Fluid circulations during collapse of an accretionary prism : Example of the Naxos Island Metamorphic Core Complex (Cyclades, Greece)

Cette thèse a pour objectif de caractériser les circulations de fluides en contexte d’effondrement d’un prisme d’accrétion crustal. Le Metamorphic Core Complex (MCC) de Naxos comprend un système de détachement/décollement caractérisé par mylonites, ultramylonites, cataclasites et failles normales dont les relations géométriques témoignent du litage rhéologique de la croûte continentale. La chimie des inclusions fluides déterminée par l’analyse microthermométrique, la spectroscopie RAMAN, l’ablation laser couplée à l’analyse spectroscopique (LA-ICP-MS), le « crush-leach », et les signatures isotopiques C et H des inclusions fluides permettent d’identifier trois grands types de fluides (1) des fluides salés riche en métaux, ii) des fluides aquo-carboniques en équilibre avec les encaissants métamorphiques, et iii) des fluides aqueux, probablement d’origine météorique. Ces données indiquent que la croûte est subdivisée en deux réservoirs séparés par la transition fragile-ductile. Les fluides météoriques circulent en association avec la déformation fragile de la croûte supérieure alors que les fluides salés et les fluides aquo-carboniques circulent en relation avec la déformation ductile. La géométrie de ces réservoirs évolue lors de la formation du MCC, conjointement avec l’exhumation et le refroidissement des roches métamorphiques. Le passage des roches du réservoir ductile au réservoir fragile est associée à un changement depuis un gradient géothermique élevé (60-100°C/km) vers un gradient géothermique plus faible (35-60°C/km). La transition fragile-ductile correspond ainsi à la fois à une limite rhéologique corrélée à une limite thermique et une limite de perméabilité.The aim of this thesis is to characterize fluid circulations in the context of the collapse of a crustal accretionary belt. The Naxos Metamorphic Core Complex comprises a detachment/decollement system characterized by mylonites, ultramylonites, cataclasites and normal faults with structural relationships reflecting the rheological layering at the crustal scale. Fluid inclusion chemistry is determined by microthermometry, Raman spectroscopy; laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), crush-leach and stable isotopes (C and H) analyses. These data characterize three different types of fluids: (1) high salinity fluids with a high metal content and high Th, (2) aqueous-carbonic fluids in equilibrium with the wall rocks and (3) aqueous probably surface-derived fluids. These data indicate that the crust is subdivided into two crustal reservoirs separated by the brittle/ductile transition. Surface-derived aqueous fluids circulate in association with the brittle deformation within the upper crust whereas aqueous-carbonic and high salinity fluids circulate in relation with ductile deformation. The characteristics of the trapped fluids indicate that as rocks have passed through the ductile/brittle transition they undergo a drastic change in geothermal gradient from 60 to 100°C/km within a lithostatic pressure regime to 35-60°C/km within a hydrostatic pressure regime. This implies that the fluid circulations are closely related to the rheological layering within the crust and its evolution during crustal extension. The ductile/brittle transition corresponds to a rheological boundary correlated to a thermal boundary and impermeable cap

Circulations fluides au cours de l'effondrement d'un prisme d'accrétion crustal : l'exemple du "Metamorphic Core Complex" de l'île de Naxos (Cyclades, Grèce)

