Petrology, geochemistry, stable and radiogenic isotopy of the Guelb Moghrein Iron Oxide-Copper-Gold-Cobalt Deposit, Mauritania [Elektronische Ressource] / von Grigorios Aarne Sakellaris

Greg

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2007
• Nombre de lectures: 12
• Langue: English
• Poids de l'ouvrage: 18 Mo

Extrait

“Petrology, Geochemistry, Stable and Radiogenic Isotopy of the Guelb
Moghrein Iron oxide-Copper-Gold-Cobalt Deposit, Mauritania”
Von der Fakultät für Georessourcen und Materialtechnik
der Rheinisch-Westfälischen Technischen Hochschule Aachen
zur Erlangung des akademisches Grades eines
Doktors der Naturwissenschaften
vorgelegte Dissertation von Dipl. Geol.
Grigorios Aarne Sakellaris
aus Thessaloniki
Berichter: Univ.-Prof. Dr.rer.nat. Franz Michael Meyer
Univ.-Prof. Dr.rer.nat. Ulrich Kramm
Tag der mündlichen Prüfung: 27. August 2007
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbarFOREWORD
I would like to express my appreciation to Prof. Dr. Franz Michael Meyer, Head of the
Institute of Mineralogy and Economic Geology, RWTH Aachen University, for initiating the
project and providing dedicated and stimulating supervision throughout my work. I am
grateful to Dr. Jochen Kolb for his continued guidance, assistance and for the many valuable
discussions. Besides that he reviewed all my manuscripts with an amazing efficiency and
rigor. Dr. Karsten Eden and Dr. Annika Dziggel are thanked for their worthy comments on
the research subject. Prof. Dr. Ulrich Kramm and Prof. Helge Stanjek are thanked for their
interest, supporting attitude and enlightening discussions, especially during the internal
seminars.
I am grateful to all scientists and departments that contributed to my work providing
not only data but also knowledge and inspiration: Dr. Axel Gerdes and Dr. Yann Lahaye,
Mineralogical Institute, Frankfurt University (Laser-Ablation Inductively-Coupled-Plasma
Mass-Spectrometry); Dr. Radegund Hoffbauer, Institute of Mineralogy and Petrology, Bonn
University (Oxygen, hydrogen and carbon isotopy); Dr. Michael Böttcher, Max-Planck-
Institute of Marine Microbiology, Bremen (Sulfur isotopy); Dr. Torsten Vennemann, Institute
of Mineralogy and Geochemistry, Lausanne University (Oxygen and carbon isotopy).
The author is further indebted to many fellow researchers and scientists who kindly
assisted during laboratory work or provided valuable data:
Dr. Sven Sindern, Petra Nikolay and Rudolf Neff (Geochemical Laboratory); Dr. Thomas
Witzke and Getrud Siebel (X-Ray Diffraction Laboratory); Dr. Annemarie Wiechowski and
Roman Klinghardt (Electron Microprobe Laboratory); Martyn Brand (Electronic/Computer
Technician); Thomas Derichs (Thin and Polished Section Laboratory); Monika Wiechert
(Photography Laboratory) as well as Hildergard Breuers and Gudrun Günther from the
Secretarial Office.
This research was made possible through grant (Me 1425/6-1/2) of the Deutsche
Forschungsgemeinschaft. The Deputy General Manager of GEMAK, Nouakchott, Mohamed El
Moctar O.M. El Hacen, and the Director of MORAK, Akjoujt, Aboubekrine O.A. dit Ebaye, are
thanked for their hospitality and support in Mauritania, for providing and allowing access to
their lease areas, and for the permission to publish the results of this study.
Love, respect and many thanks for their enduring support, patience, friendship and
enjoyable life in the office go to my 413-room buddies Amanda, Arif, Hassan, and André.
Love and appreciation also to my Greek colleagues and friends Maria and Fotis for making
me feel like home. Special thanks to Petros, Yvonne, Jörg, Bodo, Ioanna, Zeno, Irene, and
Anne, for their friendship, sincere interest and for showing me the enjoyable side of life
outside the office.
Last but not least, my thanks to my family for their unconditional support and for
always being there when needed.ABSTRACT
The Guelb Moghrein Fe oxide-Cu-Au-Co deposit is located in the central Mauritanides fold-
and-thrust belt, in the Akjoujt region, northern Mauritania. The deposit is hosted by an
Archean metavolcanosedimentary sequence at the western boundaries of the West African
Craton. Sericite-quartz- and biotite-garnet-quartz schists that belong to the Saint Barbe
Volcanic unit make up the stratigraphic base of the lithological sequence in the pit. The
sericite-quartz schist represents a metatuff of rhyodacite to rhyolite composition and the
biotite-garnet-quartz schist is an intercalating metapelite. A suite of metavolcanic rocks of
the Akjoujt Metabasalt unit including massive amphibolite lenses, biotite-actinolite- and
chlorite schists dominate the deposit area. Textural and geochemical analyses indicate that
the precursor volcanic rocks are submarine, andesites/basalts crystallized in a volcanic
arc/continental margin setting. The Fe oxide-Cu-Au-Co mineralization is hosted by a lensoid
