C-O Isotopes from the supergene Zn nonsulfide deposit of Reef Ridge, Alaska.

The Reef Ridge (RR) prospect is a typical supergene "nonsulfide" zinc mineralization, located in the Yukon-Koyukuk region of west central Alaska (USA), hosted in Lower-Middle Devonian shallow water dolomites. The useful mineralization, consisting of Zn carbonates (smithsonite) mixed with Fe-(hydr)oxides (goethite and hematite) and remnants of sulfides, commonly occurs in the oxidation zone. A geochemical study on the stable C-O isotopes of RR smithsonite and dolomite has been carried out, in order to define the genetic constrains of the supergene mineralization. In this study the results of the isotopic analyses are briefly summarized, in order to re-consider the "traditional" interpretation on the origin of Zn nonsulfide deposit in warm, semi-arid environments. Seven concretionary smithsonite and thirteen dolomite fragments were separated by mechanical hand picking, and analyzed at the University of Erlangen-Nuremberg (Germany) using mass spectrometry. Carbon and oxygen isotope values are reported in ‰ relative to V-PDB and VSMOW respectively, by assigning a δ13C value of +1.95‰ and a δ18O value of -2.20‰ to standard NBS19. Reproducibility was checked by replicate analysis of laboratory standards (>± 0.07 ‰ 1σ for both C and O). Oxygen isotope values of dolomite and smithsonite were corrected using the equation described by [1]. The results show that the δ18O values of the host dolomite are between 25.5 and 28.1‰ V-SMOW, whereas the δ13C ratios are relatively constant in the range from 0.3‰ to 1.6‰. The δ18O values of smithsonite are comprised between 19.1 and 21.9‰ VSMOW. On the contrary, the δ13C values are more variable, ranging between –0.7 and 2.1‰. The δ18O and δ13C values of dolomite have the Middle Devonian seawater signature, whereas those of RR smithsonite are quite different from the values of most supergene smithsonite worldwide. The majority of the δ13C values for smithsonite are similar to those of the host rock, suggesting that the prevailing C source was in the host carbonates, with limited contribution from organic carbon; whereas the δ18O values suggest fluids of meteoric origin. Their strong depletion in δ18O can be considered normal at sub-arctic/arctic latitudes where RR is located. Using the oxygen isotope fractionation equation between smithsonite and water [1], the formation temperature of this Zn carbonate can be approximated. Using the δ18O values of meteoric waters in the vadose zone (~ -14 and ~ -15‰), which are precipitated during the summer season (we hypothesized that no precipitation can happen at winter temperatures), the calculated precipitation temperatures would be in the range of 5 to 10° C. There are two main hypothesis for the age of smithsonite precipitation. Assuming a climate with temperatures similar to today, Zn carbonate could have precipitated from cold water during the summer in Late Tertiary. Alternatively, the supergene nonsulfides may have formed from local meteoric waters at very low temperatures, possibly related to cold/humid episodes during the warmer Pliocene or Pleistocene-Holocene.. The “traditional” interpretation of Zn nonsulfide deposit formation in warm (semi-arid?) conditions should be questioned where other climate zones are indicated. This applies not only to the nonsulfide prospects in Alaska, but also to other “oxide” deposits at high latitudes.