Iron-oxide and alkali-calcic alteration, skarn, and epithermal mineralizing systems of the Grenville Province: the Bondy gneiss complex in the Central Metasedimentary Belt of Quebec as a case example — a field trip to the 14th Society for Geology Applied to Mineral Deposits (SGA) biennial meeting
Abstract
The 2017 Society for Geology Applied to Mineral Deposits (SGA) FT-02 field trip is taking place in the east-central part of the Central Metasedimentary Belt of Quebec, Grenville Orogen. The trip is an opportunity to ponder the nature of gneiss complexes in the belt, the pathways and styles of magma emplacement at deep crustal levels, the development of skarn, magma mingling and assimilation zones, and the origin of graphite deposits. The field trip also provides an overview of the architecture and Grenvillian and pre-Grenvillian tectono-magmatic evolution of the Central Metasedimentary Belt in Quebec (Grenville Province) through sedimentation, volcanism, hydrothermal activity, magma emplacement, metamorphism, and deformation. It illustrates the rheological contrast across the different domains of Mont-Laurier area of the Central Metasedimentary Belt as recorded by styles of magma emplacement and extent of deformation of dyke swarms and intrusions.
The Central Mineral Belt (CMB) in Labrador, Canada, hosts multiple U (±base ± precious metal) showings, prospects and deposits in metamorphosed and variably hydrothermally altered Neoarchean to Mesoproterozoic, igneous and sedimentary rocks. Previous work has recognized U mineralization locally associated with Fe-Ca and alkali metasomatism typical of metasomatic iron oxide and alkali-calcic alteration systems (IOAA) that host iron oxide-copper-gold (IOCG) and affiliated critical metal deposits. However, the type, extent and temporal or genetic relationships between the diverse Fe, Ca and alkali metasomatism and the regionally distributed U mineralization remains poorly understood.
Combined unsupervised machine-learning and classification of alteration from a large geochemical dataset distinguish the main alteration phases in the CMB, identify compositional changes related to U mineralization, and infer lithological/mineralogical information from samples with censored (i.e., missing), limited and/or inaccurate metadata. Weak to intense Na and Na + Ca-Fe (Mg) metasomatism in the southwest (Two-Time and Moran Lake areas) and eastern (Michelin area) portions of the CMB pre-dates U mineralization and Fe-oxide breccia development, similar to albitite-hosted U and IOCG deposits globally.
Rare earth elements and spider diagrams highlight both preservation and disruption of normally immobile elements. Principal component and cluster analysis indicate significant variations in Fe-Mg ± Na contents in the rocks from combinations of Na, Ca, Fe, and Mg-rich alteration, while protolith REE signatures can be locally preserved even after pervasive albitization-hematization. Cluster analysis identifies mineralized felsic and mafic rocks in the Michelin deposit and Moran Lake area, facilitating inference of relevant lithological/mineralogical information from samples lacking or with limited meta-data.
The methods outlined provide rapid and relatively inexpensive means to optimize identification of mineral systems within large geochemical datasets, verify drill core or field observations, highlight potentially overlooked alteration, and refine economic mineral potential assessments. Based on our results and previous work, we suggest the mineral potential of the southwestern and eastern CMB needs to be re-assessed with modern exploration models for IOAA ore systems and their iron oxide-poor variants.
Identification of alteration facies is an important factor for Iron Oxide Copper-Gold deposit (IOCG) exploration because they show a typical regional and deposit-scale zoning and have predictable polymetallic mineralization associations. The high intensity of IOCG alteration often lead to complete transformation of precursor rocks, thus altered rock geochemical signatures are no longer controlled by protolith composition. After a regional metamorphism overprint episode, IOCG alteration facies could present a typical metamorphic parageneses that could be predicted by phase equilibria modelling using the Gibbs energy minimization methods.
The large Dahongshan Fe-Cu-(Au-Ag) deposit in the Kangdian iron oxide copper-gold (IOCG) metallogenic province, southwest China, contains approximately 458.3 Mt of ore at 41.0% Fe, 1.35 Mt Cu (metal) at 0.78% Cu, and significant amounts of Au (16 t), Ag (141 t), Co (18,156 t), and Pd + Pt (2.1 t). The deposit consists mainly of two types of ores: (1) lenses of massive or banded magnetite-(hematite) hosted in extensively Na metasomatized metavolcanic rocks, metaarenite, and brecciated rocks, and (2) strata-bound disseminated, stockwork, and banded magnetite-chalcopyrite-(bornite) in mica schist and marble. Both types of orebodies and country rocks underwent extensive hydrothermal alteration, resulting in a similar paragenesis. Pervasive stage I sodic alteration formed widespread albite and local scapolite. It was subsequently replaced by Ca-or K-rich minerals represented by actinolite, K-feldspar, biotite, sericite, and chlorite of stages II and III. Magnetite is slightly younger than and partly overlaps the sodic alteration assemblages. Hematite is texturally later than magnetite, is locally abundant within the massive Fe oxide orebody, and is closely associated with sericite. Copper sulfides are coeval with quartz, biotite, sericite, and chlorite in stage III assemblages. Widespread siderite and ankerite predominate in stages II and III, respectively. Quartz-calcite veins mark the result of waning stage IV hydrothermal alteration. In addition to widespread alteration during the major ore-forming event, the deposit has also undergone extensive overprinting and remobilization during post-ore magmatic and metamorphic events. The Dahongshan orebodies are intimately associated with abundant doleritic dikes and sills that have hydrothermal mineral assemblages similar to those in the ore-hosting rocks. One dolerite sill that cuts a massive Fe orebody