THALLIUM: A PATHFINDER ELEMENT FOR SHALE-HOSTED SEDEX ZN-PB-AG DEPOSITS, BROOKS RANGE, ALASKA

Thallium concentrations of mineralized whole rock and stream sediment samples from three large SEDEX Zn-Pb-Ag massive sulfide deposits hosted in the Mississippian Kuna Formation were compared to data from four high grade but volumetrically small Zn-Pb-Ag vein-breccia occurrences that are hosted in stratigraphically underlying clastic rocks. Mineralized samples contain the following Tl concentrations: Red Dog deposit - 0.7 to 521 ppm (n=22, mean=59 ppm), Su-Lik - 18.8 to 172.0 ppm (n=7, mean=99 ppm), and three mineralized samples from Drenchwater contain 3.6, 6.6 and 15.6 ppm. In contrast, regional unmineralized shales and vein-breccia samples contain maximum concentrations of 1.84 and 0.6 ppm Tl, respectively.

In situ trace element analyses of sphalerite, galena, and pyrite were obtained using the inductively coupled plasma-laser ablation-mass spectrometer. Results indicate that Tl resides predominantly as lattice substitutions in pyrite. Thallium concentrations in pyrite are as follows: Red Dog ore samples – 2 to 12,220 ppm Tl (n=102, mean=448 ppm), Su-Lik ore samples – 160 to 1,580 ppm Tl (n=14, mean=600 ppm), and mineralized Drenchwater samples – 2.7 to 3,434 ppm Tl (n=88, mean=831). Pyrites from the vein breccia occurrences contain less than 1.5 ppm Tl.

Data from stream sediment samples collected in the vicinity of the SEDEX deposits mimic the whole rock results, with elevated Tl concentrations (2 to ~15ppm) in proximity to mineralized areas, typically with corresponding high concentrations of Zn and/or Pb. Samples from streams flowing through the Su-Lik deposit area show elevated Tl contents up to 200 meters upstream and Tl+Zn+Pb at and downstream from the shallowest (within 45m of the surface) portion of the buried deposit.

This study shows that Tl concentrations can be used to distinguish potentially economic shale-hosted deposits from uneconomic vein breccia occurrences in northern Alaska. Anomalous concentrations of Tl are recognized in many SEDEX deposits worldwide; however, this study is one of the first to report Tl variations in stream sediment samples. The data suggest that Tl concentration can be used, along with Zn and Pb to indicate proximity to this type of SEDEX deposit.