Fluid inclusions in the mesozonal gold deposit at Atud mine, Eastern Desert, Egypt Hassan Z. Harraz Department of Geology, Faculty of Science, Tanta University, Tanta, Egypt Accepted 24 October 2002 Abstract Gold mineralization at the Atud mine occurs as fracture-filling auriferous quartz veins hosted in Neoproterozoic dioritic rocks andalongtheircontactwithmetagabbro.Goldmineralizationisassociatedwithmetasomaticalterationzonesaroundshearzones andquartz–carbonateveinarrays.Themineralizedveinsconsistofquartz,carbonateandalbitegangueenclosingminoramountsof pyrrhotite, arsenopyrite, pyrite and sphalerite. Trace amounts of galena, chalcopyrite, magnetite and rutile are also present. Para- genetically, 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(15.6–36.2at.%Ag)ispresentmainlyasdiscretegrainsofnativegold(<5–20 lmindiameter).Freegoldappearsmainlyas inclusions in quartz and as microscopic inclusions (<5 lm) in arsenopyrite and pyrite. Gold also occurs in fractures and grain boundariesofpyrite,arsenopyriteandbasemetalsulphides.Theseoccurrencesofgoldindicatethatseveralinfluxesofgoldand/or stages of remobilization took place. Based on temperatures inferred from arsenopyrite compositions by electron microprobe, the estimated temperatures for Early andMainmineralizedstagesreach340–430,and273–368 °C,respectively.Thesulphuractivity(atm)oforeformationattheAtud deposit was estimated for each stage as 10 6:5 –10 9:6 ,and10 8 –10 12:2 , respectively. Fluid inclusions in quartz intimately associated with the mineralization are dominated by aqueous H 2 O–CO 2 þ NaCl types. In most cases, these fluid inclusions co-exist in individual samples and show various CO 2 phase volume proportions at 40 °C. Co- existing H 2 O-rich liquid and CO 2 -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 in- clusions in quartz intimately associated with the early and main mineralizing events were trapped during phase separation of an originally homogenous H 2 O–CO 2 liquid, with low salinity (2.8–8.2 wt.% NaCl equiv.) and high density of 0.8–0.9 g/cc (corre- sponding to 15–76 mol% CO 2 ). 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 di- oritesareconsideredtohaveactedaspreferentialsitesforfluidflowandoreprecipitationduetotheirbrittlenatureduringregional deformation. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Fluid inclusions; Microthermometry; Mineralogy of quartz veins; Atud deposit; Gold-Egypt 1. Introduction In the Arabo–Nubian Shield, gold mineralization is widelyassociatedwithpost-tectonicgranitoidrocks(El- Gaby et al., 1988; Pohl, 1988). The majority of these deposits occur as Au-bearing quartz veins with a poly- metallicsulphideassemblage,exhibitingpinch-swellvein textures and showing evidence of multiple stages of mineralization(HilmyandOsman,1989;Hussein,1990; Harraz and El-Dahhar, 1993). P–T conditions at or below greenschist metamorphic boundary favoured es- tablishment of brittle–ductile and brittle structures in which gold deposits were selectively sited (El-Gaby et al., 1988). The source of the ore-bearing fluids is Journal of African Earth Sciences 35 (2002) 347–363 www.elsevier.com/locate/jafrearsci E-mail address: hharraz2001@yahoo.com (H.Z. Harraz). 0899-5362/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0899-5362(02)00152-5