A CASE STUDY OF USING COPPER ISOTOPE RATIOS FOR MINERAL EXPLORATION AT PEBBLE PORPHYRY CU-AU-MO DEPOSIT, ALASKA

We measured copper isotope ratios from the natural waters such as seeps, rivers, groundwater, and ponds to gain an understanding of weathering of subsurface Cu-bearing minerals that encompass the Pebble porphyry Cu-Au-Mo deposit property in Alaska. We hypothesize that copper isotope ratios can be used to determine active weathering of sulfide minerals because oxidative dissolution of Cu-rich sulfides results in solutions that have enriched δ⁶⁵Cu signatures. Therefore, enriched copper isotope ratios in surface and subsurface waters signify weathering of Cu-rich minerals at depth. Our data support this notion because it is evident on the Pebble deposit property that levels of the heavier δ⁶⁵Cu isotopes are higher in surface waters (δ⁶⁵Cu = 0.67‰ to 1.4‰) than in surface waters proximal to deposit (δ⁶⁵Cu = -0.52‰ to -0.01‰). Measurement of copper isotope ratios in natural waters aids in the exploration of new deposits and the dispersal of heavy metals during chemical weathering.