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The sandstones are generally fine to very fine grained, of low porosity, <br /> well cemented, lenticular, discordant and crossbedded. Most sandstones <br /> at the mine show a well defined system of joints at outcrops. A 25 to <br /> 30 foot-thick channel filled sandstone can be observed in cross sections <br /> along Tuttle Draw near the county road. The "upper" sandstone unit of <br /> the Dakota (Young, 1973) is absent at the mine site. <br /> Conglomerates are absent in overburden strata. However, a 2 to 4 foot <br /> conglomerate marks the base of the Dakota in the underburden. This <br /> conglomerate is the "lower" lithologic unit of the Dakota described by <br /> Young ( 1973) . <br /> E. MINERALOGY. Iron disulfides (pyrite [FeS2] and marcasite [FeS2]) <br /> are commonly the most undesirable minerals present in overburden strata <br /> at the Nucla Mine site. These minerals are associated with coal-bearing <br /> rocks in which reducing conditions are prevalent at the time of <br /> deposition. When exposed to an oxidizing environment in the presence of <br /> percolating water, iron disulfides often react to form acid leachates . <br /> According to Caruccio et al . ( 1977) , the rate at which these acid <br /> leachates are produced are dependent on the quantity and grain-size <br /> distribution of pyritic material , availability of oxygen, presence of <br /> iron bacteria , amount of calcareous material and relationship to the <br /> ground water aquifer. <br /> Another mineral commonly encountered in overburden strata at the Nucla <br /> Mine is gypsum (CaSO4-2H2O) . Gypsum is significantly soluble in water <br /> and, according to Hounslow ( 1978) , "precipitates readily from solutions <br /> produced by the oxidation of pyrite coupled with the dissolution of <br /> limestone" . As gypsum occurs both as a primary and secondary diagenic <br /> mineral in strata at the mine site, a similar cycle of solution and <br /> precipitation can be expected in spoil materials . <br /> Quartz, feldspar and clay minerals occur as primary detrital minerals in <br /> sandstones and shales. Clays , shales and organic residues commonly have <br /> higher quantities of trace metals occurring as adsorbed ions than do <br /> coarser grained strata. These ions may be released into solution as a <br /> result of oxidation and low pH. Other less commonly occurring minerals <br /> 6-16 Revised 03/13/87 <br />