A textural and chemical study of white mica in the South Mountain Batholith, Nova Scotia: primary versus secondary origin

White mica (HM) In peralumlnous granitoid rocks of the South Mountain Batholith (SMB) and East Kemptville leucogranite (EKL) of Nova Scotia have been examined to see if parameters can be used to distinguish between primary and secondary grains. Texturally very little of the UN in the SMB can unequivocally be classified as primary, whereas most of the UM In the EKL is consistent with such an origin. Although discriminant diagrams which utilize major element chemistry do not provide unambiguous divisions between primary and secondary HM, there appear to be some chemical trends which indicate that the bulk composition of the host rock is an important control. This is best exemplified by volatile (i.e. F) and trace element contents. Including the rare earth elements. For example, F, Li, Rb and Cs are systematically higher In UM from relatively more evolved units of the SMB. The importance of bulk rock composition is also indicated by the enrichment of lithophile elements In UM from the EKL. itself enriched in these same elements. The octahedral impurities in UM of the SMB are accommodated via blotltlc and pbengitic substitutions, whereas UM from the EKL is dominantly pbengitic. Comparison to experimentally determined stability fields for muscovite indicate that HM from the SMB re-equilibrated to 500-600oC In the more primitive units and 400-550oC In the more evolved units and greisens. Recent experimental data also suggest crystallization of the UM may have occurred at pressures of ca. 2 kb in melts with 2-3 wt.% H2O. RÉSUMÉ On a examiné les micas blancs (MB) dans les roches granltoldes péralumlneuses du Batholite de South Mountain (BSM) et du leucogranite d'East Kemptville (LEK) en Nouvelle-Écosse afin de déterminer si certains paramétres peuvent servlx a dlstinguer les grains primalres des grains secondaires. É l'égard de la texture, une très faible proportion des MB dans le BSM peuvent, sans l’ombre d'un doute, être classes comae primalres alors que la plupart des MB dans le LEK sont compatibles avec une telle orlglne. Bien que les diagrammes discriminant par chimle des éléments majeure ne procurent aucune Unite precise entre les MB primalres et secondaires, on semble y discemer des tendances chlmiques lndlquant une Influence préponderante de la composition totale de l'encaissant. Les contenus en éléments volatiles (i.e. F) et en traces, y comprises les terres rares, en sont la meilleure illustration. Par exemaple, la teneur en F, Li, Rb et Cs est systèmatiquement plus éleveé dans les MB provenant des unités plus évoluées, par comparaison, du BSM. L'importance de la composition totale de la roche est aussi attestée par l'enrichtssement en éléments lithophiles des MB du LEK (luimeme enrichl en ces éléments). L'accoramodation des impurités octaédriques dans les MB du BSM se fait par le biais de substitutions biotitiques et phengitiques alors que les MB du LEK sont surtout phengitiques. Une comparaison des MB du BSM aux doroaines de stabilites de la muscovite détermines experimentalement révèle que leur ré- équilibrage s'est effectue entre 500 et 600oC dans les unités les plus primitives et entre 400 et 550oC dans les unités plus évoluées et les greisens. À la lumlère de données expérimentales récentes, la cristallisation des MB aurait eu lieu àa des pressions d'environ 2 kb dans des bains ayant de 2 à 3 % en poids d'H2O. [Traduit par le journal

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)

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)

Depositional Setting of Algoma-type Banded Iron Formation

International audienc

Latest Triassic onset of the Central Atlantic Magmatic Province (CAMP) volcanism in the Fundy Basin (Nova Scotia): New stratigraphic constraints

In this paper we investigate the stratigraphic relationship between the emplacement of the CAMP basalts and the Triassic–Jurassic (Tr–J) boundary in the Fundy Basin (Nova Scotia, Canada). This is one of the best exposed of the synrift basins of eastern North America (ENA) formed as a consequence of the rifting that led to the formation of the Atlantic Ocean. The Triassic palynological assemblages found in the sedimentary rocks below (uppermost Blomidon Formation) and just above the North Mountain Basalt (Scots Bay Member of the McCoy Brook Formation) indicate that CAMP volcanism, at least in Nova Scotia, is entirely of Triassic age, occurred in a very short time span, and may have triggered the T–J boundary biotic and environmental crisis. The palynological assemblage from the Blomidon Formation is characterised by the dominance of the Circumpolles group (e.g. Gliscopollis meyeriana, Corollina murphyae, Classopollis torosus) which crosses the previously established Tr–J boundary.The Triassic species Patinasporites densus disappears several centimetres below the base of the North Mountain basalt, near the previously interpreted Tr–J boundary. The lower strata of the Scots Bay Member yielded a palynological assemblage dominated by Triassic bisaccate pollens (e.g Lunatisporites acutus, L. rhaeticus Lueckisporites sp., Alisporites parvus) with minor specimens of the Circumpolles group. Examination of the state of preservation and thermal alteration of organic matter associated with the microfloral assemblages precludes the possibility of recycling of the Triassic sporomorphs from the older strata. Our data argue against the previous definition of the Tr–J boundary in the ENA basins, which was based mainly on the last occurrence of P. densus. Consequently, it follows that the late Triassic magnetostratigraphic correlations should be revised considering that chron E23r, which is correlated with the last occurrence of P. densus in the Newark basin, does not occur at the Tr–J boundary but marks rather a late Triassic (probably Rhaetian) reversal