The aim of this thesis is to characterize fluid circulations in the context of the collapse of a crustal accretionary belt. The Naxos Metamorphic Core Complex comprises a detachment/decollement system characterized by mylonites, ultramylonites, cataclasites and normal faults with structural relationships reflecting the rheological layering at the crustal scale. Fluid inclusion chemistry is determined by microthermometry, Raman spectroscopy; laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), crush-leach and stable isotopes (C and H) analyses. These data characterize three different types of fluids: (1) high salinity fluids with a high metal content and high Th, (2) aqueous-carbonic fluids in equilibrium with the wall rocks and (3) aqueous probably surface-derived fluids. These data indicate that the crust is subdivided into two crustal reservoirs separated by the brittle/ductile transition. Surface-derived aqueous fluids circulate in association with the brittle deformation within the upper crust whereas aqueous-carbonic and high salinity fluids circulate in relation with ductile deformation. The characteristics of the trapped fluids indicate that as rocks have passed through the ductile/brittle transition they undergo a drastic change in geothermal gradient from 60 to 100°C/km within a lithostatic pressure regime to 35-60°C/km within a hydrostatic pressure regime. This implies that the fluid circulations are closely related to the rheological layering within the crust and its evolution during crustal extension. The ductile/brittle transition corresponds to a rheological boundary correlated to a thermal boundary and impermeable cap.Cette thèse a pour objectif de caractériser les circulations de fluides en contexte d?effondrement d'un prisme d'accrétion crustal. Le Metamorphic Core Complex (MCC) de Naxos comprend un système de détachement/décollement caractérisé par mylonites, ultramylonites, cataclasites et failles normales dont les relations géométriques témoignent du litage rhéologique de la croûte continentale. La chimie des inclusions fluides déterminée par l'analyse microthermométrique, la spectroscopie RAMAN, l?ablation laser couplée à l'analyse spectroscopique (LA-ICP-MS), le « crush-leach », et les signatures isotopiques C et H des inclusions fluides permettent d'identifier trois grands types de fluides (1) des fluides salés riche en métaux, ii) des fluides aquo-carboniques en équilibre avec les encaissants métamorphiques, et iii) des fluides aqueux, probablement d'origine météorique. Ces données indiquent que la croûte est subdivisée en deux réservoirs séparés par la transition fragile-ductile. Les fluides météoriques circulent en association avec la déformation fragile de la croûte supérieure alors que les fluides salés et les fluides aquo-carboniques circulent en relation avec la déformation ductile. La géométrie de ces réservoirs évolue lors de la formation du MCC, conjointement avec l'exhumation et le refroidissement des roches métamorphiques. Le passage des roches du réservoir ductile au réservoir fragile est associée à un changement depuis un gradient géothermique élevé (60-100°C/km) vers un gradient géothermique plus faible (35-60°C/km). La transition fragile-ductile correspond ainsi à la fois à une limite rhéologique corrélée à une limite thermique et une limite de perméabilité

Fluid circulations during collapse of an accretionary prism (Example of the Naxos Island Metamorphic Core Complex (Cyclades, Greece))

Cette thèse a pour objectif de caractériser les circulations de fluides en contexte d effondrement d un prisme d accrétion crustal. Le Metamorphic Core Complex (MCC) de Naxos comprend un système de détachement/décollement caractérisé par mylonites, ultramylonites, cataclasites et failles normales dont les relations géométriques témoignent du litage rhéologique de la croûte continentale. La chimie des inclusions fluides déterminée par l analyse microthermométrique, la spectroscopie RAMAN, l ablation laser couplée à l analyse spectroscopique (LA-ICP-MS), le crush-leach , et les signatures isotopiques C et H des inclusions fluides permettent d identifier trois grands types de fluides (1) des fluides salés riche en métaux, ii) des fluides aquo-carboniques en équilibre avec les encaissants métamorphiques, et iii) des fluides aqueux, probablement d origine météorique. Ces données indiquent que la croûte est subdivisée en deux réservoirs séparés par la transition fragile-ductile. Les fluides météoriques circulent en association avec la déformation fragile de la croûte supérieure alors que les fluides salés et les fluides aquo-carboniques circulent en relation avec la déformation ductile. La géométrie de ces réservoirs évolue lors de la formation du MCC, conjointement avec l exhumation et le refroidissement des roches métamorphiques. Le passage des roches du réservoir ductile au réservoir fragile est associée à un changement depuis un gradient géothermique élevé (60-100C/km) vers un gradient géothermique plus faible (35-60C/km). La transition fragile-ductile correspond ainsi à la fois à une limite rhéologique corrélée à une limite thermique et une limite de perméabilité.The aim of this thesis is to characterize fluid circulations in the context of the collapse of a crustal accretionary belt. The Naxos Metamorphic Core Complex comprises a detachment/decollement system characterized by mylonites, ultramylonites, cataclasites and normal faults with structural relationships reflecting the rheological layering at the crustal scale. Fluid inclusion chemistry is determined by microthermometry, Raman spectroscopy; laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), crush-leach and stable isotopes (C and H) analyses. These data characterize three different types of fluids: (1) high salinity fluids with a high metal content and high Th, (2) aqueous-carbonic fluids in equilibrium with the wall rocks and (3) aqueous probably surface-derived fluids. These data indicate that the crust is subdivided into two crustal reservoirs separated by the brittle/ductile transition. Surface-derived aqueous fluids circulate in association with the brittle deformation within the upper crust whereas aqueous-carbonic and high salinity fluids circulate in relation with ductile deformation. The characteristics of the trapped fluids indicate that as rocks have passed through the ductile/brittle transition they undergo a drastic change in geothermal gradient from 60 to 100C/km within a lithostatic pressure regime to 35-60C/km within a hydrostatic pressure regime. This implies that the fluid circulations are closely related to the rheological layering within the crust and its evolution during crustal extension. The ductile/brittle transition corresponds to a rheological boundary correlated to a thermal boundary and impermeable cap.NANCY1-Bib. numérique (543959902) / SudocSudocFranceF

