The Lower Carboniferous Um Bogma Formation in G. Um Hamd consists of argillaceous and
carbonate rocks. It could be subdivided into a lower siltstone-Fe, Mn ore member, a middle siltstonemudstone-
shale member and an upper dolostone-dolomitic limestone member. Thethickness of the lower
member increases in the western part of G. Um Hamd where the middle and upper members are thinnest.
The argillaceous and carbonate rocks display several primary sedimentary and secondary structures. Also,
the lower part of the upper member is highly fossiliferous. Generally, the argillaceous rocks are slightly
calcareous and variably highly ferruginous. Fe oxides and Mn- bearing minerals have several modes of
occurrence most of which indicate secondary origin. Texturally, these rocks are poorly to very- poorlysorted
and classified as mudstone, sandy mudstone, sandy siltstone, sandy claystone and muddy
sandstone. On the other hand, the carbonate rocks are represented by microsparite and dolomitized
microsparite, sparite and pel-oosparite. The Um Bogma rocks consist of a large assemblage of primaryand
secondary minerals that are allogenic and/or authigenic. They include non-radioactive, radioactive and
REEs- and base metals- bearing minerals in addition to native gold and silver. The lower and middle
members show relatively higher field radioactivity measurements. Diagenesis played a major role in
modifying the original textural and compositional characteristics of the rocks. It involved compaction,
cementation, replacement, alteration and dissolution.