Abstract

About 100 deposits of fossil black sandstone have been reported in the Rocky Mountain region. These deposits are in regressive sandstones of Late Cretaceous age and are in Alberta, Montana, Wyoming, Colorado, Utah, Arizona, and New Mexico. These deposits are similar to present black sand accumulations on beaches and most were probably formed on the back beach during storms.

Opaque iron-titanium oxides and zircon are the most abundant heavy minerals in the deposits but garnet, tourmaline, rutile, sphene, chromite, spinel, monazite, apatite, stauro-lite, kyanite, epidote, anatase, brookite, pyroxene, amphibole, chlorite, biotite, glauconite, allanite, and gold are also present. In Alberta, Montana, and Wyoming, where mineral suites, individual minerals, and rock fragments suggest volcanic sources were important, titaniferous magnetite and intergrown iron-titanium oxide minerals are the most abundant heavy minerals in the deposits. In Utah, Colorado, New Mexico, and Arizona, where mineral suites, individual minerals, and rock fragments suggest sedimentary rock sources were important, ilmenite and zircon are the most abundant minerals.

Although these deposits have large tonnages of heavy minerals both individually and as a group of deposits, they are probably not of economic value at present because of problems in mineral beneficiation. For example, in Montana and Wyoming iron-titanium oxide minerals contain too much titanium for iron ore and too much iron for titanium ore, and in Utah, Colorado, and New Mexico individual opaque minerals are so highly altered to fine aggregates of anatase and other secondary minerals that much of the TiO₂ in these fine aggregates would be lost in the sludge if standard concentration methods for black sand were employed.