5/21/2023 0 Comments Endview woods associationThese characteristics suggest that uranium was not initially precipitated within the cellular tissue, but instead arrived later in the silica-bearing groundwater that produced the opal. The highest reading (3800 CPM) was a specimen of fossil wood that had a prominent yellow streak on one surface ( Figure 30B). Radiation levels are relatively low in the silicified wood preserves cellular architecture (25 CPM), compared to samples where wood has been encrusted with or replaced by common opal (50 to 300 CPM). This quartz does not contain uranium or vanadium, suggesting that it represents late-stage mineralization. The diagenetic history of the fossil wood appears to have involved multiple episodes of silica deposition, as evidenced by the presence of fractures in the opalized wood that contain crystalline quartz. These characteristics suggest that uranium is present at trace levels within the opal-CT that is present both as a replacement for wood cells, and as botryoidal encrustations on wood surfaces ( Figure 30). The uranium contents of fossil logs are as high as 35 wt.%. During the 1950s uranium boon, in the American Southwest the ore in some small mines consisted of a single fossil log. Uranium mineralization may reach high levels in wood and organic debris. Relict organic matter may include intact logs ( Figure 1), or layers of “trash” ( Figure 2). The source of the dissolved uranium varies, but in the element has commonly been leached from trace amounts present in thick beds of volcanic ash. Uranium may also be present as organic complexes as well as inorganic minerals. Very commonly, the cause of precipitation is the halo of reducing compounds that surround decomposing organic matter. Under oxidizing conditions, uranium has relatively high solubility, and sedimentary ore deposits typically occur where uranium dissolved in groundwater encounters reducing conditions, causing the element to precipitate as “pitchblende” (uraninite, UO 2). The element is slightly more common than tin, 40 times more abundant than silver, and 500 times more abundant than gold. Uranium is present in small amounts in many geologic materials. Instead, the element is dispersed in trace amounts within the opal. Uranium-bearing common opalized wood and stratiform common opal from two prospects in Nevada, USA, contain no identifiable uranium minerals. New analyses utilizing scanning electron microscopy and X-ray fluorescence (SEM/EDS) reveal that fossil wood from oxidized ore zones may contain a diverse variety of uranium minerals, including carnotite, tyuyamunite, and zippeite, as well as various vanadate and sulfate minerals. These observations are applicable to primary ore zones that are located below the water table, where oxidation is inhibited. Previous microscopic observations of petrified wood from a few uranium mines have demonstrated that uranium in fossil wood primarily involves the oxide mineral uraninite or the silicate mineral coffinite, often in association with metal sulfides such as chalcopyrite. However, the mineralogy of uranium in fossil wood has received relatively little study. Uraniferous sandstone deposits commonly resulted when uranium in groundwater precipitated in reducing environments caused by degradation of ancient wood and organic debris.
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