Metal dispersion and mobility in soils from the Lik Zn–Pb–Ag massive sulphide deposit, NW Alaska: environmental and exploration implications

Abstract

The Lik deposit in northern Alaska is a largely unexposed shale-hosted Zn–Pb–Ag massive sulphide deposit that is underlain by continuous permafrost. Residual soils overlying the mineralized zone have element enrichments that are two to six times greater than baseline values. The most prominent elements are Ag, Mo, P, Se, Sr, V by total 4-acid digestion and Tl by a weak partial digestion (Enzyme Leach or EL) because they show multi-point anomalies that extend across the entire mineralized zone, concentration ranges are 0.5–2.6 ppm Ag, 4–26 ppm Mo, 0.1–0.3% P, 3–22 ppm Se, 90–230 ppm Sr, 170–406 ppm V, and 1.6–30 ppb Tl. Lead, Sb, and Hg are also anomalous (up to 178 ppm, 30 ppm, and 1.9 ppm, respectively), but all are characterized by single point anomalies directly over the mineralized zone, with only slightly elevated concentrations over the lower mineralized section. Zinc (total) has a consistent baseline response of 200 ppm, but it is not elevated in soils overlying the mineralized zone. However, Zn by EL shows a distinct single-point anomaly over the ore zone that suggests it was highly mobile and partly adsorbed on oxides or other secondary phases during weathering. In situ analyses (by laser ablation ICP-MS) of pyrite and sphalerite from drill core suggest that sphalerite is the primary residence for Ag, Cd, and Hg in addition to Zn, and pyrite contains As, Fe, Sb, and Tl. The level and degree of oxidation, and the proportion of reacting pyrite and carbonate minerals are two factors that affected the mobility and transport of metals. In oxidizing conditions, Zn is highly mobile relative to Hg and Ag, perhaps explaining the decoupling of Zn from the other sphalerite-hosted elements in the soils. Soils are acidic (to 3.9 pH) directly over the deposit due to the presence of acid-producing pyrite, but acid-neutralizing carbonate away from the mineralized zone yield soilsthat are near neutral. The soils therefore formed in a complex system involving oxidation and weathering (mechanical and chemical) of sulphide minerals, dissolution of carbonate minerals, and precipitation of iron and manganese oxide minerals.

Regional Fluid Flow and Basin Modeling in Northern Alaska

“…In particular, stream sediment samples from drainages near the SHMS deposits contain anomalously high concentrations of Tl (>1.3 ppm) compared to those near the vein breccia occurrences (approximately 0.5 to 0.8 ppm). Soil samples collected over buried portions of the Su-Lik deposit also show elevated Tl directly over the ore body (Kelley and Kelley, 2003).…”

“…6.2). These elements, and mercury (Hg), molybdenum (Mo), and selenium (Se), are also elevated in soil samples at the Su-Lik deposit (Kelley and Kelley, 2003), suggesting that Tl, U, P, Hg, Mo, and Se may be additional pathfinder elements for SHMS deposits (Graham and others, in press).…”

Sedimentary exhalative (sedex) zinc-lead-silver deposit model

“…Stream-sediment samples (<0.2-millimeter [mm] and <0.5-mm fractions) associated with sedex deposits in cold semiarid settings in northwestern Alaska contain anomalous concentrations of many metals, including as much as 10 milligrams per kilogram (mg/kg) Ag; tens of mg/kg As, Cd, and Sb; hundreds of mg/kg Cu and Ni; thousands of mg/kg Mn, Pb, and Zn; and hundreds of thousands of mg/kg Ba (Theobald and others, 1978;Kelley and others, 1992). Soil that overlies mineralized rock, such as at the Red Dog and Lik deposits in the Brooks Range, Alaska, contains hundreds to tens of thousands of mg/kg Pb, hundreds to thousands of mg/kg Ba and Zn, tens of mg/kg Ag, and tens to hundreds of mg/kg Cu, but concentrations of other metals, including less than 2 mg/kg Cd, hundreds of mg/kg Mn, and tens of mg/kg Ni, are low relative to stream-sediment abundances (Briggs and others, 1992;Meyer and Kurtak, 1992;Kelley and Kelley, 2003;Kelley and Hudson, 2007). Soil overlying sedex deposits in warm semiarid settings in Australia similarly contains hundreds to thousands of mg/kg Cu, Pb, and Zn and tens of mg/kg Ag (Cox and Curtis, 1977).…”