has a laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb age of 1661 ± 7 Ma, which is, within uncertainty, consistent with the age of 1653 ± 18 Ma determined for hydrothermal zircons from stockwork chalcopyrite-magnetite ore. The zircon U-Pb ages are thus considered to mark the timing of major mineralization that formed the Dahongshan deposit. Post-ore modification is recorded by an Re-Os isochron age of 1026 ± 22 Ma for pyrite in discordant quartz-carbonate-sulfide veins, and by younger Neoproterozoic mineralization dated at ca. 830 Ma using Re-Os isotopes on molybdenite. The former age is contemporaneous with late Mesoproterozoic magmatism in the region, whereas the latter is coeval with regional Neoproterozoic metamorphic events in southwest China. Carbon and oxygen isotope values of albitized marble are between those of mantle-derived magmatic carbon and dolostone end members. The ore-forming fluids that equilibrated with stage II magnetite have δ¹⁸O values of 9.1 to 9.5%, whereas fluids linked to the deposition of quartz and ankerite during stages III and IV have lower δ¹⁸O values of 2.9 to 7.3%. The oxygen isotope data indicate that the ore-forming fluids related to stage II are chiefly magmatically derived and mixed with abundant basinal brine during stages III and IV; this interpretation is consistent with sulfur isotope values of sulfides in the deposits. Pyrite and chalcopyrite from the Dahongshan deposit have a large range of Δ34S values from -3.4 to +12.4%, implying mixing of magmatic and external sulfur (likely from basinal brines) in sedimentary rocks. The Dahongshan deposit formed in an intracratonic rift setting due to underplating by mafic magmas that induced large-scale fluid circulation and pervasive sodic-calcic metasomatism in country rocks. Ore metals were derived mainly from a deep-seated magma chamber and partly from country rocks. Hydrothermal brecciation of the country rocks formed at the top of the dolerite intrusions and along zones of weakness within the country rocks owing to overpressure imposed by the ore fluids. Magnetite and hematite precipitated early near the dolerite intrusions, whereas Cu sulfides formed later in country rocks where sulfide saturation was favored. We propose that this genetic model may be widely applicable to Precambrian IOCG deposits elsewhere that formed in intracratonic rift settings.
Kwyjibo is a Mesoproterozoic Fe-Cu-REE-(Au) deposit of IOCG type in the northeastern part of the Grenville Province, on the north shore of St-Lawrence Gulf, in the Province of Québec. It consists of 10 known polymetallic mineralized zones over a strike length of 4 kilometres. The main mineralized zones (Malachite, Josette, Andradite, Fluorine, Grabuge and Gabriel) were discovered between 1993 and 1995 during regional follow-up of regional geochemical lake-bottom sediment anomalies spatially associated with a regional curvilinear magnetic anomaly. Most of the zones consist of low-grade copper and REEs, except in the Josette horizon, a deformed hydrothermal iron formation (referred to as a magnetitite) metamorphosed to upper amphibolite metamorphic grade that extends for more than 1.2 kilometres. High grade REE-mineralization, mainly of Nd, Y, Dy and Tb, has been delineated by both surface work at the Josette showing (2.95% total rare earth oxide [TREO] and 1.44% Cu over 10 m) and by close-spaced diamond drilling along the Josette horizon with reported weighted average intervals from 0.84% TREO over 65 m to 3.64% TREO over 33 m.
The Morin Anorthosite Mass underlies an area of 2 500 square kilometers; it consists of three tectonic units: a dome, a diapir and a nappe. The units have in common a large part of their magmatic history, but their tectonic evolution is different. Under the influence of gravity, the dome and the diapir behaved as buoyant masses, while the nappe and part of the diapir spread laterally. Texture and structure of the anorthosite are correlated with these differences in tectonic evolution. Jotunitic and mangeritic rocks envelope the dome and part of the diapir, whereas a troctolite sill occurs in the zone of weakness between the dome and the nappe. The jotunites and mangerites shared a large part of their tectonic evolution with that of the dome, but while the dome was emplaced in an advanced stage of crystallization, its envelope was largely liquid. It is probable that anorthosites, jotunites and mangerites are comagmatic, in the widest sense of the word, and it is possible that troctolite was an early differentiate of the parent magma. The structural relations between the plutonic rocks and the surrounding supracrustal rocks have many characteristics in common with basement-cover relations in polycyclic otogenic zones. However, the plutonites can be considered as a basement complex only if a rather improbable wholesale remobilization of at least the jotunites and mangerites is postulated. The main phase of penetrative deformation in the supracrustal rocks gave rise to gently inclined and recumbent folds (F2) east and northeast of the Morin Mass, and is correlated with the lateral spreading of the anorthosite nappe. This phase overprinted most older structures, but relicts of the latter (F1) are preserved locally. A set of major open upright folds (F3) is restricted to the area adjacent to the anorthosite nappe and is interpreted as a set of second-order compression folds related to the emplacement of the nappe. A late phase of deformation gave rise to very gentle major and minor folds (F4) that slightly deform the axial lineation of F2-folds. The regional metamorphism is transitional between the amphibolite and granulite facies; it is roughly contemporaneous with the main phase of deformation, thus with the lateral spreading of the anorthosite nappe. The only well-defined isograd is one that separates orthopyroxene-quartz bearing granulites on the side of the Mass from hornblende-quartz bearing gneisses away from the Mass. Late metamorphism took place in and around the anorthosite dome and produced garnet and clinopyroxene from plagioclase and iron-rich mafic minerals.
SUMMARY: Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs.