metacarbonate body in the central part of the pit area. The metacarbonate is a massive,
very coarse-grained rock, consisting of up to 5 cm large siderite crystals. A line of evidence
including geochemical and stable isotopic signature of the siderite indicates that the
metacarbonate represents marine metasediment deposited in a continental shelf setting.
Thermobarometric analyses on the regional lithologies record a clockwise retrograde
P-T path subsequent to thrust deformation events. Peak, amphibolite facies metamorphism
for the amphibolite of the Akjoujt Metabasalt unit was calculated at 580 ±40°C and 5 kbar.
Retrograde shearing of the amphibolite during the northeastward D regional thrust event2
resulted in the development of the S foliation and the formation of the biotite-actinolite2
schist. This retrograde overprint occurred at 410 ±30°C and ca. 3 kbar. The D event2
resulted in overthrusting of the Akjoujt Metabasalt unit by the Saint Barbe Volcanic unit as
well as the IOCG hydrothermal mineralization in the metacarbonate. Subsequent eastward
D thrusting resulted in the retrogression of amphibolite to chlorite schist under retrograde3
lower greenschist facies metamorphism. The shape analysis of the metacarbonate body
indicates that its lensoid geometry is the result of planar alignment and thin-stacking during
the D event.2
The economic Fe oxide-Cu-Au-Co mineralization at Guelb Moghrein is confined to
discrete D shear zones that truncate the metacarbonate body. The ore bodies occur in2
breccia zones developed in the metacarbonate and form multiple, up to 30 m wide, tabular,
foliation (S ) parallel lenses that dip moderately SW. The breccia zones have a characteristic2
geometry of central, ductile Fe-Mg clinoamphibole-chlorite phyllonite surrounded by a
progressively developed breccia of fractured siderite. In these damage zones, a massive
sulfide-arsenide-gold assemblage formed together with low-Ti magnetite, Fe-Mg
clinoamphiboles, and graphite in more distal parts replacing the brecciated siderite.
Geochemically, the breccia zones are characterized by substantial enrichment of several
elements that include Fe, Cu, Au, and Co as well as Si, S, As, U, Th, F, REE, Bi, Ag, Bi, andTe. Peripheral to the metacarbonate in the Akjoujt Metabasalt unit a ca. 40 m wide biotite-
chlorite-grunerite-calcite alteration halo developed during hydrothermal alteration. The bulk
of the alteration assemblage occurs in the recrystallized feldspar matrix; however this zone
is not significantly mineralized. Towards the ore bodies, the biotite-chlorite-grunerite-calcite
alteration halo records progressive enrichment in K, Fe, and Mg, REE, S and Cu and
depletion in Na, and Sr relative to least altered amphibolite.
The primary ore mineralogy in the breccia zones is dominated by pyrrhotite and
chalcopyrite and includes Fe-Co-Ni arsenides, arsenopyrite, cobaltite, Bi-Au-Ag-Te minerals
as well as magnetite, Fe-Mg clinoamphiboles, chlorite, graphite, apatite, xenotime and
monazite. The paragenetic assemblages indicate a relative high formation temperature
o(400-450 C) in agreement to the calculated retrograde metamorphic conditions. The
principal metal, gold, is found either in native form, in solid solution with silver (electrum)
or bismuth (maldonite) or in a complex Bi-Au-Ag-tellurides and is associated with
arsenopyrite and clinosafflorite. Exsolution mineralogy includes troilite, pentlandite,
cubanite, mackinawite and nickeline. This assemblage was formed in a temperature range
oof 200-250 C, under lower greenschist facies metamorphism, during D .3
The isotopic composition of siderite from the breccia is characterized by depletion of
13 18 C values and enrichment of  O values attributed to the partial decarbonation of siderite
and subsequent equilibration with the hydrothermal fluid. The occurrence of graphite with
siderite and magnetite, and the very light isotopic composition of graphite are consistent
with an abiogenic origin of graphite by carbonate reduction of siderite at temperatures ca.
o430 C. The isotopic composition of the hydrothermal fluid responsible for the mineralization
18 34at the Guelb Moghrein was calculated from the measured  O,  D and  S values of
paragenetic minerals from the breccia zones. The calculated isotope values obtained are
consistent with a single fluid of metamorphic origin.
Radiometric U-Pb and Pb-Pb dating applied on hydrothermal REE-phosphates from the
main ore zones addresses the hydrothermal activity at Guelb Moghrein and consequently
the deformational events on regional scale. Phosphates associated with the primary sulfide
ore assemblage are type I monazite and xenotime. The age obtained for type I phosphates
of 2492 ±9 Ma is interpreted to reflect the age of hydrothermal fluid flow