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In the Sunday Group and Van 4 deposits, uranium mineralization is associated with <br />enrichments of vanadium. Primary vanadium minerals include montroseite • <br />[Vo.7FeO(OH)], doloresite (H8V6016) and other vanadium oxides (Weeks and Thompson <br />1953). <br />In the presence of vanadium, U(VI) combines with vanadium to form various vanadate <br />minerals. The two most important vanadate minerals are carnotite (K2(U02)2V208.3H20) <br />and tyuyamunite (Ca2(UO2)2V208.5-8H2O) (Garrels and Christ 1956). Other important <br />minerals in this group include rauvite (Ca(UO2)V10O28.16H2O) and uvanite <br />(U2V6021.15H20). The yellow to greenish-yellow minerals observed as oxidation rinds <br />on areas of mineralized rock at the Sunday Group and Van 4 mines are commonly <br />identified as carnotite by the field geologist, but this mineral may be tyuyamunite or <br />other uranium-vanadium minerals with uranium in the U(VI) oxidation state. Weeks <br />and Thompson (1953) identified 43 different uranium and vanadium minerals in the <br />Colorado Plateau, so field identification of the specific uranium-vanadium minerals is <br />difficult. The electron microprobe is a good tool to facilitate identification of the various <br />uranium-vanadium minerals. <br />Deposition of uranium deposits is thought to result from reductive precipitation of <br />uranium from solution as uranium enriched groundwater percolates through permeable <br />sandstone units such as the Top Rim Sandstone. The presence of organic matter within <br />the sandstone may contribute to the reducing conditions necessary for precipitation of <br />uranium. Other metals are commonly enriched in addition to uranium within these <br />deposits. These metals include vanadium, molybdenum, copper, and arsenic (Rose et al. • <br />1979). In addition, pyrite (FeS2) may be precipitated under these conditions if the <br />concentrations of iron and sulfur are high enough in the percolating groundwater. <br />The mineralogical occurrence of arsenic is an important consideration in understanding <br />the potential bioavailability of arsenic at the Sunday Group and Van 4 mines. Arsenic <br />enrichment is associated with uranium-vanadium mineralization, and therefore it is <br />likely that the arsenic is associated with either the uranium-vanadium minerals or <br />minerals such as; pyrite that occur in association with the uranium-vanadium minerals. <br />Arsenic commonly substitutes for iron in the pyrite mineral lattice and it is not <br />uncommon for pyrite to contain trace quantities of arsenic. Uranium (VI) can also form <br />arsenate minerals if the concentration of arsenic in the deposit is high enough. Garrels <br />and Christ (1956) describe the crystal structure of the uranium vanadate and uranium <br />arsenate minerals as similar sheet structures in which a sheet of uranyl ions is <br />interlayered with sheets of vanadate or arsenate ions. Arsenic at the Sunday Group and <br />the Van 4 mines occurs at very low concentrations, so uranium arsenate minerals may <br />not occur. However, it is possible that arsenic may substitute in the mineral lattice or be <br />adsorbed with the various uranium vanadate minerals. <br />1.2 Objectives <br />The objectives of the Electron Microprobe analysis included: <br />• Determination of the mineral phases with which arsenic is associated. • <br />3 <br />T:\64986-Denison Mines\Task Order 3 - DMO Sampling and Analysis Plan\Task 3.12 - Soil, Ore, Rock Data Assessment Report\Bioavailability\DcnisonEMPText02l7O9 (2).doc