Pyrite rich sandstone, weathering

Computer simulation of the weathering of a pyrite rich sandstone ... 

Figure 13. Simulation of the genesis of a roll-type uranium deposit and of the weathering of a pyrite-rich sandstone. Twenty boxes model at logi = —1 with Vx = 10, In the upper part we present an instant view of the alteration profile with neoformed minerals. Numbers in brackets correspond to the molar percentage of the neoformed minerals vs. total neoformed minerals in each box. In the lower part we present the major physicochemical parameters PE, pH, and [O2] (mol/1) in the percolating water in the above boxes. The scale is the same in the upper part and the lower part. IPW = initial percolating water.

Gourmelon C., Nichard G., and Boulegue J., Netastable equilibria in sulfide rich waters. Application to a computer simulation of the weathering of pyrite rich sandstones and sulfide ore formation, p. 56-65, in "Proceedings of 2nd Intern. Symp. on Water-Rock Interaction , vol. 4, Sciences Geologiques, Uni-versite de Strasbourg, 1977. 

If pyrite is the source of arsenic in the Marshall Sandstone aquifer, its oxidation must have occurred at surface or near-surface conditions. Weathering of detrital Marshall Sandstone and Michigan Formation shale in tills could explain the presence of arsenic-rich wells in glacial aquifers. However, high levels of arsenic are more common in the bedrock aquifer than in glacial aquifers, and in both aquifers, the distribution of such wells is scattered. Bedrock samples show the distribution of authigenic pyrite to be very patchy, and not all of the pyrite is arsenic-rich. Therefore, even in the case of pyrite oxidation at depth in a bedrock aquifer, we would expect local... 

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