In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (≤1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (≤2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by
The Mont-Tremblant gneiss is a granulite-facies quartzofeldspathic orthogneiss unit in Morin terrane of the southwestern Grenville Province. These gneisses are the oldest rocks in the Morin terrane and country rocks to intrusions of the 1.15 Ga anorthosite-mangerite-charnockite-granite (AMCG) suite. The Mont-Tremblant gneiss may also be basement to the Grenville Supergroup, but until now it has not been directly dated. Sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) geochronology of zircon from two penetratively deformed samples of Mont-Tremblant gneiss yields igneous ages of 1324 +/- 38 Ma and 1333 Mont-Tremblant 32 Ma (2 sigma), and one sample shows metamorphic zircon growth during the Shawinigan orogeny at 1159.4 +/- 15.6 Ma. Geochemically, Mont-Tremblant gneiss samples are calc-alkaline, granitic, and show hallmarks of an evolved arc environment. These characteristics are consistent with the Mont-Tremblant gneiss being correlative with the Geon 14 Lacoste and Bondy suites in the Central Metasedimentary Belt of Quebec to the west, but contrast with the more juvenile tonalitic Geon 14 calc-alkaline suites in the Adirondack Highlands and other Grenville outliers in the Appalachian Mountains. This transition may reflect differences in the lithosphere along strike of the Laurentian margin during Geon 14 subduction.
The integrated approach of field work, microscopy, whole-rock and mineral-scale geochemistry, and in situ U-Th-Pb zircon geochronology has proven to be useful for recognizing the type, timing, and sequence of complex Na and K fluid alteration related to the development of Kiruna-type magnetite-apatite deposits and the tectonic evolution of the granites that host these deposits. The Lyon Mountain Granite in the northeastern Adirondack Mountains of New York State has undergone multiple episodes of hydrothermal fluid alteration and Fe mineralization. Perthite granite containing ubiquitous 1060-1050 Ma zircon grains was overprinted by potassic alteration, which in turn was overprinted by pervasive Na alteration. During the Na alteration, preexisting orebodies, consisting of magnetite, clinopyroxene, and apatite, were overprinted and remobilized to form new deposits that contain magnetite, apatite, quartz, and zircon. The U-Th-Pb zircon geochronology data suggest that the Lyon Mountain Granite intruded the Adirondack Highlands during the Ottawan orogeny between ca. 1060 and 1050 Ma. However, subsequent alteration obscured much of the prehistory of the LMG. Amphibolite layers within the Lyon Mountain Granite and granitic dikes and pegmatites that crosscut the foliation in the Lyon Mountain Granite have been dated between 1045 and 1016 Ma. These ages coincide with previous published zircon age data from second-generation orebodies associated with Na alteration.
The Pyrites Complex in the Adirondack Lowlands domain of the Grenville Province forms the core of a large NE-trending, elongate, winged structure similar to 15 km long dominated by highly deformed metagabbro, amphibolite, and hornblende schist. A previously unrecognized km-scale boudin of metamorphosed ultramafic rocks associated with the belt is described. It is largely replaced by secondary hydrous minerals, but retains relict igneous textures and some primary minerals such as augite and chromite, and is cut by several 1-2-m-wide lamprophyre dikes. The ultramafic rocks are interpreted as part of an obducted ophiolite complex on the basis of its structural contact with a belt of rocks including marine metasedimentary rocks, pyritic gneisses, metagabbros, and amphibolites with mid-ocean ridge basalt chemistry and were emplaced within a collapsing backarc basin of Shawinigan age. Small (50-200 mu) zircon crystals separated from peridotite and pyroxenite yield minimum ages (1140 +/- 7 and 1197 +/- 5 Ma) and constrain the timing of metamorphic and thermal events associated with the Shawinigan orogenesis and anorthosite-mangerite-charnockite-granite (AMCG) plutonism. Neodymium T-DM ages from the Pyrites Complex range from 1440 to 2650 Ma, are not compatible with derivation from a typical depleted mantle reservoir, and suggest, along with incompatible element concentrations, that these rocks record mantle enrichment, presumably during subduction beneath the leading edge of Laurentia. Rifting and development of oceanic crust between the southern Adirondack Highlands and Lowlands, coincident with a similar backarc rifting in the Central Metasedimentary Belt at ca. 1300 Ma, are proposed. Three mafic suites in the Adirondack Lowlands are distinguished by their field relations, age, geochemistry, and Nd isotopic systematics and reflect the various stages of evolution of the Trans-Adirondack backarc basin. Within the Lowlands interleaved evaporites, metasedimentary and possible metavolcanic rocks, calc-alkaline and transitional plutonic rocks, and ophiolitic rocks of the Pyrites Complex provide constraints on the tectonic processes and sedimentary response to development of a backarc basin, magmatic arc, fore-deep sedimentation, and ophiolite obduction during Shawinigan convergence from ca. 1200-1160 Ma, which culminated in slab breakoff and plate delamination resulting in intrusion of the AMCG suite throughout the Frontenac-Adirondack terrane and beyond.
