Traitement de compilations de données issues d'analyses au LA-ICP-MS effectuées sur des sulfures aurifères

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Characterization of banded iron formations associated with gold mineralization: primary geochemical signatures and exploration implications

Algoma-type banded iron formations (BIFs), which represent chemical sedimentary rocks characterized by alternating layers of iron-rich minerals and chert intercalated with Eoarchean to late Paleoproterozoic volcano-sedimentary sequences, act as a favorable host rock for orogenic gold mineralization within several Archean cratons (i.e., Pilbara, Kaapvaal, Superior, Slave and Churchill). Besides this economic aspect, these Fe-rich sequences have long been appreciated as an important contributor to furthering our understanding of the geochemical evolution of the Earth. However, these deposits are in general tectonically deformed, metamorphosed and dismembered, thus making reconstruction of their depositional setting and overall geologic setting difficult. Based on four Canadian BIF-hosted gold deposits (the Meadowbank, Meliadine, Musselwhite and Beardmore-Geraldton deposits), this thesis aims to establish the depositional setting of the Algoma-type BIF using the abundance of REE+Y of chert material used as proxy of the primary signature, as well as assess gold enrichment processes based on textures and trace element zoning of variable sulfides (i.e., pyrite, arsenopyrite and pyrrhotite) and finally study if there is a particular geochemical type of Algoma-type BIF associated with gold mineralization. Laser ablation-inductively coupled plasma-mass spectrometric (LA-ICP-MS) analyses performed on chert material suggest that BIFs from the four deposits show common depositional settings illustrated by deposition in semi-closed to closed basin under variable influence of high-temperature (>250ºC) hydrothermal fluids input and detrital contamination. Moreover, evidence of late diagenetic processes involving O isotopic exchange between chert precursor (i.e., opaline material) and seawater origined fluid have been documented. According to their primary v signature, it appears that barren versus mineralized BIFs do not show any geochemical divergence suggesting that the depositional setting may influence the epigenetic gold mineralization. Based on quantitative element distribution maps combined with line traverse and spot analyses by LA-ICP-MS on sulfides, a common gold mineralizing event characterized by intense stratabound sulfide-replacement of Fe-rich material was reported in three studied deposits exhibiting a Au-As-Se-Te-Ag element association. This result suggests that metamorphic/hydrothermal orogenic processes driven by devolatilization of a common weakly to unmetamorphosed source rock have ledto generation of gold-bearing fluid which channelled into Algoma-type BIF via major crustal faults and/or shear zones within low tensile strength rocksDoctor of Philosophy (PhD) in Mineral Deposits and Precambrian Geolog

Typology of hard-rock Li-hosted deposits in Europe

International audienceLithium became a strategic metal in the last decade due to its widespread use in electromobility and green technologies. Consequently, demand has increased significantly reviving European interest in lithium mining and leading many countries to assess their own mineral resources/reserves to secure their own supplies. A compilation of European lithium hard-rock occurrences and a systematic assessment of metallogenic processes related to Li-mineralization have been produced. Accordingly, it appears that lithium is well represented through various deposit types related to several orogenies from Precambrian to Miocene ages. Thus, these deposits have been identified as mostly resulting from endogenous processes such as lithium-cesium-tantalum (LCT) pegmatites (e.g. Sepeda in Portugal; Aclare in Ireland; Läntta in Finland), rare-metal granites (RMG; Beauvoir in France; Argemela in Portugal) and greisens (e.g. Tregonning-Godolphin, Meldon in UK; Cinovec in Czech). Local exogenous processes may be related to significant Li-endowments such as jadarite precipitation in the Jadar Basin (Serbia) but are rarely related to economic grade and tonnage of lithium. Moreover, common parameters are identified in the Li endowment processes including: 1) a pre-existing Li-rich source; 2) a lithospheric thickening; and 3) an extensional regime

Deciphering multi-stage ore-forming processes in metasedimentary-rock-hosted orogenic gold deposit settings using LA ICP-MS sulphide analysis

