At the Caribou mine, near Nederland, Colorado, a vein of uraninite-bearing sulfide ore in monzonite is enveloped by an alteration halo up to four feet thick. Alteration began with the development of chlorite, calcite, epidote, and pyrite in pyroxene and biotite followed by an argillic phase of montmorillonite and kaolinite formation chiefly in plagioclase and orthoclase, and finally a sericitic and silicic phase strongly developed adjacent to the vein. The zonal arrangement is complicated by a sericitized zone lying outside the argillized zone. The alteration was reflected chemically by strong leaching of lime and soda, increases in potash and silica adjacent to the vein, and little change in magnesia and alumina.Deposition of vein minerals took place in two stages separated by a period of brecciation. In Stage A, quartz, calcite, and siderite were followed successively by pyrite, chalcopyrite, sphalerite, and galena. Uraninite was deposited early in Stage B, following gersdorffite and chalcedony. Sphalerite, chalcopyrite, pyrite, argentite, proustite, and native silver completed the sequence. Uranium leakage from the vein extended to the outer limit of altered wall rock, beyond lead and zinc dissemination. The metals appear to have been transported by ionic diffusion through porous wall rock, and penetration was controlled by the amount of porous rock formed by hydrothermal alteration, rather than by abundance and spacing of fractures.Uraninite at Caribou has characteristic colloform texture, and a colloidal origin appears probable. Sulfides showing similar forms are believed not to have had a colloidal origin. Measurement of the three best x-ray powder patterns of uraninite gave lattice constant values of 5.368 A degrees , 5.385 A degrees , and 5.398 A degrees . The variation may be due to differences in U 6 / total U ratio in the three different specimens.