2.3 WEATHERING
37
Fig. 2.13. Diagrammatic section through a weathered sulfide-ore body. Secondary ore minerals are concentrated near the water table by oxidation and leaching in the upper gossan zone.
and Park, 1986, pp. 796-831). Four zones can be distinguished in the weathering profile
of such ores (Fig. 2.13). At the top is an upper leached zone, or "gossan," composed of
residual silica and limonite. This grades down into the "supergene" zone, adjacent to
the water table. Immediately above the table oxidized ore minerals are reprecipitated,
such as malachite, azurite, and native copper. Secondary sulfide ores, principally chalcocite, are precipitated in the lower part of the supergene zone. This grades down into
the unweathered hypogene ore, mainly composed of pyrite and chalcopyrite.
2.3.4.3 Uranium Roll-Front Deposits
Uranium ores occur in several settings. One of the most common is the roll-front ore
body. The origin of this type of deposit was once in dispute, but it is now generally agreed
that its genesis is intimately connected to weathering. Uranium roll-front ores occur in
the Colorado Plateau, Wyoming, New Mexico, and the Texas Gulf Coast of the United
States (Turner-Peterson and Fishman, 1986; Granger and Warren, 1969; Galloway and
Hobday, 1983; Hobday and Galloway, 1998). In these areas uranium mineralization occurs in rocks ranging in age from Permian to Tertiary. It is generally absent from marine and eolian sands. Typically it occurs in poorly sorted arkosic fluvial sands that are
rich in carbonaceous detritus. Plant debris and fossil tree trunks are not uncommon.
Pyrite and carbonate cement are typical minor constituents. The host sandstones are
generally cross-bedded with conglomeratic erosional bases and typical channel geometries. The usual ore body is in the form of a roll-front (Fig. 2.14). This is an irregular cupshaped mass lying sideways with respect to the channel axis. The sandstone on the concave side of the roll is highly altered and bleached to a white color. Pyrite, calcite, and
carbonaceous material are absent; matrix and feldspars are extensively kaolinized.
These observations have suggested to many geologists that mineralization occurred
37
Fig. 2.13. Diagrammatic section through a weathered sulfide-ore body. Secondary ore minerals are concentrated near the water table by oxidation and leaching in the upper gossan zone.
and Park, 1986, pp. 796-831). Four zones can be distinguished in the weathering profile
of such ores (Fig. 2.13). At the top is an upper leached zone, or "gossan," composed of
residual silica and limonite. This grades down into the "supergene" zone, adjacent to
the water table. Immediately above the table oxidized ore minerals are reprecipitated,
such as malachite, azurite, and native copper. Secondary sulfide ores, principally chalcocite, are precipitated in the lower part of the supergene zone. This grades down into
the unweathered hypogene ore, mainly composed of pyrite and chalcopyrite.
2.3.4.3 Uranium Roll-Front Deposits
Uranium ores occur in several settings. One of the most common is the roll-front ore
body. The origin of this type of deposit was once in dispute, but it is now generally agreed
that its genesis is intimately connected to weathering. Uranium roll-front ores occur in
the Colorado Plateau, Wyoming, New Mexico, and the Texas Gulf Coast of the United
States (Turner-Peterson and Fishman, 1986; Granger and Warren, 1969; Galloway and
Hobday, 1983; Hobday and Galloway, 1998). In these areas uranium mineralization occurs in rocks ranging in age from Permian to Tertiary. It is generally absent from marine and eolian sands. Typically it occurs in poorly sorted arkosic fluvial sands that are
rich in carbonaceous detritus. Plant debris and fossil tree trunks are not uncommon.
Pyrite and carbonate cement are typical minor constituents. The host sandstones are
generally cross-bedded with conglomeratic erosional bases and typical channel geometries. The usual ore body is in the form of a roll-front (Fig. 2.14). This is an irregular cupshaped mass lying sideways with respect to the channel axis. The sandstone on the concave side of the roll is highly altered and bleached to a white color. Pyrite, calcite, and
carbonaceous material are absent; matrix and feldspars are extensively kaolinized.
These observations have suggested to many geologists that mineralization occurred