International audienceOrogenic gold deposits span a spectrum in regards to setting and style of mineralization, nature of gold (e.g., invisible, coarse) and favourable host rock. In addition, deposit formation is often attributed to protracted multi-stage hydrothermal processes. That gold mineralization also depends on a variety of features, such as metal-source reservoirs, metal-transport processes and wall-rock stratigraphy, which add to the challenge of investigating origin of paleo-mineralized settings. Recent work using quantitative laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS) element distribution maps/profiles and their corresponding time slice datasets (TSD) provides new insight into identifying and assessing elemental paragenesis, multi-dimensional element coupling/decoupling processes, and corresponding mineralizing events. To further assess complexities of mineralization, application of geostatistical tools (e.g., multidimensional scaling, principal component analysis, linear discriminant analysis) and various innovative multi-element binary plots (e.g., Ag versus Au, Ni versus Co) is advised. To illustrate the application of this methodology, the results LA ICP-MS mapping and data processing for Fe sulphides from several metasedimentary-rock-hosted Canadian gold systems are presented: three Archean Algoma-type BIF-hosted gold deposits (~ 4 Moz Au Meadowbank, ≥ 2.8 Moz Au Meliadine district, ~ 6 Moz Au Musselwhite) and eight slate-belt style vein gold deposits from the Paleozoic Meguma terrane (Nova Scotia). The maps and derived elemental plots generated from the various settings demonstrate that: 1) the gold mineralization present is the product of multi-stage processes; 2) elemental associations vary as mineralization progresses, such as the early growth history of sulphides versus later coupled dissolution-precipitation reactions; and 3) different metal-source reservoirs and stratigraphy influence the fluid signature. These results contribute to better deciphering the complex processes involved in the protracted evolution of these and other orogenic-type gold systems (e.g., remobilization of invisible gold from early sulphide, precipitation of visible gold in later sites, increase of Au fineness)

Do Magnetite Layers In Algoma-Type Banded Iron Formations (BIF) Preserve Their Primary Geochemical Signature? A Case Study of Samples From Three Archean BIF-Hosted Gold Deposits

International audienceThe geochemistry of chert layers in Algoma-type banded iron formations (BIF) has been used to constrain the depositional setting of the BIFs, as rare earth element (REE) and yttrium (Y) systematics are a function of their chemical environment of formation. In contrast, the chemistry of the interbedded oxide-rich layers (i.e., magnetite) has not been analyzed for this purpose because of the presumed potential effects related to diagenetic changes during conversion from primary iron-bearing mineralogy to magnetite. Here, we explore the validity of this latter hypothesis by examining the results of LA-ICP-MS analysis of iron-oxide layers at three Canadian BIF-hosted gold deposits (i.e., Meadowbank, Meliadine, Musselwhite) to assess whether the primary REEþY systematics of the oxide layers are preserved and, if so, what are the implications. The results indicate that, regardless of their diagenetic, later metamorphic, and hydrothermal histories, the chemistry of the iron oxides retains a primary signal in all cases with the following indicated: (1) interaction of the primary Fe-oxyhydroxide phases with seawater, as reflected by heavy REE enrichment relative to light REE depletion that is coupled with variable La and Y enrichment; and (2) some input of moderate-temperature (.250 8C) hydrothermal vent fluids, as suggested by positive Eu anomalies. The chemical data are also used to evaluate currently used classification diagrams for ore deposits based on magnetite chemistry. Our new data indicate that the fields for magnetite geochemistry in BIF are too restricted or lead to misclassification of samples. In the case of the latter, it may be that interaction of fluids with the immediate substrate influences the chemical signature of samples. Therefore, caution is suggested in using these diagrams where hydrothermal fluids are involved in magnetite formation

Insights into the gold metallogeny of the Meguma terrane of Nova Scotia from LA-ICP-MS arsenopyrite geochemistry

