Louise Corriveau

Geochemistry of the granulitic Bondy gneiss complex: a 1.4 Ga arc in the Central Metasedimentary Belt, Grenville Province, Canada

The 1.4 Ga Bondy gneiss complex (BGC) is one of a series of gneiss complexes that outcrops as tectonic domes structurally below Grenvillian, 1.3/1.25 Ga, marble and quartzite assemblages in the Central Metasedimentary Belt (CMB) of Quebec, western Grenville Province. The complex comprises a variety of tonalitic to granitic orthogneiss intercalated with thin units of layered metabasite and laminated felsic gneiss that host a Cu/Au/Fe oxides hydrothermal system. A metatonalite pluton outcrops at the southern end of the complex. These rocks were metamorphosed at granulite-facies at 1.20 Ga. In spite of this metamorphism and variable Rb, U and/or Th loss, LILE depletion in the mafic and felsic gneisses is subordinate, and was mainly controlled by the mineralogy of the protoliths during percolation of metamorphic fluids. In most cases, trace element signatures of the granulites do not show evidence for REE and HFSE mobility. Their major, trace element and Nd-isotope geochemistry displays systematic and highly reproducible signatures attributable to volcanic and plutonic rocks formed in a mature island-arc setting. The arc event is inferred from the association of mafic granulite, intermediate gneiss, and tonalite with similar geochemistry to that of modern mature calc-alkaline island-arc rocks. Some of the mafic granulites and laminated quartzofeldspathic gneisses display geochemical signatures characteristic of modern tholeiitic basalts and high-silica rhyolites related to back-arc rifting. The NdT values suggest a depleted mantle source with the addition of a crustal component to form the felsic and intermediate magmas. Taking as an example the Sunda, Lesser Antilles and New Zealand island-arcs, we advocate that 1.4 Ga magmatism occurred in mature island-arc, back-arc system along the Laurentian margin. # 2002 Elsevier Science B.V. All rights reserved.

Coexisting K-rich alkaline and shoshonitic magmatism of arc affinities in the Proterozoic: a reassessment of syenitic stocks in the southwestern Grenville Province

Biotite-rich syenitic stocks in the Mont-Laurier area of the southwestern Grenville Province are shown to belong to the first recorded Proterozoic example of an ultrapotassic, K-rich alkaline and shoshonitic rock association with clear arc affinities. The plutons investigated were previously considered mostly syenitic, typical of nepheline syenite alkaline suites, slightly metamorphosed and late-tectonic with respect to the Grenville orogeny. We find that they postdate the regional metamorphism and comprise a felsic to ultramafic range of rock types belonging to two series: (1) a potassic-to-ultrapotassic, silica-undersaturated series of biotite-rich nepheline-bearing syenite, syenite, monzonite, diorite and pyroxenite, and (2) a shoshonitic, critically silicasaturated series of quartz syenite and amphibole-bearing syenite, with rare monzonite and diorite. The ubiquitous biotite, previously regarded as metamorphic, is reinterpreted as igneous and diagnostic of the potassic character. The shoshonitic and potassic series display the strong enrichment in Al, Ca, K and large-ion-lithophile elements relative to the high-field-strength elements (e.g. Ba/Nb≤722, La/YB∼45) and the low contents in Mg that are characteristic of arc-related magmas. The syenitic rocks consistently share the distinctive arc-related geochemical signature of their mafic counterparts. Syenites may thus represent a potential source of paleotectonic information for high grade terranes. Geochemical discriminants (NbN/TaN and HfN/TiN ratios) indicate that the shoshonitic and potassic series are unrelated by closedsystem fractionation processes. Rather, the chemical differences between the two series probably reflect differences in source characteristics and conditions of melting. Similar plutons occur throughout the Central Metasedimentary Belt of the southwestern Grenville Province. They define a 1089 to 1076 Ma, 450-km-long grenvillian potassic alkaline plutonic (PAP) province. The presence of this K-rich alkaline province indicates that the scarcity of K-rich rocks in the Precambrian could be only apparent and a consequence of misidentification of K-rich plutons in metamorphosed Precambrian terranes. These 1.1 Ga ultrapotassic to shoshonitic plutonic rocks are geochemically similar to shoshonites and leucitites of the Sunda arc. This similarity suggests that subduction-type enrichment processes were operating in the Proterozoic in ways similar to those of modern settings.

