GRANITOID MAGMATISM AND FLUID REGIME OF THE TAGRAGRA-DE-AKKA GOLD DEPOSITION (MOROCCO)

Lebedev V.I., Lebedev N.I., Lebedeva M.F., Lebedeva S.V.

Tuvinian Institute for Exploration of Natural Resources SB RAS, Kyzyl, Russia, samina51@inbox.ru

The “butonjera” Tagragra d’Akka is one of the richesr gold-containing ore regions that are localized in the northern African Platform in western part of the Anti-Atlas fold-block area. It is represented by hard protrusion of the Proterozoic deeply metamorphosed formations within the Cambrian deposits like box folds. The revealed here gold objects are confined to the erosional “window”, where the intensely dislocated greenstone altered metaterrigenous rocks of the Early Proterozoic were found.. They are cut by leucogranites (2049±5 Ma) and granophyres (1755±25 Ma), as well as numerous dykes of diabases with the absolute datings on muscovite from metasomatites that accompany them (600 ± 10 Ma), and also rare ones of aplites, panafrican tectono-magmatic activization (Juery, 1976; Rapport Relatif…, 1994). The volcanogenic, evaporate and carbonate-terrigene subplatform sediments of the Late Proterozoic unconformably occur on metamorphites and granitoids of the Early-Middle Proterozoic.

A determination of spatial-temporal regularities of ore formation is one of the expert study problems and that of evaluating the perspectives of new metal-bearing structures discovery with rich gold content. They can serve a carcass and effective means of ore forming system fragment correlation in developing the geological and genetic models of specific gold-quartz low sulfide objects of arsenic formation and integral model of the Tagragra d’Akka gold ore knot. At the same time, the information about peculiarities of geodynamic, metallogenic and paleogeochemical process occurrence should be considered that characterize the conditions of the unique ore forming system functioning and in the end, elucidate the reasons of gold concentrated emplacement in local spatial-temporal volume of the Tangagra d’Akka “butonjera”.

The geologists of Goldmine de Akka (Zouhair, 1992) established that structural control was the most prominent factor of gold deposit dislocation in this ore knot. Ore shoots of steep subvertical dipping with high gold concentrations are confined to intervals of quartz breccias on the step windings of sinistral latitudinal and dextral diagonal shift zones that are predominantly developed along the dyke contacts of chloritized diabases and dolerites with host greenstone altered carbonaceous metaargillites. It testifies to postconsolidational formation of ore content zones. The following is mostly favorable for localization gold economic concentrations and accompanying veinlet-impregnated arsenopyrite-pyrite-pyrrhotite mineralization:

quartz and carbonate-quartz veins of multistage formation in fault-shifts of near meridian, sublatitude and north-eastern trendings (320-340°, 10-35°, 60-80°, 90-110є);

systems of intersecting «ladder veins in knots of dyke crossing of intensely chloritized and sulfidized diabases and dolerites;

zones of quartz-veined stockworks flexures of shift zones and vertical curves of folds of graphitized metaargillites in tectonically dislocated contacts of main composition dykes;

borders of zonal oxide (reddening of rocks) and protoxide (green color) settings of paleohydrogeochemical epithermal influence on chloritized and sulfidized dykes of diabases and dolerites.

the analysis of search-evaluation, prospecting and exploitation works in the Tagragra d’Akka ore knot, expert estimate of geological situation testify to the fact that the studied ore forming systems are characterized by similar mineral parageneses. Metal-bearing fluids, with their physical, chemical and thermobarometric parameters having been determined in the process of sample mineralogical and geochemical studies from Iouririne Centre, Ifarar, Angaraf, Tinsirheme, Alous, F-132, F-15, Owim Tikada и Bougadrour (Zouhair, 1992) deposits and conserved as fluid inclusions in quartz of various generations are differed both by composition and properties. The presence of contrast water-salt fluid in the conserved inclusions is the main criterion that allows to predict intervals of gold economic concentrations with high probability in quartz veins of the studied objects. Along with NaCl, CaCl₂ and CO₂, it is enriched by a group of such specific components as CH₄, N₂ и KCl. Gold in native form and laminated hematite are present in some fluid inclusions. This relatively low temperature (200-350ºС) stage of fluid generation was accompanied by alteration of host rocks with synchronous deposition of successive generation (Qtz-2, Qtz-3 и Qtz-4) quartz large masses in association with chlorite, later muscovite and iron-magnesium carbonates with minor sulfides and iron sulfoarsenides in the fissure shift systems S₁, S₂ и S3. At the same time, near ore metasomatosis was generally of alkaline character and expressed by formation of chlorite, actinolite, tremolite, feldspars, pyrite, marcasite and late carbonates. The mixture of chloride exogenic brines with deep (juvenile) arsenic fluids saturated by methane, nitrogen and carbon dioxide contributed to destruction of chloride and sulfide gold complexes and its precipitation in native form. The mixture process of juvenile fluids and exogenic brines was just responsible for sharp change in P-T-V parameters of the forming hydrothermal solutions and their chemism (Lebedev, 1998). The intensity of the formed metal-bearing fluid introduction in the zone of ore emplacement and changeability of its composition were largely determined by the peculiarities of ore magmatic system evolution and pulsation of intensive tectonic movements in shift zones being synchronous to stages of ore process.

