Coffinite formation

High amounts of Ca (and potentially Ra), Sr, and LREE were co-precipitated by Al-phosphates from the crandallite group, which crystallized in the clay halo. Clay minerals (mainly chlorite and to a lower extent illite) and Ti-oxide were found to have sorbed significant amounts of uranium. Sorption onto mineral surfaces was followed by the formation of coffinite, USi04-nH20, rimming clay, and rutile particles. 

Figure 3.37 Change in uranium existence forms in carbonate rich ground water and formation of crystalline and amorphous coffinite (USiO ) due to Eh decline. Pourbais diagram (Eh-pH) constructed for a system with content, mg-1of uranium 240, carbonates 200 and El SiO 60 (Galloway W. E., Kaiser W. R., 1980)...

At Montmassacrot (Fig. 5) the mineralized formations trend WSW-ENE and dip 60-80° to the north. They consist of very brecciated seams of disseminated pitchblende and coffinite (average grade, 0.8%o). Radon clearly indicates the mineralization with anomalous values close to the ore deposit. There is a good correlation between these results and those of the dry residual uranium content. 

At La Borderie (Fig. 6) the formations are similar (subvertical or dipping 70° to the north), but with a less brecciated seam. The mineralization occurs as coffinite (grade, 2.8 Too), mostly concentrated in the intersection of tectonic trends. 

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