Previous

Contents

Next

Length-slow chalcedony and origin of quartz geodes in Lower Eocene limestones, Gebel Duwi Egypt.
Ahmed M. ElShishtawy, Abd ElMonem T. Abd ElHameed, Anter A. Abd ElWahab1 and Aziz A. Abu Shama1

Geology Dept., Fac. Sciences, Tanta Univ., Egypt, 1Biology Dept., Fac. Education, Kafr ElSheikh, Egypt.

Quartz geodes in the Thebes Formation limestones (Lower Eocene), Gebel Duwi, Quseir area, are spherical to sub-spherical, 5 to 9 cm masses. Most of the geodes are lined with alternatively two (in parts three) zones:

An outer zone composed mainly of quartz, calcite and dolomite, and
an inner milky zone with knobby surface of carbonates and less quartz.

In some cases, sparry calcite in the quartz geodes interior forms the third zone.

Length-slow chalcedony spherulites, quartz rosettes, zebraic chalcedony, flamboyant and euhedral megaquartz containing anhydrite inclusions appear along the contact between the two zones. These varieties are excellent indices for paleosaline environment.

These quartz geodes are here suggested to have been formed by silicification of anhydrite nodules. This is based on presence of:
  1. Anhydrite inclusions in the middle zone of geodes and their absence in the outer one or matrix surrounding geodes;
  2. Length-slow chalcedony;
  3. Authigenic quartz fabric of replacement evaporite precursor; and
  4. Macrocell structure, inherited from chicken-wire anhydrite, in the milky middle zone of geodes.

Five stages of silicification of anhydrite nodules to form quartz geodes can be suggested. These include:

  1. Volume-by-volume replacement of anhydrite by silica;
  2. formation of quartz rosettes and megaquartz;
  3. development of isopachous quartz and drusy quartz;
  4. development of length-fast chalcedony spherulites; and
  5. development of large sparry calcite.