Control of Shear-Zone-Induced Pressure Fluctuations on Gold Endowment: The Giant El Callao District, Guiana Shield, Venezuela

The El Callao district, with a total endowment of more than 2000 t Au, is considered to be the most prolific gold resource in Venezuela. Mineralization is hosted by a vein system that is genetically associated with the El Callao transpressional shear zone. This vein system consists of a network of interconnected quartz–albite–ankerite veins enveloping a large number of metabasaltic fragments that host gold-bearing pyrites. Based on detailed mineralogical, microstructural, and fluid inclusion studies, a pressure-temperature pathway was established for the evolution of the mineralizing fluid during shear-zone development and exhumation. This path is characterized by repeated episodes of fluid pressure fluctuation from lithostatic (higher than 1.6 kbar) to near-hydrostatic values (<0.4 kbar), recorded throughout the transition from the quasi-plastic to frictional deformation cortical domains. Each successive pressure drop induced boiling of the hydrothermal fluid, with the resulting fluid phase separation controlling: (i) pyrite and invisible gold crystallization, which occurred during ductile and ductile-brittle transition strain conditions, and (ii) primary gold remobilization with consequent native-refined gold precipitation, occurring mainly under brittle conditions. The metallogenic framework that was proposed for the El Callao shear zone can be used as a vector to explore and characterize other mineralized shear zones in the Guiana Shield and analogous orogenic systems worldwide

Primary gold deposition linked toaqueous fluids in the Inata gold deposit(Burkina Faso) : Evidence from sulphidesand fluid inclusions geochemistry

International audienceThe Inata gold deposit is hosted by metavolcano-sedimentary units thatwere affected by several pre-, syn- and post-mineralization deformationstages and greenschist-facies metamorphism during the Eburnean orogeny.Early-D1 E-W oriented structures host anhedral pyrite of metamorphicorigin, rich in Ni, Co, As and Cu (concentrations measuredby in-situ LA-ICPMS). The Au/Ag ratio of this early pyrite generationis very low (1). Concentrations of arsenopyrite are highestclose to quartz-ankerite-albite veins and decrease gradually withdistance, whereby pyrite becomes the dominant sulphide furthest fromthe veins. Visible gold is essentially found within fractures crosscuttingsulphides associated to late stage transpressive D3 structures. This goldis interpreted to be the result of remobilization of early, disseminatedgold, related to the D2 ore stage.Analyses of fluid inclusions (FI) revealed different H2O/CO2 ratios aswell as D values of H2O for each deformation stage. Fluid geochemistry,obtained by in-situ LA-ICPMS analysis of FI, allowed distinguishingdifferent fluid compositions. Early D2 ore-stage fluids are characterizedby low salinities, low CO2, and low K and Ca concentrations,and are rich in B, Ag, Pb, As and REE. Zones in quartz veins containingthese aqueous fluids are also marked by higher 18O values (>15‰)than D1, late D2 and D3 related quartz veins. In contrast, late D3 fluidsrelated to remobilization are rich in CO2 and K and do not contain mostof the latter trace elements. Trapping conditions of FI related to earlyD2 stage are of 410 (30) °C and 1.75 (0.25) kbar, whereas those relatedto the D3 event where trapped from 320 to 360°C and between 1.1to 1.5 kbar. The Inata gold deposit thus represents a very rare examplewhere nearly pure aqueous mineralizing fluids are documented