Recent investigations in geochronology and tectonics provide important new insights into the evolution of the Grenville Orogen in North America. Here, we summarize results of this research in the USA and focus upon ca. 1.4–0.98 Ga occurrences extending from the Adirondack Mountains to the southern Appalachians and Texas. Recent geochronology (mainly by U/Pb SHRIMP) establishes that these widely separated regions experienced similar tectonomagmatic events, i.e., the Elzevirian (ca. 1.25–1.22 Ga), Shawinigan (ca. 1.2–1.14 Ga), and Grenvillian (ca. 1.09–0.98 Ga) oroge-nies and associated plate interactions. Notwithstanding these commonalities, Nd model ages and Pb isotopic map-ping has revealed important differences that are best explained by the existence of contrasting compositions of deep crustal reservoirs beneath the Adiron-dacks and the southern Appalachians. The isotopic compositions for the Adirondacks lie on the same Pb–Pb array as those for the Grenville Province, the Granite-Rhyolite Province and the Grenvillian inliers of Texas suggesting that they all devel-oped on Laurentian crust. On the other hand, data from the southern Appalachians are similar to those of the Sunsas Terrane in Brazil and sug-gest that Amazonian crust with these Pb–Pb characteristics was thrust onto eastern Laurentia during its Grenvillian collision with Amazonia and subse-quently transferred to the latter during the late Neoproterozoic breakup of the supercontinent, Rodinia, and the formation of the Iapetus Ocean. The ca. 1.3–1.0 Ga Grenville Orogen is also exposed in the Llano Uplift of Texas and in small inliers in west Texas and northeast Mexico. The Llano Uplift contains evidence for a major collision with a southern continent at ca. 1.15–1.12 Ga (Kalahari Craton?), mag-matic arcs, and back-arc and foreland basins, all of which are reviewed. The Grenvillian Orogeny is considered to be the culminating tec-tonic event that terminated approxi-mately 500 m.y. of continental margin growth along southeastern Laurentia by accretion, continental margin arc magmatism, and metamorphism. Accordingly, we briefly review the tec-tonic and magmatic histories of these Paleoproterozoic and Mesoproterozoic pre-Grenvillian orogens, i.e., Penokean, Yavapai, and Mazatzal as well as the Granite-Rhyolite Province and discuss their ~5000 km transcontinental span.
Granitic rocks can have a wide range of sources, over all parts of the spectrum from pure mantle to pure crust. These sources show a significant correlation with tectonic setting. Granites from ocean ridges usually have characteristics that indicate depleted mantle sources. Granites in volcanic arcs usually have depleted mantle sources modified by a component from subducted oceanic crust and sediment. Granites from intraplate settings usually show evidence of enriched mantle (lithosphere andlor asthenosphere) sources together with rare crustal melts. Granites from syn-collision settings are usually characterised by pure crustal sources or by mantle sources containing large subducted crustal components. Granites from post-collision settings usually carry signs of enriched lithospheric mantle sources together with rare crustal melts. Further interaction between mantle-derived magmas and crust is a function of the thickness, temperature and composition of the crust and the residence time and temperature of the magma-varables that are also linked to tectonic setting. These relationships between source and setting provide the basis for the geochemical fingerprinting of granites which, when combined with geological considerations, enable granites to be assigned to their most probable setting of intrusion.
Stable carbon isotope geochemistry provides important information for the recognition of fundamental isotope exchange processes related to the movement of carbon in the lithosphere and permits the elaboration of models for the global carbon cycle. Carbon isotope ratios in fluid-deposited graphite are powerful tools for unravelling the ultimate origin of carbon (organic matter, mantle, or carbonates) and help to constrain the fluid history and the mechanisms involved in graphite deposition. Graphite precipitation in fluid-deposited occurrences results from CO2- and/or CH4-bearing aqueous fluids. Fluid flow can be considered as both a closed (without replenishment of the fluid) or an open system (with renewal of the fluid by successive fluid batches). In closed systems, carbon isotope systematics in graphite is mainly governed by Rayleigh precipitation and/or by changes in temperature affecting the fractionation factor between fluid and graphite. Such processes result in zoned graphite crystals or in successive graphite generations showing, in both cases, isotopic variation towards progressive 13C or 12C enrichment (depending upon the dominant carbon phase in the fluid, CO2 or CH4, respectively). In open systems, in which carbon is episodically introduced along the fracture systems, the carbon systematics is more complex and individual graphite crystals may display oscillatory zoning because of Rayleigh precipitation or heterogeneous variations of δ13C values when mixing of fluids or changes in the composition of the fluids are the mechanisms responsible for graphite precipitation.
Foreword by Peter A. Cawood Acknowledgements. 1. Water and hydrothermal fluids on Earth 2. Hydrothermal processes and wall rock alteration 3. Tectonic settings, geodynamics and temporal evolution of hydrothermal mineral systems 4. Intrusion-related hydrothermal mineral systems 5. Porphyry systems fossil and active epithermal systems 6. Skarn systems 7. Submarine hydrothermal mineral systems 8. Metalliferous sediments and sedimentary rock-hosted stratiform and/or stratabound hydrothermal mineral systems 9. Orogenic, amagmatic and hydrothermal mineral systems of uncertain origin 10. Hydrothermal systems and the biosphere 11. Hydrothermal processes associated with meteorite impacts 12. Hydrothermal processes and systems on other planets and satellites 13. Uranium hydrothermal mineral systems References. Index
A 1.2 Ga association of aluminous gneisses, garnetites, and white felsic gneisses of andesitic composition in the southern Manicouagan area (central Grenville Province) provides evidence consistent with protolith formation and hydrothermal alteration in a submarine volcanic environment. In addition to field relations, potential relics of quartz phenocrysts in the aluminous gneisses, revealed by SEM-MLA (scanning electron microscope with a mineral liberation analysis software) imaging, are consistent with a volcanic precursor. Moreover, in these rocks, aluminous nodules and seams of sillimanite are considered to represent metamorphosed hydrothermal mineral assemblages and to reflect former pathways of hydrothermal fluid. These features are preserved despite the Grenvillian granulite-facies metamorphic overprint and evidence of partial melting. In addition, the garnetites are inferred to represent hydrothermally altered products of the white gneisses, based on the gradational contacts between the two rock types. The compositional ranges of minerals are generally similar to those of granulite-facies metapelites, but moderately elevated contents of Mn in garnet from the garnetites, and Zn in spinel from the aluminous gneisses, are consistent with hydrothermal addition of these elements to the protolith. The most prominent alteration trends are an increase in Fe-Mg-Mn from the white gneisses to the aluminous gneisses and the garnetites, and a trend of increasing alumina index in some white gneisses, suggesting mild argillic alteration. The new findings highlight the preservation of early hydrothermal alteration in high-grade metamorphic belts in the Grenville Province, and these altered rocks are potential targets for exploration.
