Environmental geochemistry of shale-hosted Ag [sbnd]Pb [sbnd]Zn massive sulfide deposits in northwest Alaska: natural background concentrations of metals in water from mineralized areas

Abstract

Red Dog, Lik and Drenchwater are shale-hosted stratiform Ag Pb Zn massive sulfide deposits in the northwestern Brooks Range. Natural background concentrations of metals in waters from the undisturbed (unmined) Drenchwater prospect and Lik deposit were compared to pre-mining baseline studies conducted at Red Dog. The primary factors affecting water chemistry are the extent of exposure of the deposits, the grade of mineralization, the presence of carbonate rocks in the section, and the proportion of Fe-sulfide in the ore. Surface water samples from the Drenchwater prospect, which has pyrite-dominant mineralization exposed in outcrop, have pH values as low as 2.8 and high dissolved concentrations of metals including as much as 95 mg l −1 Al, 270 mg l −1 Fe, 8 μl −1 Cd, 10 μl −1 Pb, and 2600 μl −1 Zn, with As up to 26 μgl −1. Surface waters from the Red Dog deposit prior to mining were also acidic and metal-rich, however, dissolved metal concentrations in Red Dog waters were many times greater. The higher metal concentrations in Red Dog waters reflect the high Zn grades and the abundant sphalerite, pyrite, and galena that were present in outcrop prior to mining. In contrast,l despite significant mineralization at the Lik deposit, carbonate rocks in the section buffer the system, resulting in less acidic, mostly near-neutral pH values with low concentrations of most metals except Zn.