Syntectonic fluids redistribution and circulation coupled to quartz recrystallization in the ductile crust (Naxos Island, Cyclades, Greece)

International audienceThe presence of external fluids in metamorphic rocks has been shown to have a profound impact on rock rheology as high fluid pressure processes promote embrittlement and favor ductile deformation by recrystallization. Moreover, it has been proposed that brittle deformation guides fluid circulation and that intracrystalline deformation is responsible for fluid redistribution at the grain scale. Nevertheless, the amount of fluid present in the metamorphic ductile crust is debated and the nature of the interaction between fluids and recrystallization processes are not clearly identified. The aim of this study is to document the spatial distribution of fluid inclusions relative to microstructures in quartz grains and aggregates from veins sampled in amphibolite facies metamorphic rocks, exposed in the island of Naxos in the center of the Attic-Cycladic Metamorphic Complex in Greece. The veins, ranging from discordant structures with sharp contacts to totally transposed structures into the metamorphic foliation, display a large variety of microstructures and fluid evidences interpreted as recording exhumation processes through the ductile/brittle transition: (i) remnants of primary quartz grains contain abundant CO2-H2O fluid inclusions, decrepitated for the most part, distributed in clusters and in fluid inclusion trails, (ii) fluid inclusions with a similar composition are less abundant in recrystallized zones and in subgrains but are concentrated along grain boundaries indicating that grain boundary migration is responsible for redistribution of CO2-H2O fluids, (iii) subgrains of the last generation are almost devoid of fluid inclusions and are characterized by thick grain boundaries with abundant metamorphic fluids locally forming a continuous film. CO2-H2O fluid inclusions aligned in parallel, regularly spaced intragranular trails, locally rooted into grain boundaries, are interpreted as reflecting the spatial redistribution of these fluids in quartz slip planes owing to the increase of fluid pressure in grain boundaries. This proposition is corroborated by the parallelism between slip planes and fluid inclusion trails. Perpendicular sets of fluid inclusion trails formed at the junctions of subgrains suggest that redistribution of fluids along quartz slip planes contributes to the initiation of subgrain rotation recrystallization. Transgranular fluid inclusion planes oriented perpendicular to the regional mineral and stretching lineations of the host metamorphic rocks mark a transition from grain to rock scale brittle behavior associated with the infiltration of H2O fluid, mixing to various degrees with the initial CO2-H2O fluid already present in the metamorphic rocks. These features indicate that, in the ductile-metamorphic crust, fluid redistribution is intimately linked to recrystallization mechanisms allowing fluids circulation under lithostatic pressure