High-temperature metasomatism driven by ascent of voluminous, saline fluid columns in the upper crust plays a major role in the genesis of iron oxide-alkali alteration ore systems but fundamental questions remain on genetic linkages among iron oxide copper-gold (IOCG), iron oxide-apatite (IOA), albitite-hosted uranium, and skarn deposits that they produce. Excellent surface exposures of such systems in the Great Bear magmatic zone of northernwestern Canada record the depth to paleosurface, prograde evolution of iron oxide-alkali alteration facies, and mineralization. Across the belt, albitite corridors that are tens of kilometers in length record the earliest reactions between highly saline fluids and host rocks along fault zones and subvolcanic intrusions. Pervasive albitization partitioned metals from the host rocks into the ascending fluid column, leaving behind structurally weakened corridors of porous albitite. These corridors were cut, replaced, and overprinted by amphibole- and magnetite-bearing, calcic-iron alteration assemblages. In extreme cases, the discharge of calcium, iron, and specialized metals formed iron oxide-apatite deposits (±vanadium ± rare earth elements) while recharging the outgoing fluids in sodium, potassium, and base and precious metals. As temperatures declined and fluid chemistry evolved through fluid-rock reactions, the formation of potassic-iron alteration assemblages, breccias, and sulfides resulted in magnetite- and hematite-group IOCG mineralization. Within carbonate units, skarns formed prior to, are replaced by, and evolved to calcic-iron alteration facies. Skarns can locally host base metal mineralization. Tectonically uplifted albitite breccias replaced by potassic-iron alteration assemblages became a preferential host for uranium mineralization. The results of this study also illustrate that permutations and cyclical build-up of alteration products can arise from a combination of faulting, differential uplift, and renewed magmatism. Framed within an alteration-facies deposit model, alteration zones and mineral occurrences play a pivotal role in predicting the mineral potential of iron oxide and alkali-altered systems at district to deposit scales.
This special issue is focused on the geology and mineral deposits in the Mesoproterozoic St. Francois Mountains terrane of southeast Missouri, United States, and in the Paleoproterozoic Great Bear magmatic zone of Northwest Territories, Canada. Contributions in this special issue emphasize iron oxide-apatite (IOA) ± rare earth element (REE) and iron oxide-copper-gold (IOCG) and affiliated deposits (e.g., Hitzman, 2000; Barton, 2014). The volume includes results of studies by the U.S. Geological Survey (USGS) and Geological Survey of Canada (GSC), derived from research done for the USGS Mineral Resources Program, and from GSC research completed under auspices of the Geomapping for Energy and Minerals and Targeted Geoscience Initiative programs. These studies took advantage of extensive historic drilling at and near IOA and IOCG deposits in Missouri, and of exceptional field exposures in the Great Bear magmatic zone. Collectively, results of this cumulative body of work provide new insights into the geological, geochemical, geophysical, and temporal characteristics of IOA ± REE, IOCG, and affiliated deposits, leading to enhanced understanding of diverse magmatic-hydrothermal systems, and refining of genetic models and exploration strategies for such deposits, within these terranes and elsewhere in the world.
Both North American terranes contain iron oxide-alkali alteration zones that formed during predominantly I-type, calc-alkaline (including S-type and shoshonitic in the Great Bear zone) volcano-plutonic magmatism. Later magmatism in both terranes was mainly of A-type affinity. Locally, the IOA deposits are spatially associated with high REE concentrations. The IOCG and affiliated deposits include Co- and Birich variants. Skarns, polymetallic veins, and albitite-hosted U deposits are also present within the footprints of the regional iron oxide-alkali alteration systems. Mineralization occurred contemporaneously with mafic to felsic volcanism and local volcanogenic sedimentation, including within or marginal to large ash-flow calderas. Proterozoic rocks in both the St. Francois Mountains terrane and the Great …
The Great Bear magmatic zone in northwestern Canada is a Paleoproterozoic volcano-plutonic belt of high K, calc-alkaline to shoshonitic affinity interpreted as a continental arc that formed between 1.87 and 1.85 Ga following the short-lived Calderian orogeny. Tectonomagmatic evolution of this magmatic zone favored the formation of multiple iron oxide and alkali alteration systems within a time frame of 10 m.y., as constrained geochronologically within error between 1875 and 1865 Ma. This illustrates a temporal and genetic relationship between shoshonitic to high K, calc-alkaline continental arc magmatism and the formation of iron oxide-rich deposits and alkali alteration associated with iron oxide-copper-gold (IOCG) mineralization. Early rhyolitic magmatism formed the basal sequence of the Lou assemblage under a compressive (or transpressive) tectonic regime. Between 1872 and 1867 Ma, an apparent higher magmatic production is linked to a marked change in composition of the volcanic and plutonic rocks, from rhyolite to intermediate/felsic and locally mafic. Compositional homogeneity of the intrusive and volcanic rocks of the Mazenod and Bea assemblages, termination of ductile to brittle-ductile deformation along the main deformation zones, and transition to widespread brittle fracturing and breccia formation are interpreted to reflect a change in the regional stress regime, from compressional/ transpressional to extensional/transtensional. This change in stress regimes is associated with iron oxide-rich mineralization that initially formed the NICO Au-Bi-Co deposit, followed by IOCG mineralization in the NICO and Sue-Dianne deposits as well as albitite-hosted U mineralization in the Southern Breccia.
