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This chapter deals with the geologic controls on the distribution of arsenic in rocks.Specifically, it focuses on rare but geographically extensive paleohydrologic events that produced widespread arsenic enrichments in the earth's crust in the form of elevated contents of arsenic-rich (arsenian) pyrite. We summarize evidence documenting the existence of ancient large-scale hydrothermal fluid migration events in the central and eastern United States and discuss impacts on the arsenic content of aquifer rocks through which the fluid migrated. There are two specific geologic settings discussed. One is the midcontinent region, and the other is the Appalachian region (with a focus on coal in the Appalachian Basin). Evidence for a hydrothermal fluid flow event in the midcontinent comes largely from studies on the genesis of large zinclead deposits (the so-called Mississippi Valley-type or MVT ores) of the region that are hosted in Paleozoic carbonate rocks. These studies demonstrate that ore-fluids for MVT deposits (brines derived from adjacent sedimentary basins) were driven towards the craton by gravity flow during a late stage of the Ouachita orogeny about 270 million years ago. These warm brines migrated laterally for hundreds of kilometers. Arsenian pyrite was deposited along these fluid pathways. The same aquifers that were pathways for ore fluid migration are exploited today as drinking water aquifers, raising the possibility that given conditions favorable for release of arsenic from arsenian pyrite, drinking water arsenic concentrations might become elevated. Further east, Paleozoic sediments of the Appalachian basin were aquifers for westward migrating fluids during a late phase of the Appalachian orogeny (also about 270 million years ago). Coal beds were particularly favorable sites for deposition of arsenian pyrite. In marked contrast to the midcontinent flow event, the fluids involved in the Appalachian region were likely derived from deep-seated metamorphic processes and were not as saline as those in the midcontinent. We argue that in the southernmost Appalachians of Alabama, the metamorphic fluids impacting coal were related in time and composition to 128Chapter 5 metamorphic fluids that formed arsenic-bearing gold deposits in the metamorphic rocks to the east. Pennsylvanian bituminous coals of Alabama locally contain abundant arsenian pyrite, which also is enriched to a lesser extent in copper, selenium, molybdenum, mercury and antimony. Mining of Alabama coal has resulted in elevated arsenic content in stream sediments, and in non-potable (saline) waters produced during coalbed methane recovery, but arsenic enrichment in drinking water supplies is apparently localized to individual water wells. Invasion of metamorphic fluids into coal-bearing Pennsylvanian rocks was widespread throughout the Appalachian basin, and arsenian pyrite occurrences are also widespread but sporadic. Large fluid flow systems are not restricted to the examples described in detail in this chapter, and additional areas impacted b...