40Ar/39Ar ages of CAMP in North America: implications for the Triassic-Jurassic boundary and the 40K decay constant bias

The Central Atlantic magmatic province (CAMP) is one of the largest igneous provinces on Earth (>107 km²) and spanning four continents. Recent high-precision 40Ar/39Ar dating of mineral separates has provided important constraints on the age, duration, and geodynamic history of CAMP. Yet, the North American CAMP is strikingly under-represented in this dating effort. Here we present 13 new statistically robust plateau, mini-plateau and isochron ages obtained on plagioclase and sericite separates from lava flows from the Fundy (n = 10; Nova Scotia, Canada) and Hartford and Deerfield (n = 3; U.S.A.) basins. Ages mostly range from 198.6 ± 1.1 to 200.1 ± 1.4 Ma (2σ), with 1 date substantially younger at 190.6 ± 1.0 Ma. Careful statistical regression shows that ages from the upper (199.7.0 ± 1.5 Ma) and bottom (200.1 ± 0.9 Ma) units of the lava pile in the Fundy basin are statistically indistinguishable, confirming a short duration emplacement (<< 1.8 Ma; ≤1 Ma). Three ages obtained on the Hartford (198.6 ± 2.0 Ma and 199.8 ± 1.1 Ma) and Deerfield (199.3 ± 1.2 Ma) basins were measured on sericite from the upper lava flow units. We interpret these dates as reflecting synemplacement hydrothermal activity within these units. Collectively, CAMP ages gathered so far suggest a short duration of the main magmatic activity (2-3 Ma), but also suggest the possibility of a temporal migration of the active magmatic centers from north to south. Such a migration challenges a plume model that would postulate a radial outward migration of the magmatism and is more compatible with other models such as the supercontinent global warming hypothesis. When compared to the age of the Triassic-Jurassic boundary, the filtered CAMP age database suggests that the onset of the magmatic activity precedes the limit by at least few hundred thousand years, therefore suggesting a causal relationship between CAMP and the end of Triassic mass extinction. An age at 191 Ma possibly suggests a minor CAMP late tailing activity (190-194 Ma) which has already observed for dykes and sills in Africa and Brazil. We speculate that, if real, this late activity can be due to a major extensional event, possibly heralding the oceanisation process at ~190 Ma. Comparison between high quality U/Pb and 40Ar/39Ar ages of pegmatite lenses from the North Mountain basalts confirms a ~1% bias between the two chronometers. This discrepancy is likely attributed to the miscalibration of the 40K decay constants, in particular the electron capture branch

Application of LA ICP-MS analysis of arsenopyrite to gold metallogeny of the Meguma Terrane, Nova Scotia, Canada

International audienceInvestigation of gold metallogeny in the Paleozoic Meguma terrane (Canada) is conducted through LA-ICP-MS analysis of arsenopyrite collected from eight slate-belt style vein gold deposits using a novel approach integrating elemental distribution maps and their derived elemental paragenesis with multi-element binary plots. The data reveal two distinct gold events: 1) an early event characterized by a Co-Ni-Mo-Sb-Se elemental association related to initial growth of arsenopyrite that reflects the presence of invisible gold (>10 ppm); and 2) a second event, spatially associated with late fracture sets, that is characterized by an Al-Ti-V-Mn element association and reflects either remobilization or upgrading of primary invisible gold and is manifest as visible gold. The results of this study indicate a complex and protracted history of gold mineralization which has important ramifications for the Meguma gold deposits, as well as other orogenic gold districts globally. In the case of the Meguma Terrane, it involves an initial gold event that is followed by element mobilization and, in the case of precious metals (Au, Ag), an upgrading through a zone refining process. In addition, the variable coupling and decoupling of elements is only revealed using in-situ derived LA-ICP-MS data