International audienceThe time-space distribution of ore deposits in regards to the evolution of geological terranes is known as metallogeny. In the case of the Phanerozoic Meguma terrane of southern Nova Scotia, the origin of its classic slate-belt hosted orogenic gold systems (i.e., Meguma gold deposits) has been the focus of study for well over a century. Despite considerable effort involving many sub-disciplines (e.g., field studies, structural analysis, lithogeochemistry, geochronology), many unanswered questions remain about these deposits, such as fluid and metal reservoirs, single or multiple ore-forming events, and nature of the mineralization. The advent of in situ LA-ICP-MS analysis has provided the means to assess the geochemical evolution of single minerals using a large elemental database at a spatial resolution (10s μm) not available before. Here we apply a novel approach to processing and interpreting such LA data generated from mapping arsenopyrite from eight gold deposits across the central and eastern Meguma terrane (i.e., the Ovens to Upper Seal Harbour) to further investigate its gold metallogeny. Using our approach, we first establish an elemental paragenesis for the mineralization in each deposit, assess the number of gold events and their nature (i.e., refractory vs non-refractory), characterize elemental associations and elemental abundances through time, and assess elemental reservoirs (e.g., Co, Ni) using a new set of discriminant diagrams. The results document a similar geochemical fingerprint for gold mineralization across the terrane based on a similar elemental paragenesis in deposits, but also indicates several gold events. Using Au-Ag binary plots with false-color coding to highlight elemental associations, the earliest gold event (>10 ppm Au; Au:Ag=10) is identified by a Co-Ni-Mo-Sb-Se association which equates spatially to primary growth zones in arsenopyrite. In contrast, a second event, characterized by an Al-Ti-V-Mn element association, Au:Ag>100, and spatially associated with late fractures, is attributed to remobilization and upgrading of earlier refractory gold. A third event, characterized by a Bi-Pb-Cd-In-Ag elemental association, predates the second gold event and is also reflected in archived whole-rock lithogeochemical data. Collectively these results indicate a protracted history of precious-metal mineralization in all deposits from an early refractory event to subsequent remobilization events, which may be similar or separate temporally. In addition, the Co-Ni data may reflect interaction of ore-forming fluids with mafic rocks at depth. This study demonstrates therefore the important application of our approach to long standing problems in gold metallogeny, such as the Meguma terrane and also elsewhere

Results of LA-ICP-MS sulfide mapping from Algoma-type BIF gold systems with implications for the nature of mineralizing fluids, metal sources, and deposit models

International audienceQuantitative laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) element distribution maps combined with traverse mode analyses have been acquired on various sulfides (pyrite, pyrrhotite, arsenopyrite) from three Canadian Algoma-type BIF-hosted gold deposits (~4 Moz Au Meadowbank, ≥ 2.8 Moz Au Meliadine district,~6 Moz Au Musselwhite). These data, in conjunction with detailed petrographic and SEM-EDS observations, provide insight into the nature and relative timing of gold events, the presence and implication of trace element zoning regarding crystallization processes, and elemental associations that fingerprint gold events. Furthermore, the use of an innovative method of processing the LA-ICP-MS data in map and traverse modes, whereby the results are fragmented into time-slice data, to generate various binary plots (Ag versus Ni) provides a means to identify elemental associations (Te, Bi) not otherwise apparent. This integrated means of treating geochemical data, along with petrography, allows multiple gold events and remobilization processes to be recognized and their elemental associations determined. The main gold event in each of these deposits is characterized by the coupling of an As-Se-Te-Ag element association coincident with intense stratabound sulfide-replacement of the Fe-rich host rock. Additionally, the data indicate presence of a later remobilization event, which upgraded the Au tenor, as either non-refractory or refractory type, along fracture networks due to the ingress of subsequent base metal-bearing metamorphic fluids (mainly a Pb-Bi association). Furthermore, the data reveal a stratigraphic influence, as reflected in the elemental associations and the elemental enrichments observed and the nature of the sulfide phase hosting the gold mineralization (arsenopyrite versus pyrite)

Environnements de dépôt des formations de fer rubanées archéennes de type Algoma et leurs implications sur la minéralisation aurifère