Tectonics, mineralisation and regolith evolution of the West African Craton

The Leo Man Craton in West Africa is host to numerous economic gold deposits. If some regions, such as the SW of Ghana, are well known for world-class mineralizations and have been extensively studied, gold occurrences elsewhere in the craton have been discovered only in the last half a century or so, and very little is known about them. The Julie gold deposit, located in the Paleoproterozoic Birimian terrane of NW Ghana, is one such case. This deposit is hosted in a series of granitoid intrusives of TTG composition, and consists of a network of deformed, boudinaged quartz lodes (A-type veins) contained within an early D-J1 E-W trending shear zone with dextral characteristics. A conjugate set of veins (C-type) perpendicular to the A-type veins contains low grade mineralization. The main ore zone defines a lenticular corridor about 20-50 in in width and about 3.5 km along strike, trending E-Wand dipping between 30 and 60 degrees N. The corridor is strongly altered, by an assemblage of sericite + quartz + ankerite + calcite + tourmaline + pyrite. This is surrounded by a second alteration assemblage, consisting of albite + sericite + calcite + chlorite + pyrite + rutile, which marks a lateral alteration that fades into the unaltered rock. Mass balance calculations show that during alteration overall mass was conserved and elemental transfer is generally consistent with sulfidation, sericitization and carbonatization of the host TTG. Gold is closely associated with pyrite, which occurs as disseminated grains in the veins and in the host rock, within the mineralized corridor. SEM imagery and LA-ICP-MS analyses of pyrites indicate that in A-type veins gold is associated with bismuth, tellurium, lead and silver, while in C-type veins it is mostly associated with silver. Pyrites in A-type veins contain gold as inclusions and as free gold on its edges and fractures, while pyrites from C-type veins contains mostly free gold. Primary and pseudosecondary fluid inclusions from both type veins indicate circulation in the system of an aqueous-carbonic fluid of low to moderate salinity, which entered the immiscibility PT region of the H2O-CO2 NaCl system, at about 220 degrees C and <1 kbar

Multistage Mineralization of the Inata Gold Deposit, Burkina Faso: Insights from Sulphide and Fluid Inclusion Geochemistry

International audienceThe Inata gold deposit formed during the Eburnean orogenesis and is hosted in meta-volcano-sedimentary rocks. Gold mineralization is directly linked to disseminated arsenopyrite and pyrite assemblages which are associated to a quartz-ankerite-albite-tourmaline vein system that formed inside a N-NNE-striking, steeply dipping, D2 deformation zone. LA-ICP-MS analyses reveal that early sulphides, which formed parallel to the metamorphic D1 foliations, have low Au/Ag ratios of 40) in contrast to late D2 sulphide overgrowths which show more or less equal Au/Ag ratios. These assemblages are locally crosscut by narrow D3 fault corridors, where visible gold is observed within fractured sulphides and is interpreted being the result of late stage remobilization during D3. Extraction of fluids indicates that veins associated to D1 and D3 are characterized by high CO2 content whereas especially early D2 veins are dominated by H2O-rich fluids. δD values of fluid inclusions versus δ 18 O isotopic compositions of hosting quartz veins indicate two distinct sources, one of metamorphic/magmatic origin, rich in H2O, and the other one being the result of extensive fluid-rock interactions with C-rich volcano-sediments

Formation and deformation of pyrite and implications for gold mineralization in the El Callao district, Venezuela

