Vein uranium deposits

Vein uranium deposits are epigenetic concentrations of uranium minerals, typically pitchblende and coffinite, in fractures, shear zones and stockworks. The uranium minerals are either the sole metallic constituents in the veins or, in polymetallic veins, they are accompanied by other metals, such as bismuth, cobalt, nickel, arsenic, silver and copper.

The deposits are hosted by (1) granitic or syenitic rocks (intragranitic veins), (2) rocks surrounding granitic plutons (perigranitic or peribatholithic veins), or (3) sheared or mylonitized, usually metamorphosed, sedimentary or igneous complexes (veins in shear and fault zones).

Intragranitic uranium veins are typically developed in highly differentiated granitic rocks, e.g., in two-mica leucocratic granites that were subjected to preceding alteration, such as albitization and desilicification (episyenitization). The deposits are spatially related to regional faults. The principal uranium minerals, pitchblende and coffinite, are commonly associated with sulphides and gangue minerals, such as carbonates, quartz, chalcedony, fluorite and barite. A region with such veins is the La Crouzille district of the Massif Central, France.

Perigranitic uranium veins are commonly developed in metasedimentary and metavolcanic rocks, at their contacts with intrusive granitic plutons. The host rocks are often cut by lamprophyre and aplite dykes. The deposits consist of subvertical veins, breccia zones, stockworks and irregular bodies spatially associated with major faults. The mineralization is either monometallic or polymetallic. The gangue minerals include carbonates (calcite, dolomite) and quartz. The wall rocks and the gangue in the vicinity of the uranium minerals are commonly hematitized. A region with perigranitic monometallic veins is the Příbram uranium mining district, Czechoslovakia. Typical regions with deposits of polymetallic type are the Jáchymov and Aue districts in Czechoslovakia and East Germany, respectively.

Uranium veins in shear and fault zones are typically developed in areas affected by repeated orogenic deformations, which have resulted in reactivation of major fault systems, and mylonitization and metasomatic alteration of the rocks that became the hosts for the uranium mineralization. A region with such deposits is the former Beaverlodge mining district in Saskatchewan, Canada.

A conceptual genetic model for vein uranium deposits usually includes the following factors. Formation of the veins is commonly related to late phases of orogenic cycles. Deposition of the ore-forming minerals takes place from fluids due to changes in the pH, Eh, pressure and temperature of these fluids. The ore-forming fluids can be of various types, including juvenile, post-magmatic, connate, diagenetic, ground and meteoric waters. The mineralization often occurs in several stages, in which the physical-chemical characteristics of the fluids are different. Localization of the mineralization is structurally and lithologically controlled. Interaction between the fluids and the host rocks and ionization effects of the radionuclides result in alteration of the wall rocks and of the vein material.

Classification of the deposits as of vein-type is based on the traditional morphologic, rather than genetic, criteria. From the associational point of view many of the deposits are closely associated with unconformities and, conversely, many uncoformity-related deposits exhibit, at least in part, features typical of veins. Therefore both deposit types could be classified as discordant hydrogenic uranium/polymetallic deposits.