International audienceLes formations de fer rubanées (FFR) de type Algoma sont des roches sédimentaires d'origine chimique, constituées d'une alternance de minéraux riches en fer et de chert, stratigraphiquement associées à des roches volcaniques sous-marines et situées dans des ceintures de roches vertes archéennes. Ces roches sont parfois associées à des gisements aurifères et l'hypothèse d'un enrichissement primaire en or des FFR a été proposée par certains pour expliquer la présence de gisements d'or dans certaines FFR et non dans d'autres. Les conditions menant à la formation des FFR et leurs caractéristiques géologiques demeurent controversées, principalement en raison de la superposition des effets de la déformation post-sédimentaire et du métamorphisme associé, mais également en raison de l'absence d'analogues modernes permettant de réaliser des études comparatives. Récemment, il a été suggéré que l'abondance des éléments du groupe des terres rares (ETR) et de l'yttrium dans les bandes de chert, lesquelles sont peu réactives et très résistantes aux modifications post-déposition et ainsi beaucoup plus susceptibles de conserver leurs caractéristiques géochimiques primaires, pouvait fournir des informations primordiales sur l'environnement de dépôt des FFR. L'analyse géochimique par ablation laser et spectrométrie de masse à plasma couplé par induction (LA ICP-MS) a été effectuée sur des bandes de chert de quatre gisements aurifères canadiens (i.e., Meadowbank, Meliadine, Musselwhite et Beardmore-Geraldton) associés à des FFR. Le chert de ces quatre sites est caractérisé par : (1) un enrichissement en ETR lourdes couplé à une anomalie positive en La et Y indiquant l'interaction de l'eau de mer avec des oxyhydroxydes de fer; (2) l'anomalie positive en Eu indiquant une contribution variable de fluides hydrothermaux de haute température (>250°C); et (3) un rapport Y/Ho près des valeurs chondritiques (i.e., Y/Ho≈27) et des concentrations en ETR cohérentes témoignant d'une contamination détritique variable. Les valeurs de pH de la colonne d'eau au moment de la précipitation du chert ont été évaluées en utilisant le rapport Ce/Ce* MUQ. Ainsi, un ratio positif suggère des conditions relativement acides (i.e., pH ≤ 5) pour la plupart des échantillons, alors que des conditions plus alcalines (i.e., pH ≥ 5) illustrées par un rapport négatif, sont indiquées pour les échantillons montrant une précipitation d'oxyhydroxyde de fer dans les bandes de chert. L'étude comparative de ces quatre sites indique que la composition primaire, l'environnement de dépôt et les conditions de formation des FFR ne semble pas influencer la formation de gisements d'or, appuyant l'origine épigénétique des gisements aurifères associés aux FFR

Environnements de dépôt des formations de fer rubanées archéennes de type Algoma

International audienceLes formations de fer rubanées (BIF) de type Algoma sont des roches sédimentaires d'origine chimique, constituées d'une alternance de minéraux riches en fer et de chert, strati-graphiquement associéesà des roches volcaniques sous-marines et situées dans des ceintures de roches vertes archéennes. Ces roches sont parfois associéesà des gisements aurifères. Les conditions menantà la formation des BIF et leurs caractéristiques géologiques demeurent controversées, principalement en raison de la superposition des effets de la déformation post-sédimentaire et du métamorphisme associé, maiségalement en raison de l'absence de formations analogues modernes permettant de réaliser desétudes comparatives. Récemment, il á eté suggéré que l'abondance deséléments du groupe des terres rares (ETR) et de l'yttrium dans les bandes de chert, lesquelles sont peu réactives et très résistantes aux modifications post-déposition et ainsi beaucoup plus susceptibles de conserver leurs caractéristiques géochimiques primaires, pouvait fournir des informations fiables sur l'environnement de dépôt des BIF. L'analyse géochimique par ablation laser et spectrométrie de masseà plasma couplé par induction (LA ICP-MS) aété effectuée sur des bandes de chert de trois gisements aurifères canadiens (Meadowbank, Meliadine et Musselwhite) associésà des BIF. Le chert de ces trois sites est caractérisé par : 1) un enrichissement en ETR lourdes coupléà une anoma-lie positive en La et Y indiquant l'interaction de l'eau de mer avec des oxyhydroxydes de fer; 2) l'anomalie positive en Eu indiquant une contribution variable de fluides hydrother-maux de haute température (> 250•C); et 3) un rapport Y/Ho près des valeurs chondri-tiques (Y/Ho≈27) et des concentrations en ETR cohérentes témoignant d'une contamination détritique variable. Les valeurs de pH de la colonne d'eau au moment de la précipitation du chert ontétéévaluées en utilisant le rapport Ce/Ce*MUQ. Ainsi, un ratio positif suggère des conditions relativement acides (pH ≤ 5) pour la plupart deséchantillons, alors que des conditions plus alcalines (pH ≥ 5) illustrées par un rapport négatif, sont indiquées pour leś echantillons montrant une précipitation d'oxyhydroxydes de fer dans les bandes de chert

Depositional Setting of Algoma-type Banded Iron Formation

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