Zoheir and Akawy reported in 2010 that 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). Sulfide-bearing quartz veins and related hydrothermal breccia bodies display a range of textures including sheared, boudinaged and recrystallized quartz, open space ï¬lling and microbreccia. These variable textures imply a complex history of crack-seal mechanism characterizing 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 siliciï¬ed shears and related quartz veins.
The auriferous quartz lodes are characterized 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 siliciï¬cation, pyritization, sericitization, carbonatization conï¬ned 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 low salinity (1.4–6.7 wt.% eq. NaCl) aqueous solution and a carbonic fluid. Evidence for fluid immiscibility during ore formation includes variable liquid/vapor ratios in inclusions along individual trails and bulk inclusion homogenization into liquid and occasionally to vapor 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 sulfides in the mineralized 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.
The authors concluded that based on their ï¬eld, microscopic and fluid inclusion studies, the Abu Marawat gold deposit formed as a result of interaction between iron-rich host rocks and structurally focused hydrothermal fluids under mesothermal conditions. The nature and mechanism of the ore fluids and formation conditions of the Abu Marawat gold deposit are similar to those of the Archaean BIF-hosted mesothermal lode-gold deposits. The presence of refractory gold in pyrite and chalcopyrite may evoke similarity to the orogenic gold deposits in the Eastern Desert and elsewhere.
Based on the fluid inclusion data and the high geothermal gradient, gold mineralization and the related hydrothermal alteration have been concurrent with an active volcanism. The structural evolution of the area leads to assume a genetic link between gold mineralization and the post-Hammamat felsites intrusions (a potential fluid source) in view of their communal relationship with the N–S fault/shear trend.
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