The study aims to present new information and illustrations about sulfidic sedimentary rocks in
order to provide a better understanding of how the sulfide minerals might have been formed. It also
presents detailed information on the major and trace element contents of various sulfide phases
hosted within the sediments.
The K/T sediments at Gabal Gifata (Weina and Birbaya sections), hold different morphologic
patterns of sulfides including tubular burrow fills, flat sulfidic thin bands and films, fossil
replacement and fills, as well as disseminated grains and framboids. The sulfide minerals are
composed of pyrite, sphalerite and galena with minor greenockite set in a groundmass composed of
gypsum and clays with celestite.
These sulfide minerals are believed to have originated in reduced bottom-water conditions via
sulfate reducing bacteria after rapid burial of organic matter. The sulfide minerals seam to develop
through successive stages by segregation, filling and replacement processes in early and
intermediate diagenesis. The framboidal pyrite and its associations of fine pyrite crystals were
formed in early diagenesis by rapid growth. The sulfide minerals are arranged within the burrow
fills in concentric shells. The outer shells are composed of pyrite while the cores are composed
mostly of large sphalerite and in some cases with galena. The grain size and crystallinity of the
mineral crystals increase inwards through the shells suggesting progressive formation of the
sulfides from the margins to the centers. The minerals were crystallized in a sequence: pyrite,
sphalerite-galena in early (pyrite) and intermediate (sphalerite-galena) diagenetic stages. The
sulfidic thin bands are composed mostly of pyrite with subordinate sphalerite. The preservation of
biogenic cells by pyrite within these bands and the survival of pyrite within sphalerite suggest
earlier formation of pyrite by replacement of organic matter (in early diagenesis) followed by
sphalerite through cavity filling (in intermediate diagenesis).
The microprobe analyses of sulfides showed that the pyrite, which occurs close to the sphalerite,
displays much higher concentrations of the trace elements Zn and Ni than the pyrite in the outer
shells. Distinct chemical differences exist among the different colors of sphalerite.
Several observations suggest that benthic conditions during the deposition of black shales were
sufficiently O2-depleted as to exclude benthic organisms (anoxic/euxinic), and that a possibly sharp
rise in dissolved O2 levels coincided with the onset of the overlying sediments deposition, crossing a
critical threshold to dysoxic levels that allowed benthic rise.