Uranium in submarine metalliferous deposits

Hydrothermal submarine metalliferous deposits, common in areas of the ocean floor with high heat flow, contain generally about 10 ppm U as an order of magnitude. The $\text{U}{}^{234}\text{U}{}^{238}$ ratio is in the majority of cases close to that of seawater; only in a few cases is it anomalously high. Anomalous $\text{U}{}^{234}\text{U}{}^{238}$ ratios are coupled with low U concentrations. These data are explained by a model where thermal water (essentially heated seawater) in its sub-bottom circulation often is unable to leach U from the basaltic oceanic crust; in fact, these thermal waters may in some cases lose U. When leaching of U from the basalt does take place, probably during shallow stages of the sub-bottom circulation, the resulting anomalous $\text{U}{}^{234}\text{U}{}^{238}$ ratio can be preserved in the hydrothermal deposit only if mixing with ‘seawater’ U is prevented.