Matthew Gaudet

The Lofdal carbonatite complex, Namibia, Africa, intruded into basement rocks of the Huab Metamorphic Complex in the late Neoproterozoic during rifting between the Congo and Kalahari cratons.  The complex is currently exposed in a basement inlier of the Congo craton called the Welwitschia inlier.  The Lofdal carbonatite complex consists of nepheline syenite and carbonatite as plugs and dykes, with associated breccias and fenites, an alteration rock.  The Main intrusion, the focus area of this study, consists of a sovite plug, intruding into overlying nepheline syenite body accompanied by associated contact phases, potassic fenite and siliciocarbonatite.  Potassic fenites are dominantly composed of potassium feldspar, and are metasomatised from nepheline syenite.  Siliciocarbonatites are dominantly composed of magmatic coarse-grained aegirine, calcite, and apatite and are interpreted to have formed from a residual carbonatite melt.

            A total of twenty-two representative end-member samples were collected from various lithologies at the Main intrusion.  Petrographic descriptions, whole rock geochemistry, and mineral chemistry analyses of these samples provides insight into the magmatic history of the Main intrusion, and the nature and origin of niobium and uranium mineralization.  Pyrochlore, a relatively common accessory mineral in potassic fenites and siliciocarbonatites, with a unit cell structure A₂B₂O₆Z is determined to be the main host mineral for niobium and uranium. The cores of pyrochlores from siliciocarbonatites are interpreted to have grown in a calm magmatic environment.  The rims of pyrochlores from siliciocarbonatites are uranoan pyrochlore and uranpyrochlore and are interpreted to have crystallized from a residual carbonate melt, rich in volatiles and silica, in the presence of magmatic fluids.  The growth of these rims is interpreted to have been enhanced by rapid crystallization of fluorapatite.  The siliciocarbonatites lithology, then, is interpreted to be late-magmatic in origin, while the potassic fenite  lithology is interpreted to be the result of an early homogeneous potassium-rich metasomatic event, overprinted by a later carbonate and sodium-rich heterogeneous alteration event.    Metasomatic fluids are interpreted to have originated from the carbonatite body, released during various stages of crystallization.

Keywords: Lofdal, carbonatite, pyrochlore, uranium, siliciocarbonatite, Fenite.
Pages: 110
Supervisor: Yana Fedortchouk / Scott Swinden