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.
Pan-African dextral transpressive duplex and flower structure in the Central Eastern Desert of Egypt
Abstract The NE–SW trending Mubarak shear belt in the Central Eastern Desert (CED) of Egypt records the structural features of a transpression regime. Structural interpretation of satellite data and field observations indicates the presence of two episodes of deformation. The less dominant D1 deformation is related to sinistral movement along NW–SE trending Wadi Abu Dabbab shear zone and formation of F1 tight to isoclinal non-cylindrical folds and thrust fan as a consequence of transpression. The thrust fan includes ESE- to SE-dipping thrusts in the ophiolitic melange and S- to SSW-dipping thrusts in the northern part of the metagabbro–diorite complex. The transpression-related sinistral shear regime is superimposed by the dominant D2 dextral transpression along NE–SW trending Wadi El-Umra shear zone. This dextral shearing is characterized by development of NNE- to NE-trending S2 cleavage, strike-slip duplex, NNE- and NE-trending F2 folds, and NNW-directed thrusts. These two events represent a single progressive phase associated with sinistral transpressional deformation, which is related to a younger E–W shortening event. E–W directed shortening is due to oblique convergence between East and West Gondwana along the Mozambique belt. Transpressional structures in the NE–SW trending Mubarak–Barramiya shear belt indicate highly oblique convergence leading to wrench-dominated dextral transpression and development of a major flower structure between Wadi Mubarak and Hafafit dome occupying the whole width of the CED.
Natural Radioactivity of Intrusive-Metamorphic and Sedimentary Rocks of the Balkan Mountain Range (Serbia, Stara Planina)
Stara Planina (also known as the Balkan mountain range) is known for numerous occurrences and deposits of uranium and associated radionuclides. It is also famous for its geodiversity. The geologic framework is highly complex. The mountain is situated between the latitudes of 43° and 44° N and the longitudes from 22°16′ to 23°00′ E. Uranium exploration and radioactivity testing on Stara Planina began back in 1948. Uranium has also been mined in the zone of Kalna, within the Janja granite intrusive. The naturally radioactive geologic units of Stara Planina are presented in detail in this paper. The main sources of radioactivity on Stara Planina can be classified as: 1. Granitic endogenous—syngenetic–epigenetic deposits and occurrences; 2. Metamorphogenic—syngenetic; and 3. Sedimentary, including occurrences of uranium deposition and fluctuation caused by water in different types of sedimentary rocks formed in a continental setting, which could be classified under epigenetic types. The area of Stara Planina with increased radioactivity (higher than 200 cps), measured by airborne gamma spectrometry, is about 380 square kilometers. The highest values of measured radioactivity and uranium grade were obtained from a sample taken from the Mezdreja uranium mine tailing dump, where 226Ra measures 2600 ± 100 Bq/kg and the uranium grade is from 76.54 to 77.65 ppm U. The highest uranium (and lead) concentration, among all samples, is measured in graphitic schist with high concentrations of organic (graphitic) material from the Inovska Series—99.47 ppm U and 107.69 ppm Pb. Thorium related radioactivity is the highest in granite samples from the Janja granite in the vicinity of the Mezdreja granite mine and the Gabrovnica granite mine tailing dump, and it is the same—250 ± 10 Bq/kg for 232Th, while the thorium grade varies from 30.82 to 60.27 ppm Th. In gray siltstones with a small amount of organic material, the highest radioactivity is related to potassium—2080 ± 90 Bq/kg for 40K.