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Origin of coffinite in sedimentary rocks by a sequential adsorption-reduction mechanism.

Martin B. Goldhaber·B. S. Hemingway·A. Mohagheghi·R. L. Reynolds·H. R. Northrop
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Abstract

Coffinite is the dominant ore mineral in the V-U ores of the Tony-M mine in the Henry Mts mineral belt of the Colorado Plateau. This orebody was formed at a density-stratified solution interface between uranyl-ion-bearing meteoric water and a saline fluid which was locally reducing. The localization of U at this solution interface occurred by adsorption onto the surfaces of detrital minerals, this adsorption being related to the pH difference between the two fluids. Experimental evidence is presented showing that the adsorption facilitated the reduction of uranium to U(IV). This adsorbed, reduced uranium bonded with aqueous silica in the ore zone to form coffinite. The high concentration of silica (as a monomeric species) in the ore-forming solution stabilized coffinite in preference to uraninite.-R.A.H.

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Martin B. Goldhaber, B. S. Hemingway, A. Mohagheghi, R. L. Reynolds, H. R. Northrop. 1987. Origin of coffinite in sedimentary rocks by a sequential adsorption-reduction mechanism.. https://doi.org/10.3406/bulmi.1987.7975

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USGS1987

The solubilities of some major and minor element minerals in ground waters associated with a sandstone-hosted uranium deposit

Ground-water samples from 41 wells penetrating basal Oakville sandstone (Miocene) in S Texas were chemically analysed to identify chemical changes related to nearby U orebodies. The coverage included a 240 km 2 area which contains several fault-related U deposits. Factors affecting the hydrochemistry include: 1) relatively high permeabilities of buried fluvial-channel sediments; 2) upwards leakage of brines along growth faults into the aquifer; 3) development of a redox interface (Eh = 0 volts) within the aquifer; and 4) the semi-arid climate. Variations in the saturation index (SI) for chemically reduced minerals of U, As, Mo, Se and for associated minerals such as pyrite outlined the position of known deposits. The SI increases towards zero as the deposits are approached from updip distances of 3-4.5 km, then decreases again downdip. The radiogenic pathfinders Ra and Rn showed very strong anomalies with ore, but diminished to background levels at short distances from ore. A strong He anomaly is deflected in the direction of ground-water flow away from the ore.-R.A.H.

Bulletin de Mineralogie↗
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Zimbabweite, a new alkali-lead arsenic tantalate from St Anns mine, Karoi district, Zimbabwe

Zimbabweite (Na,K) 2 PbAs 4 (Ta,Nb,Ti) 4 O 18 , a new mineral with trivalent arsenic, has been recognized at St Anns mine, southeast of Miami, Karoi district, Zimbabwe, in kaolinized pegmatite and dump material. The mineral is honey yellow-brown in large crystals, which are as much as 1 x 1 x 2 cm, and is clear pale yellow in thin fragments. Oxidation results in darkening of the color. There is one excellent cleavage, {010}. Zimbabweite has a Mohs hardness of 5 to 5.5, a white streak, adamantine luster, brittle fracture, is not magnetic, and does not fluoresce under either short wave or long wave ultraviolet light. d m = 6.20(3) and d c = 6.16 g/cm3. Optically, the mineral is biaxial (+), 2V Z = about 80°. Mean reflectances in air for an {010} = X-Z cleavage plate are : 589 nm--16.6 %, 470 nm--17.7 %, 546 nm--17.1 %, 650 nm--16.4 %. The indices of refraction determined by immersion methods are all greater than 2.10. Optic axis dispersion is very strong with ν > r, a = Z, b = Y, and c = X. The mineral is very pale yelow and is moderately pleochroic and X = pale yellow brown, Y = light reddish brown, and Z =reddish brown. Zimbabweite is insoluble in common acids or bases. A chemical analysis yielded, in weight percent, Ta 2 O 5 46.5, As 2 O 3 26.5, PbO 15.0, Nb 2 O 5 4.8, Na 2 O 3.1, K 2 O 1.5, TiO 2 1.4, BaO 0.4, UO 2 0.3, Bi 2 O 3 0.2, H 2 O (total) 0.19, SnO 2 0.1, F 0.04, SrO 0.02, total 100.05. A calculated formula is : (Na 1.51 K 0.48 Ba 0.04 ) Σ2.03 Pb 1.01 (As 4.03 Bi 0.01 ) Σ4.04 (Ta 3.17 Nb 0.55 Ti 0.26 U 0.02 Sn 0.01 ) Σ4.01 O 18 . Zimbabweite is orthorhombic, space group is Ccma or Cc2a , with a = 12.233(2)Å, b = 15.292(2)Å, c = 8.665(2)Å, V = 1621. 0(4) Å 3 , with Z = 4. No structural relationship between zimbabweite and any other tantalate minerals is apparent.

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