NATURAL ARSENIC ENRICHMENT: EFFECTS OF THE DIAGENETIC-TECTONIC-HYDROTHERMAL CYCLE

Arsenic is only 41^st in abundance (2.5 ppm) in the bulk continental crust (Rudnick and Gao, 2005), but groundwater occurrences seem disproportionately common. An examination of high-arsenic and low-arsenic rocks confirms the overall geologic processes that mobilize and enrich arsenic naturally. Arsenic in continental crust is enriched compared to the bulk earth or the solar nebula (1.7 and 1.8 ppm, respectively) reflecting As partitioning during early differentiation of the earth. Arsenic is enriched from mantle to lower to middle to upper crust (0.066→0.2→3.1→4.8 ppm, respectively) which includes the effects of the rock cycle over billions of years. The average concentration of arsenic in surface seawater is only 1.7 μg/L, but the total mass of arsenic in the oceans is about 2,380 Tg. Thick marine sediments can contain substantial As. Arsenic is concentrated naturally by: (1) sorption onto fine-grained clays and sediments and precipitation of arsenian pyrite in strongly reducing environments undergoing sulfate reduction (carbonaceous marine shales), (2) leaching and expulsion by hydrothermal fluids from large volumes of shales, greywackes, and slates into low-porosity zones of faults and fractures at temperatures exceeding 100°C, (3) transport during hydrothermal circulation largely through sulfide complexation with gold to lower temperatures where precipitation as arsenian pyrite, arsenopyrite, realgar, and orpiment occurs, (4) weathering and erosion of arsenic-sulfide minerals exposed by uplift with transport to river deposits where continued redox cycling can occur and accumulation back into deltaic sediments. Arsenic from the Tibetan Plateau, Kamchatka, Alaska, Japan, New Zealand, Italy, and the Andes appear as hot springs and arsenic-sulfide mineralization resulting from large heat fluxes in plate collisions and substantial quantities of sediments. Tectonic hot spots mobilize arsenic in hydrothermal fluids depending on the amount of fine-grained terrestrial sediments or volcanic metasediments. Hence, Yellowstone National Park has high-arsenic hot springs but Hawaii does not. Iceland’s thermal waters have more arsenic than Hawaii but much less than Yellowstone. Tectonics, hydrothermal circulation, and diagenesis play important roles in the natural enrichment of arsenic.