The El Callao mining district is the most important gold-producing region in Venezuela. It is hosted in the Paleoproterozoic Guasipati-El Callao greenstone belt, which forms part of the Guayana craton, the Venezuelan extension of the Guiana Shield of South America. It consists of volcanic and volcanosedimentary sequences that were affected by several deformation events, particularly localized shear zones. The Colombia mine, the largest active mine in the district, produces 4 tonnes (t) (128,600 oz) of gold annually with reserves estimated at 740 t (24 Moz) and grades of up to 60 g/t. Gold mineralization is concentrated within a vein network in the Colombia corridor, a shear fracture-hosted mesh of interconnected quartz-ankerite-albite veins enclosing fragments of altered metabasaltic host rocks. Gold occurs mostly in the metabasaltic fragments and is spatially associated with pyrite, in which it occurs as invisible gold, micron-sized native gold inclusions, and filling fractures. Based on optical and scanning electron microscopy-backscattered electron observations, two types of pyrite are recognized: a simple-zoned pyrite and a less common oscillatory-zoned pyrite. Both types consist of a mineral inclusion-rich core and a clearer rim; however, in oscillatory-zoned pyrite, the latter is composed of complex rhythmic overgrowths of alternating As-rich and As-poor bands. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis and elemental mapping reveal the presence of invisible gold in all generations of pyrite. The highest concentrations (5-23 ppm Au) are found in oscillatory-zoned pyrite rims, which correlate with the highest As concentrations (16,000-23,000 ppm). In As-poor bands, Au (up to 1.5 ppm) and As (300-6,000 ppm) concentrations decrease by about an order of magnitude. Copper, Bi, Te, Sb, Pb, and Ag always occur with invisible gold, particularly in pyrite cores, suggesting that at least part of the gold occurs in sulfosalt nanoparticles of these metals and metalloids. Visible native gold grains occur as small inclusions throughout core and rim of both pyrite types, as well as in fractures within it. In both occurrences, chalcopyrite, sphalerite, tellurobismuthite, ankerite, albite, and chlorite accompany native gold, and gold fineness ranges between 900 and 930. At an early stage of vein mesh formation, pyrite formed in the metabasaltic fragments at the expense of ankerite, which, in turn, resulted from alteration of Fe-Ti oxides. Gold, together with other chalcoplaile elements, was incorporated within the structure of pyrite, most likely by destabilization of metal sulfide complexes during ankerite replacement. Subsequent cyclic reactivations of the shear zone caused development of pressure shadows around pyrite, generating local and repeated decreases in pressure, which triggered local boiling of the hydrothermal fluid, as evidenced by the presence of primary fluid inclusions containing immiscible liquid-rich and vapor-rich aqueous-carbonic fluids. This process was responsible for a number of physical-chemical changes in the liquid, all of which contributed to the formation of the As- and Au-rich overgrowths in pyrite: (1) removal of H2O into the vapor phase, inducing saturation of dissolved metals in the remaining liquid; (2) an increase in pH due to partition of H2S and CO2 into the vapor, thus decreasing the solubility of sulfide minerals; and (3) an adiabatic decrease in temperature, lowering the solubility of As and Au in the liquid. Waning of this process restored precipitation of As-poor pyrite, until the onset of a new cycle. Because pressure drops are more significant adjacent to open spaces, oscillatory-zoned pyrite probably crystallized near newly formed veins whereas simple-zoned pyrite formed away from them. Previously formed pyrite underwent fracturing during reactivation of the deformation, especially through the brittle deformation events that postdated shearing, resulting in local pulverization of pyrite. This newly created porosity facilitated fluid circulation and r

Deformation of the lithosphere : how small structures tell a big story

We document the interplay between meteoric fluid flow and deformation processes in quartzite-dominated lithologies within a ductile shear zone in the footwall of a Cordilleran extensional fault (Kettle detachment system, Washington, USA). Across 150 m of shear zone section, hydrogen isotope ratios (delta D) from synkinematic muscovite fish are constant (delta D similar to -130 parts per thousand) and consistent with a meteoric fluid source. Quartz-muscovite oxygen isotope thermometry indicates equilibrium fractionation temperatures of similar to 365 +/- 30 degrees C in the lower part of the section, where grain-scale quartz deformation was dominated by grain boundary migration recrystallization. In the upper part of the section, muscovite shows increasing intragrain compositional zoning, and quartz microstructures reflect bulging reaystallization, solution-precipitation, and microcracking that developed during progressive cooling and exhumation. The preserved microstructural characteristics and hydrogen isotope fingerprints of meteoric fluids developed over a short time interval as indicated by consistent mica Ar-40/Ar-39 ages ranging between 51 and 50 Ma over the entire section. Pervasive fluid flow became increasingly channelized during detachment activity, leading to microstructural heterogeneity and large shifts in quartz delta O-18 values on a meter scale. Ductile deformation ended when brittle motion on the detachment fault rapidly exhumed the mylonitic footwall