at depth up to the mineralized sinter at the paleosurface 325m above the veins at the epithermal Buckskin National mine, Nevada. The total variation in ²⁰² Hg/¹⁹⁸Hg ratios range from –2.48‰ to +2.51‰, the largest reported variation in the Hg isotope system. Cinnabar from mineralized sinter that formed above a boiling epithermal Au-Ag vein system in the Ivanhoe district, Nevada range from –0.69‰ to +0.05‰. This significant amount of fractionation is thought to be the product of boiling of hydrothermal solutions related to ore formation and partitioning of lighter Hg into the vapor phase during transport to the surface.