Compositional data analysis of hydrothermal alteration in IOCG systems, Great Bear magmatic zone, Canada: to each alteration type its own geochemical signature

Iron oxide copper-gold (IOCG) systems are characterized by a wide range of hydrothermal alteration types that can indiscriminately and intensively replace their host rocks over areas of > 100 km2. Element mobility and chemical changes associated with alteration can be of a magnitude beyond that of many other types of hydrothermal systems, and may also affect normally immobile elements. Principal component analysis of whole-rock geochemical data on hydrothermally altered samples coming from the Great Bear magmatic zone IOCG systems has enabled the characterization of sodic, calcic-iron, to high to low temperature potassic-iron and potassic alteration types of IOCG systems. Results show that potassic and potassic-iron alteration features are enriched in K, Al, Ba, Si, Rb, Zr, Ta, Nb, Th and U, with potassic-iron alteration being richer in Fe. In contrast, calcic-iron alteration is enriched in Ca, Fe, Mn, Mg, Zn, Ni and Co. These compositional variations can be portrayed by IOCG alteration index and discriminant diagrams. Combined with an IOCG alteration sequencing model, the lithogeochemical footprint of IOCG systems provides a useful tool to assess the potential fertility and maturity of IOCG systems and ultimately a vector towards ore zones during exploration

An orientation study of the heavy mineral signature of the NICO Co-Au-Bi deposit, Great Bear magmatic zone, NW Territories, Canada

ABSTRACT An orientation study around the NICO Co-Au-Bi deposit in the Great Bear magmatic zone of NW Territories, Canada, was initiated in 2007 to establish a practical guide to geochemical and mineralogical exploration for iron oxide copper-gold deposits in glaciated terrain. Bedrock and till samples were collected up-ice, proximal and down-ice from mineralization and host rocks, to characterize their indicator mineral signatures. Results demonstrate that gold grain abundance, size and shape, as well as magnetite and hematite composition, have the best potential to fingerprint the mineralization at NICO. Pristine-shaped gold grains indicative of a local bedrock source and a short distance of glacial transport are relatively abundant in till samples collected immediately down-ice from several mineral occurrences at NICO and none were recovered up-ice. Iron oxide composition using preliminary discriminant diagrams shows some potential, using Ni/(Mn+Cr) versus Ti+V plots. In particular, magnetite and hematite from till samples collected over, or directly down-ice of, the NICO deposit have lower Ti+V compositions compared to magnetite and hematite from till collected up-ice from mineralization. Potential non-ferromagnetic indicator minerals are either not chemically stable in surface sediments (arsenopyrite, chalcopyrite, pyrite), not sufficiently coarse-grained or resistant to glacial transport (bismuthinite, tourmaline, ferroactinolite), not abundant enough in the mineralized bedrock (scheelite, molybdenite, cobaltite, allanite), or not sufficiently heavy (tourmaline) to be useful at NICO but may be at other deposits in the region or elsewhere in glaciated terrain. The development of indicator mineral methods, together with till geochemistry, will be tested with further sampling over the Great Bear magmatic zone.

Trace element composition of iron oxides from IOCG and IOA deposits: relationship to hydrothermal alteration and deposit subtypes

Trace element compositions of magnetite and hematite from 16 well-studied iron oxide–copper–gold (IOCG) and iron oxide apatite (IOA) deposits, combined with partial least squares-discriminant analysis (PLS-DA), were used to investigate the factors controlling the iron oxide chemistry and the links between the chemical composition of iron oxides and hydrothermal processes, as divided by alteration types and IOCG and IOA deposit subtypes. Chemical compositions of iron oxides are controlled by oxygen fugacity, temperature, co-precipitating sulfides, and host rocks. Iron oxides from hematite IOCG deposits show relatively high Nb, Cu, Mo, W, and Sn contents, and can be discriminated from those from magnetite + hematite and magnetite IOA deposits. Magnetite IOCG deposits show a compositional diversity and overlap with the three other types, which may be due to the incremental development of high-temperature Ca–Fe and K–Fe alteration. Iron oxides from the high-temperature Ca–Fe alteration can be discriminated from those from high- and low-temperature K–Fe alteration by higher Mg and V contents. Iron oxides from low-temperature K–Fe alteration can be discriminated from those from high-temperature K–Fe alteration by higher Si, Ca, Zr, W, Nb, and Mo contents. Iron oxides from IOA deposits can be discriminated from those from IOCG deposits by higher Mg, Ti, V, Pb, and Sc contents. The composition of IOCG and IOA iron oxides can be discriminated from those from porphyry Cu, Ni–Cu, and volcanogenic massive sulfide deposits.