ABSTRACT
There can be large variations in As concentration for any given rock type but the most important factor is the type and extent of hydrothermal alteration.
2 NATURAL ARSENIC ENRICHMENT OVER GEOLOGIC TIME
2.1 Enrichment in fine-grained sediments
Arsenic is concentrated naturally by sorption onto clay and silt size particles, environments that typically produce shales. Shallow marine environments often have a large organic loading which leads to sulfate-reducing conditions and pyrite formation. Arsenic is scavenged by pyrite (arsenian pyrite) with concentrations up to 10 wt.%. The As in these
1 INTRODUCTION
1.1 Arsenic in the earth
The bulk earth (and the solar nebula) has an approximate arsenic (As) concentration of 1.7 mg/kg (McDonough, 2003) which puts it about 52nd in abundance; however, As is not evenly distributed throughout the planet. In the upper continental crust As is 42nd in abundance at 4.8 ppm (Rudnick & Gao, 2003) and decreases with depth to 3.1 mg/kg in the middle crust, 0.2 mg/kg in the lower crust, and 0.066 mg/kg in the mantle. Hence, the conclusion that As was enriched in the upper crust during the formation and differentiation of the earth seems warranted although distinguishing original fractionation from evolutionary fractionation can be challenging. Further enrichment occurred over 4.55 billion years of geochemical cycling which is apparent from comparing arsenic concentrations in major rock types.