Fig.1. Ratio of structural and physical-chemical parameters of gold objects formation in the Tagragra d’Akka ore cluster.

Since the part of the arsenic complexes was insignificant in the ore forming gold-containing solutions with relatively not high density of СО₂ (from 0,14 to 0,77 g/cm³) during the hydrothermal process of the Tagragra d’Akka ore knot deposit formation, and amount of the introduced arsenic and As : (Fe, Co, Ni) ratio had low values, all that contributed to precipitation of crystals and aggregates of pyrite, arsenopyrite, with their mineral forms being largely depended on As : S : Fe ratios in solution. With the progressive increase in the part of juvenile fluids in ore forming solution, an extent of its carbon oxide and methane oversaturation sharply increased in the mixture zone. It caused the origin of numerous centres of crystallization and formation of fine-grained aggregates of ores that cemented breccia of veined quartz (Qtz-2 и Qtz-3). The formation of arsenopyrite splitted and acicular crystals is peculiar for these conditions. On the whole, the ore stage took place on the background of juvenile fluid pulsational introduction into the field of chloride exogenic brines. Discontinuity of their introduction in the space of ore deposition was determined by dynamics of endogenic “hearth” development and pulsation of exogenic brine inflow that can be explained by the screening effect of the newly formed fissures, change in rate of fissure “opening” and also rate and mass of chloride brine entering the zone of ore deposition. The more intense involvement of higher hydrodynamic level exogenic solutions in process of ore forming fluid development likely occurred in the final stages of hydrothermal process, decrease in press of low hydrodynamic level chloride brines. They were characterized by higher Eh, low pH values, presence of hydrogen sulphide and sulphate-anion. Participation of such solutions in development of ore forming fluid resulted in deposition of sulfide paragenesis in association with silicate minerals and calcite.

The hydrothermal process that caused formation of quartz veins with rich gold content was characterized by evolution of physical and chemical parameters reflected by sharp decrease in concentrations of methane, nitrogen and potassium, carbon dioxide density, intense lowering of temperature, pressure, oxidized-reduced potential and acidity-alkalinity of metal-bearing fluid as a result of juvenile ans exogenic solution mixture in the fissure systems S_1-2, S₂ и S₃. The analysis of study results of fluid inclusions in quartz of various generations from the Iourine Centr, Ifarar, Angaraf, Tinsrhene, Alous, F-132, F-15 deposits is the support of this conclusion.

The integral information on composition of hydrothermal solutions, paragenetic association of veined minerals is set in the plot (Fig.1.) built in coordinates of temperature and relative succession of dislocation formation, quartz emplacement there with various types of conserved fluid inclusions in each of its generations has been plotted in the diagram that was built in coordinates of temperature and relative succession of dislocation formation, emplacement of quartz with various types of the conserved fluid inclusions in each of its generations. The field of fluid inclusion homogenization temperatures that indicates differences in development of hydrothermal process in space and time has been shown for each studied deposit. The plot reveals the prediction peculiarity of fluid regime: mass extract of gold and accompanying ore minerals occurred synchronously to disappear of methane, nitrogen and potassium chloride from the fluid column. Gold precipitated from the water-salt carbon dioxide-calcium-chloride solution-brine of relatively high density. Gold precipitation was resumed many times and synchronously with the fluid repeated enrichment in nitrogen, destruction of carbon dioxide complex in presence of chloric iron.

The authors of the paper are deeply grateful for the presented possibility of acquaintance and cooperation in studying the Tagragra d’Akka ore knot deposits to director of FGUP “All-Russian foreign economic association ZARUBEZHGEOLOGIA” I.T.Gavrilov, leader of experts’ group S.N.Zverev and colleagues-experts, leadership of the firm O.N.A. of Tifnoute Tiranimine company and geologists of the mine Goldmine d’Akka.

The work has been carried out with financial support of Russian Foundation for Basic Research (grant № 07-05-00601).

References

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