A genetic model for a mesothermal Au deposit: evidence from fluid inclusions and stable isotopic studies at El Sid Gold Mine, Eastern Desert, Egypt

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Abstract

The El Sid Au mineralisation in the Fawakhir area, Eastern Desert, Egypt, is comprised of hydrothermal quartz veins cutting a Neoproterozoic granitoid pluton. The mineralisation is divided into Au-bearing, transitional and late carbonate vug stages. Pyrite- arsenopyrite and streaky pyrite-sphalerite-galena assemblages characterise the early and late episodes of the Au-bearing stage, respectively. These sulphide minerals sometimes contain Au as inclusions. Fluid inclusions from the Au-bearing stage are H2Osingle bondCO2 rich fluids (ca 29–62 mole% CO2 and density, ca 0.51-0.66 g cm−3) with moderate to high salinities (ca 12–19 wt% NaCl equiv.). The trapping temperature of the ore occurred between 280 and 350°C, at pressures between 120–170 MPa (∼800–1800 m depth). However, fluid inclusions from the pyrite-arsenopyrite assemblage reflect lower homogenisation temperature measurements (Th) (265–295°C) than those in the streaky pyrite-sphalerite-galena assemblage (330–365°C). The minerals of the transitional and late carbonate vug stages (quartz and calcite) were formed between 180 and 265°C.

Oxygen and H isotope data of fluid inclusions hosted in the quartz and calcite from the Au-bearing stage indicate a rather wide range of calculated δ18OH20 and δDH20 (i.e. +3.2 to + 7.8%‰ and −75 to −32‰ , respectively). The data for the streaky pyrite-sphalerite-galena assemblage exhibit higher δ180H20 values (+ 5.3 to + 6.8%‰) and lower δDH20 (−75 to −48‰) relative to that of the pyrite-arsenopyrite assemblage. Calculated isotopic temperatures of quartz-calcite fractionation range from 282 to 353°C and are consistent with the trapping temperatures (280–350°C).

The fluid inclusion and stable isotope data imply that Au at the El Sid Gold Mine has been transported as a bisulphide complex. The high salinity and inferred occurrence of CH4 in some fluid inclusions collected from the graphite-rich zone along a serpentinite-granitoid contact suggest that the mineralising solutions gained their metal contents through circulation in the fractured zones and incorporation with a geothermal convective system by wall rock interaction. Deposition of Au sulphides has taken place at shallow crustal levels (∼800–1800 m) as a result of meteoric/magmatic-metamorphic water exchange with wall rocks through H2Osingle bondCO2 immiscibility during fluid pressure drop and decreasing ligand activity.

Résumé

Le gisement d'Au d'El Sid dans la région de Fawakhir, Désert Oriental, comprend des veines de quartz hydrothermal recoupant un pluton granitique néoprotérozoïque. La minéralisation s'est formée au cours de trois stades:àAu, transitionnel etàgeodes de carbonates tardifs. Les assemblagesàpyrite- arsénopyrite etàpyrite-sphalérite-galène en bandes caractérisent, respectivement, lesépisodes précoces et tardifs du stadeàor. Les sulfures contiennent parfois de l'Au en inclusions.

Les inclusions fluides du stadeàAu sont riches en H2O et CO2 (de 29à62 moles% de CO2 et density de 0.51à0.66 g cm−3) avec une salinitémodéréeàforte (12–19 poids% enéquivalent NaCl). La température de capture du minerai se situe entre 280 et 350°C pour une pression entre 120 et 170 MPa (soit une profondeur de 800à1800 m). Cependant, les températures d'homogénéisation (Th) des inclusions fluides de l'assemblageàpyrite-arsénopyrite sont plus basses (265–295°C) que celles de l'assemblageàpyrite-sphalérite-galène en bandes (330–365°C. Les minéraux (quartz et calcite) des stades transitionnel etàgéodes de carbonates tardifs se sont formés entre 180 et 265°C.

Les valeurs isotopiques de l'O et de l'H des inclusions fluides du quartz et de la calcite du stadeàAu montrent une très grande variation des valeurs calculées de δ18OH20 et δDH20 (soit + 3.2à+ 7.8%‰ et −75à−32‰, respectivement). L'assemblageàpyrite-sphalérite-galène en bandes présents des valeurs de δ18OH20 plus fortes (+5.3à+6.8%‰) et de δDH20 plus faibles (−75à−48‰) que l'assemblageàpyrite-arsénopyrite. Les températures calculées du fractionnement isotopique quartz-calcite varient de 282à353°C, en accord avec les températures de capture (280–350°C). Les données sur les inclusions fluides et les isotopes stables impliquent que l'Au de la mine d'El sid aététransportésous forme de complexes bisulfurés. La forte salinitéet la présence supposée de CH4 dans quelques inclusions fluides prises dans la zoneágraphite le long d'un contact entre serpentinite et granitoïde suggèrent que les solutions minéralisatrices ont acquis leurs teneurs en métaux en circulant dans les zones de fractures et en réagissant avec les roches encaissantesàl'intérieur d'un systèms géothermique convectif. Le dépoˆt de sulfures d'Au a pris placeàdes niveaux crustaux superficiels (800–1800 m)àla suite d'échanges entre les eaux météoriques/magmatiques-métamorphiques et les roches encaissantes, par immiscibilitédans le système H20single bondCO2 par chute de la pression de fluides et baisse de l'activitédes ligands.

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