Gold mineralisation at the Abu Marawat mine, central Eastern Desert of Egypt, is related to a system of
massive and sheared, milky quartz veins cutting a sequence of Neoproterozoic island arc metavolcanic/
volcaniclastic rocks and related banded iron formation (BIF). Sulphide-bearing quartz veins and related
hydrothermal breccia bodies display a range of textures including sheared, boudinaged and recrystallised
quartz, open space filling and microbreccia. These variable textures imply a complex history of crack-seal
mechanism characterising the relation between mineral deposition and a major N–S-trending shear zone,
during a late brittle–ductile deformation event which affected the area at about 550 Ma. Gold-base metal
mineralisation is associated with brecciation and fracturing of the iron ore bands, close to silicified shears
and related quartz veins. The auriferous quartz lodes are characterised by the occurrence of visible pyrite–
chalcopyrite ± pyrrhotite ± sphalerite ± galena mineralisation. Gold is refractory in pyrite and chalcopyrite,
but rare visible gold/electrum and telluride specks were observed in a few samples. Hydrothermal
alteration includes pervasive silicification, pyritisation, sericitisation, carbonatisation confined to a delicate
set of veins and altered shears, and a more widespread propylitic alteration assemblage
(quartz + chlorite + pyrite + calcite ± epidote).
Fluid inclusion petrography and microthermometric studies suggest heterogeneous trapping of a lowsalinity
(1.4–6.7 wt.% eq. NaCl) aqueous solution and a carbonic fluid. Evidence for fluid immiscibility
during ore formation includes variable liquid/vapour ratios in inclusions along individual trails and bulk
inclusion homogenisation into liquid and occasionally to vapour at comparable temperatures. The trapping
conditions of intragranular aqueous-carbonic inclusions approximate 264–378 C at 700–1300 bar.
Similar temperature estimates have been obtained from Al-in-chlorite geothermometry of chlorite associated
with sulphides in the mineralised quartz veins. Fracturing enhanced fluid circulation through the
wallrock and related BIF, allowing reaction of the S-bearing ore fluid with iron oxides. This caused pyrite
formation and concomitant Au precipitation, enhanced by fluid immiscibility as H2S partitioned preferentially
into the carbonic phase. The ore fluids may have originated from granitoid intrusions (likely
the post-Hammamat felsites, whereas gold and base metals might have been leached from the Abu Marawat
basic metavolcanics. |
