PRELIMINARY TEST OF MERCURY ISOTOPIC COMPOSITIONS AS A TRACER OF METAL SOURCE IN HYDROTHERMAL ORE DEPOSITS

The ability to make direct isotopic measurements of ore-forming metals has introduced the possibility for their use as tracers of metal sources in ore deposits. The isotopic composition of Hg varies by over 5 ‰ d^202/198Hg (relative to our in-house Almaden Hg standard) in a wide variety of hydrothermal ore deposits. This variation could be caused by differences in the isotopic composition of mercury in source rocks, by fractionation during volatilization and redox reactions that take place during ore deposition, or some combination of these factors and it might complicate the use of Hg isotopic compositions as a tracer of metal source. To test this possibility, we have compared isotopic compositions of ore and possible source rocks in three important ore deposit types where possible fractionation mechanisms are known: 1) epithermal Au-Ag veins and sinter where boiling took place; 2) silica-carbonate-type Hg deposits in the California Coast Range, where cinnabar deposition involved oxidation and 3) MVT Pb-Zn deposits, where Hg substituted in sphalerite without redox or boiling.

Epithermal ores (–3.14 to +2.49 ‰) and silica-carbonate-type ores (–1.05 to +1.32 ‰) have Hg isotopic compositions that show much larger variations than MVT ores (–0.49 to +0.57 ‰).  The large variations might reflect fractionation by boiling and oxidation in epithermal and silica-carbonate-type deposits.  Preliminary tests of Hg as an isotopic tracer were undertaken at the Buckskin National epithermal deposit, where unaltered andesite, felsic dike and phyllite basement rock have identical isotopic compositions of –1.3 ‰ in the middle of the epithermal ore range, as might be expected if the light and heavy ends of the range were produced by fractionation related to boiling. At the Mayacmas silica-carbonate Hg district meta-greywacke and serpentinite from the Franciscan Complex and Coast Range Ophiolite range from –2.44 to 0.01 ‰ compared to a range of –0.19 to +1.25 ‰ for ores, as might be expected if kinetic isotope effects related to oxidation concentrated heavy isotopes in the ore. These results suggest that boiling and redox reactions can produce fractionation of more than 5 ‰ for mercury in hydrothermal systems, and that isotope analyses will be most useful as tracers in deposits where these processes did not take place.