Rare earth element (REE)-rich breccia pipes (600,000 t @ 12% rare earth oxides) are preserved along the margins of the 136-million metric ton (Mt) Pea Ridge magnetite-apatite deposit, within Mesoproterozoic (1.47 Ga) volcanic-plutonic rocks of the St. Francois Mountains terrane in southeastern Missouri, United States. The breccia pipes cut the rhyolite-hosted magnetite deposit and contain clasts of nearly all local bedrock and mineralized lithologies. Grains of monazite and xenotime were extracted from breccia pipe samples for SHRIMP U-Pb geochronology; both minerals were also dated in one polished thin section. Monazite forms two morphologies: (1) matrix granular grains composed of numerous small (<50 μm) crystallites intergrown with rare xenotime, thorite, apatite, and magnetite; and (2) coarse euhedral, glassy, bright-yellow grains similar to typical igneous or metamorphic monazite. Trace element abundances (including REE patterns) were determined on selected grains of monazite (both morphologies) and xenotime. Zircon grains from two samples of host rhyolite and two late felsic dikes collected underground at Pea Ridge were also dated. Additional geochronology done on breccia pipe minerals includes Re-Os on fine-grained molybdenite and 40Ar/39Ar on muscovite, biotite, and K-feldspar. Ages (±2s errors) obtained by SHRIMP U-Pb analysis are as follows: (1) zircon from the two host rhyolite samples have ages of 1473.6 ± 8.0 and 1472.7 ± 5.6 Ma; most zircon in late felsic dikes is interpreted as xenocrystic (age range ca. 1522-1455 Ma); a population of rare spongy zircon is likely of igneous origin and yields an age of 1441 ± 9 Ma; (2) pale-yellow granular monazite-1464.9 ± 3.3 Ma (no dated xenotime); (3) reddish matrix granular monazite-1462.0 ± 3.5 Ma and associated xenotime-1453 ± 11 Ma; (4) coarse glassy-yellow monazite-1464.8 ± 2.1, 1461.7 ± 3.7 Ma, with rims at 1447.2 ± 4.7 Ma; and (5) matrix monazite (in situ)- 1464.1 ± 3.6 and 1454.6 ± 9.6 Ma, and matrix xenotime (in situ)-1468.0 ± 8.0 Ma. Two slightly older ages of cores are about 1478 Ma. The young age of rims on the coarse glassy monazite coincides with an Re-Os age of 1440.6 ± 9.2 Ma determined in this study for molybdenite intergrown with quartz and allanite, and with the age of monazite inclusions in apatite from the magnetite ore (Neymark et al., 2016). A 40Ar/39Ar age of 1473 ± 1 Ma was obtained for muscovite from a breccia pipe sample. Geochronology and trace element geochemical data suggest that the granular matrix monazite and xenotime (in polygonal texture), and cores of coarse glassy monazite precipitated from hydrothermal fluids during breccia pipes formation at about 1465 Ma. The second episode of mineral growth at ca. 1443 Ma may be related to faulting and fluid flow that rebrecciated the pipes. The ca. 10-m.y. gap between the ages of host volcanic rocks and breccia pipe monazite and xenotime suggests that breccia pipe mineral formation cannot be related to the felsic magmatism represented by the rhyolitic volcanic rocks, and hence is linked to a different magmatichydrothermal system.
The Basin Lake copper-gold prospect lies in western Tasmania's Mount Read Volcanics and is hosted in a series of calc-alkaline andesites, quartz-feldspar porphyries, mudstones, carbonates, and sandstones between the Tyndall Group and the Central Volcanic Complex. Alteration at the Basin Lake prospect occurs over a strike length of 1.4 km and includes thin, strata-bound pyrophyllite-quartz-paragonite-kaolinite-pyrite-alunite alteration zones, up to 12 m wide and containing up to 50 wt percent pyrophyllite, with local fluorite veining. These zones grade out to paragonite-muscovite-kaolinite-quartz-pyrite and muscovite-carbonate-chlorite alteration zones. Extensive propylitic alteration (chlorite-carbonate-epidote) affects most other rocks outside these zones. Mineralization consists of thin strata-bound zones of massive and vein pyrite, tennantite, and chalcopyrite, with trace covellite and galena, hosted mainly within an intensely silicified core of the pyrophyllite-quartz-sericite alteration zone. Pyrite has delta(34)S values of -1.4 to +6.9 per mil, although marginal vein pyrite in the propylitic zone has delta(34)S values around 12.4 per mil. Large silicified glacial erratic boulders at surface contain massive and vein pyrite, enargite, and tennantite, with minor barite, and trace covellite, stannoidite, and mawsonite. Pyrite and enargite have delta(34)S values of 1.7 to 6.8 per mil; barite has delta(34)S values around 35.2 per mil with Sr-87/Sr-86 around 0.7108. The alteration and mineralization at the Basin Lake prospect is similar to that associated with high-sulfidation copper-gold systems formed by acidic, relatively oxidized fluids. A new geochemical vector, here termed the "advanced argillic alteration index" [AAAI=100 (SiO2)/(SiO2+10MgO+10CaO+10Na(2)O)], has been devised to help quantify the intensity of alteration. The values of the AAAI at Basin Lake are similar to those of several high-sulfidation epithermal deposits. The low sulfide delta(34)S values are similar to those at other sulfide occurrences in the Mount Read Volcanics that have previously been considered to be barren, are lower than those of nearby volcanic-hosted massive sulfide deposits, and may indicate a magmatic fluid component. However, the delta(34)S and Sr-87/Sr-86-values of Basin Lake barite at the assumed highest exposed level of the system and higher delta(34)S values in pyrite from marginal veins are similar to those of Cambrian volcanic-hosted massive sulfide systems, indicating the involvement of reduced seawater sulfate at these locations. Calcite carbon and oxygen isotope values, silicate oxygen isotope values, and the unusual abundance of carbonate close to advanced argillic alteration indicate fluid mixing and suggest that acidic, magmatic fluids were likely neutralized by seawater. This occurrence strengthens the case for prospecting the Mount Read Volcanics and other similar submarine volcanic belts for copper-gold and gold-only deposits that formed by the actions of hyperacid oxidized fluids.
