Fluid inclusions in the mesozonal gold deposit at Atud mine, Eastern Desert, Egypt
Gold mineralization at the Atud mine occurs as fracture-filling auriferous quartz veins hosted in Neoproterozoic dioritic rocks and along their contact with metagabbro. Gold mineralization is associated with metasomatic alteration zones around shear zones and quartz–carbonate vein arrays. The mineralized veins consist of quartz, carbonate and albite gangue enclosing minor amounts of pyrrhotite, arsenopyrite, pyrite and sphalerite. Trace amounts of galena, chalcopyrite, magnetite and rutile are also present. Paragenetically, the mineralization is divided, with decreasing temperature, into three stages, namely: (1) Early (Au-poor, pyrrhotite–arsenopyrite–pyrite–quartz vein); (2) Main (Au-rich, sphalerite–arsenopyrite–pyrite–galena ± chalcopyrite–quartz vein); and (3) Late (quartz–carbonate–pyrite ± galena). 
 
Gold (15.6–36.2 at.% Ag) is present mainly as discrete grains of native gold (<5–20 μm in diameter). Free gold appears mainly as inclusions in quartz and as microscopic inclusions (<5 μm) in arsenopyrite and pyrite. Gold also occurs in fractures and grain boundaries of pyrite, arsenopyrite and base metal sulphides. These occurrences of gold indicate that several influxes of gold and/or stages of remobilization took place. 
 
Based on temperatures inferred from arsenopyrite compositions by electron microprobe, the estimated temperatures for Early and Main mineralized stages reach 340–430, and 273–368 °C, respectively. The sulphur activity (atm) of ore formation at the Atud deposit was estimated for each stage as 10−6.5–10−9.6, and 10−8–10−12.2, respectively. 
 
Fluid inclusions in quartz intimately associated with the mineralization are dominated by aqueous H2O–CO2 + NaCl types. In most cases, these fluid inclusions co-exist in individual samples and show various CO2 phase volume proportions at 40 °C. Co-existing H2O-rich liquid and CO2-rich vapour fluid inclusions homogenized into liquid and carbonic vapour phases, respectively, over the same temperature range (270–490 °C). The petrographic observations and microthermometric data suggest that fluid inclusions in quartz intimately associated with the early and main mineralizing events were trapped during phase separation of an originally homogenous H2O–CO2 liquid, with low salinity (2.8–8.2 wt.% NaCl equiv.) and high density of 0.8–0.9 g/cc (corresponding to 15–76 mol% CO2). These data are consistent with transportion of gold as a bisulphide complex. Gold deposition occurred over a temperature range of 270–430 °C and pressures of 160–272 MPa (∼6–11 km depths), likely due to decreases in sulphur activity (10−6.5–10−12.2) accompanying fluid unmixing. Gold deposition in the Atud diorites was related to fluid phase separation, sulphidization and carbonatization of host dioritic rocks during hydrothermal alteration and mineralization. The diorites are considered to have acted as preferential sites for fluid flow and ore precipitation due to their brittle nature during regional deformation.
Mineral chemistry of polymetallic mineralization associated with altered granite, Hangaliya area, South Eastern Desert, Egypt
The Hangaliya gold deposit, located in the South Eastern Desert of Egypt, comprises a series of milky quartz veins along NW-trending Hangaliya shear zone, cutting through granitic rocks of Gabal Nugrus monzogranite. This shear zone, along with a system of discrete shear and fault zones, formed in the late deformation history of the area. The quartz vein emplacement took place under a brittle-ductile shear regime. Auriferous quartz veins are slightly sheared and boudinaged within the shear zone, especially in the hematized granite. Hydrothermal alteration is pervasive in the granitic wall rocks including sericitization, chloritization, fluoritization, sulphidization and minor carbonatization. The altered zones and associating quartz veins contain sulphides, gold, silver, cobalt, bismuth, and uraninite minerals. The Hangaliya gold veins include gold, silver, cobaltite, native bismuth, chalcopyrite, pyrite, galena, ferrocolumbite, fergusonite and uraninite. The Au-ore occurs in the quartz veins and adjacent wall rocks as dissemination in chalcopyrite and pyrite. Presence of refractory native silver, bismuth and cobalt in chalcopyrite is inferred from microprobe analyses. Wall rock sulphidization also likely contributed to destabilising the gold-silver, cobalt, bismuth assemblages and precipitation of the minerals in the hydrothermal alteration zone adjacent to the quartz veins. Gold occurs in two main modes: “invisible gold” in sulphides and native gold. Most of the “invisible gold” occurs in chalcopyrite and bismuth. The altered granites in the Hangaliya shear zone are enriched in Au, Ag, Bi, Co, and Ni with chalcopyrite, which suggests derivation of these metals from serpentinites due to interaction with the felsic Nugrus granite.
Aspects of veining in the Welsh Lower Palaeozoic Basin
Summary In the Welsh Lower Palaeozoic marginal basin, veins of diverse origin are widely distributed in space and time. The veins are categorized as pre-, syn- and post-tectonic with respect to folding and cleavage development; these sub-divisions denoting different types of veining processes. Among the pre-tectonic veins are those described as regional veins which were emplaced diachronously by hydraulic jacking and related processes due to overpressured pore fluids affecting newly lithified rocks while sedimentation continued above. Other pre-tectonic veins are those around Snowdon which were due to processes closely associated with the development of an Ordovician resurgent caldera. Syn-tectonic veins are uncommon. They include the auriferous lodes of Ogofau in south-central Wales. Post-tectonic veins are the most widespread type and are exemplified mainly by those of the Harlech and Plynlimon Domes. These late veins are attributed largely to regional extension and dewatering of the Lower Palaeozoic pile caused by the early Variscan extensional regime.