The Montauban stratiform Zn-Pb-Cu-Ag-Au deposit forms part of the Montauban Group, which lies east of the Allochtonous Monocyclic Belt in the Proterozoic Grenville Province. Metal zoning is prominent in the alteration zone and outlines a Au-Cu-rich core plunging to the south, and parallel to the mineral lineation. The mine sequence consists of massive sulfide ores, marble, calc-silicate rocks, biotite-muscovite quartzofeldspathic gneiss, cordierite-anthophyllite gneiss, nodular sillimanite gneiss, and quartzitic gneiss. The Log (Ratio) method and other geochemical discriminants indicate these gneisses were derived from intermediate to felsic volcanic rocks or related volcaniclastic sediments. The cordierite-anthophyllite rocks and associated gneisses are the metamorphosed equivalents of hydrothermally altered intermediate volcanic protoliths. The calc-silicate rocks were probably derived from Mg-Ca carbonates and marl deposited coevally with the ores. The Montauban ores and alteration resemble Kuroko-type massive sulfide deposits and were likely formed in a volcanic arc environment. -from Authors
The Carajas Mineral Province, northern Brazil, represents an Archaean cratonic block that contains the
world’s largest known concentration of large-tonnage IOCG deposits (e.g., Sossego, Salobo, Igarapé Bahia/Alemão,
Cristalino, Alvo 118, Igarape Cinzento/Alvo GT46). These deposits are hosted by 2.76 to 2.73 Ga metavolcanosedimentary
units, 2.70 to 2,65 Ga gabbro/diorite, granitoids and porphyry dykes, within brittle-ductile and ductile
shear zones. Geochronologic data suggest that formation of the Carajas IOCG deposits may possibly be linked to three
metallogenic events: -2.74, 2,57 and 1.8 Ga.
In general, the Carajas IOCG deposits display a hydrothermal alteration sequence characterised by early sodic
and sodic-calcic assemblages, followed by potassic alteration, magnetite-(apatite) formation, chloritic, copper-gold
mineralisation and hydrolytic alteration. Tourmaline is particularly common in deposits hosted by metavolcanosedimentary
units (e.g., Salobo and Igarapé Bahia/Alemão), The development of fayalite, garnet and sillimanite
represents higher temperature alteration assemblages of some deposits hosted in ductile shear zones"such as Salobo
and Igarapé Cinzento/Alvo GT46. Silica and carbonate alteration are important in deposits formed in brittle-ductile
conditions (e.g., Sossego and Alvo 118).
Extensive zones of scapolite alteration (>20 km2) represent sodic alteration around IOCG deposits (e.g., Sossego),
reflecting high salinity and buffered activity gradients in Cl in the early regional hydrothermal fluids. Metal leaching
from the host rocks was probably enhanced by the high salinity of fluidsa driven by heat from the intrusive episodes
recorded in the Carajas Mineral Province. As a consequence, geochemical ore signatures defined by the Fe-Cu-Au-
REE-(U-Y-Ni-Co-Pd-Sn-Bi-Pb-Ag'Te) association is variably developed in the Carajás IOCG deposits, and strongly
dependent upon the chemistry of the leached host rocks.
Fluid inclusions in ore-related minerals point to a fluid regime in which hot brine (>30 wt.% NaCl eq.) solutions,
represented by salt-bearing aqueous inclusions, were progressively cooled and diluted by lower temperature, low-sahmty
(<10 wt.% NaCl eq.) aqueous fluids. This mixing process was likely responsible for a trend of salinity and temperature
decrease (>550 to <300 °C), accompanied by an increase towards the mineralisation stages. This process tends to
favour the predominance of hematite-bomite in more oxidised deposits (e.g., Alvo 118) over magnetite-chalcopyrite
(e.g., Sossego).
Extensive fluid-rock interactions, possibly involving basinal/evaporite and magmatic fluid components, resulted in
lB0-enriched fluids (δ18Ofluid = 5 to 15 ‰), typical of most Carajas IOCG deposits. In addition, calculated fluid isotopic
compositions for shallow-emplaced deposits, such as Sossego and Alvo 118 (δ18Ofluid= -5.2 ‰, δDfluid = -35 ‰ of at 300 °C)
also reinforce the importance of the significant structurally-controlled influx of meteoric fluids for ore deposition related
to high fluid pressure release and brecciation* Chlorine and boron isotopes, combined with Cl/Br vs. Na/Cl systematics,
strongly suggest that fluid regimes responsible for the formation of the Carajds IOCG deposits involved a significant
contribution from residual evaporative fluids (e.g., bittem fluids generated by seawater evaporation) that may have
mixed with magma-derived brines.
Sulphur isotope compositions for the Carajds IOCG deposits vary from values close to that expected for a mantle
source (e.g., δ34S = 0 ‰ at Salobo) to 34S enriched values (>7 ‰) in deposits in which contribution of meteoric fluids
was significant, reflecting distinct physic-chemical conditions or input of heavy sulphur from surficial reservoirs.
We report sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages from high-grade gneisses of the Bondy gneiss complex, a volcano-plutonic arc and back-arc edifice hosting a Cu-Au-Fe oxides hydrothermal system in the Central Metasedimentary Belt, Grenville Province, Québec. Samples of quartzofeldspathic gneiss gave broadly similar results, with zircon cores indicating ages between 1.59 and 1.21 Ga and mantles or whole new zircon crystals giving Grenvillian metamorphic ages. A few analyses indicate minor involvement of Paleoproterozoic and Archean crustal material. Zircon cores are interpreted to have grown during crystallization of the quartzofeldspathic gneiss protoliths, but precise ages could not be determined for any of the samples. The spread of ages is attributed mainly to isotopic disturbance owing to hydrothermal alteration, high-pressure-temperature (P-T) metamorphism, and/or recent Pb loss. Only a sample of tonalitic gneiss yielded a well-defined, igneous crystallization age of 1386 ± 10 Ma. Younger zircons from the dated lithologies provide evidence for two episodes of Grenvillian metamorphism: A period of high-P-T granulite-facies metamorphism and partial melting at 1.21-1.18 Ga, and a younger, albeit localized, metamorphic overprint at ca. 1.15-1.13 Ga. The new SHRIMP U-Pb data indicate that the composite volcano-plutonic edifice likely formed, at least in part, at ca. 1.39 Ga. Together with recently published geochemical and Nd data, the new data (1) extend the known distribution of ca. 1.39 Ga arc-related magmatism in the Grenville Province, and (2) suggest that a major portion of the source region to the Bondy gneiss complex was produced by the addition of voluminous juvenile Mesoproterozoic material.
Modes of acidic volcanic activity preceding kuroko deposition provide with a clue for prospecting targets of kuroko deposits. Nature of acidic volcanism was made clear in the area of Fukasawa kuroko deposits, Hokuroku district. Chemical analyses of underlying volcanics were also made. The results of this study are summarized below.
(1) The Neogene Yukisawa dacite lavas in the Fukasawa area should be classified into 4 types, D1 lava dome, D2 lava flow, D3 lava flow, and D4 lava dome.
(2) The Fukasawa kuroko deposits were formed in connection with the activity of the Fukasawa D4 lava dome.
(3) Chemical analyses of 628 core samples of the Fukasawa D4 lava dome revealed a distinct distribution pattern of Na, K, Ca, Mg, Cu, Pb, Zn, Fe, and Mn making haloes around the orebodies.
The effective delineation of prospecting targets may be done based on these geological features.
The Prominent Hill iron oxide-copper-gold (IOCG) deposit, located in the Gawler craton of South Australia, contains ca. 278 million metric tons (Mt) of ore at 0.98% Cu, 0.75 g/t Au, and 2.5 g/t Ag. In contrast to the predominantly granite-hosted Olympic Dam IOCG deposit, Prominent Hill is mainly within unmetamorphosed sedimentary rocks comprising coarse clastic to laminated argillaceous lithologies with some volcaniclastic components and variable carbonate, including local massive dolomite. Essentially unmetamorphosed sedimentary rocks and structurally underlying mafic to intermediate-composition lavas, inferred to be members of the lower Gawler Range Volcanics, host the economically mineralized hematite breccias. The volcanic-sedimentary package was downfaulted and tilted along a major east-west fault, north of which similar but regionally low-grade metamorphosed rocks were affected by subeconomic skarn mineralization, and (on a more regional scale of the Mount Woods domain) intruded by granitic and gabbroic bodies. Hydrothermal alteration and mineralization at Prominent Hill involved pervasive and texturally destructive replacement of formerly calcareous, dolomitic, and siliciclastic breccia components. Hydrothermal alteration minerals comprise hematite, magnetite, siderite, ankerite, quartz, sericite, chlorite, kaolinite, fluorapatite, fluorite, barite, REE-U minerals (including monazite), uraninite, and coffinite, together with Cu sulfides including chalcopyrite, bornite, and chalcocite in the highest grade ore. Brecciation and replacement caused mechanical mixing as well as chemical alteration of primary lithologies, such that sedimentary contacts became obscured. Mass-balance calculations identify Al, Ti, Si, and Zr as least mobile components during hematite-chlorite-sericite to weak hematite-quartz alteration. Because Zr was not regularly assayed in drill cores, we use concentration ratios of Ti, Al, and Si from the deposit-scale assay database to delineate the distribution of lithochemical units prior to hydrothermal alteration and Cu mineralization. The resulting lithochemical model, based on one horizontal and five vertical cross sections, is used as a basis for mapping alteration patterns calculated from molar (Fe + Si)/(Fe + Si + Al), K/Na, and K/Al ratios. These chemical patterns, in conjunction with mineral stoichiometry, indicate that the spatial distribution of hematite, chlorite, variably phengitic sericite (and/or illite) ± kaolinite ± quartz-bearing alteration is superimposed on the pattern of interpreted lithologic contacts. The alteration patterns confirm visual logging results, showing that hematite enrichment correlates only partially with the distribution of Cu grades of ≥0.25 wt %. A subvertical body of complete replacement by hematite and quartz with consistent but subeconomic gold enrichment forms a Cu-barren core in the central and eastern parts of the deposit. Zones of increasing K/Al and K/Na ratios extend upward and westward from this Cu-barren core, transgressively overprinting lithologic contacts. The degree of hematite-quartz replacement can be measured by a hematite-quartz alteration index, here termed the HMSI value [(Fe + Si)/(Fe +Si + Al)], which inversely correlates with the normal probability for Cu grade. Areas of highest Cu grade (≥1 wt %) spatially correlate with irregular zones having intermediate molar alteration indices: 0.34 < K/Al < 0.40, 20 < K/Na < 36, and HMSI < 0.98. Hematite breccias and Cu ore deposition developed after tilting of the host sequence into its present steep orientation, as indicated by geopetal structures within the breccia matrix. Thus, the economic mineralization occurred late in the deformational history of the region and after extrusion of the lower Gawler Range Volcanics. The formation of the Prominent Hill orebodies occurred during or after upthrusting of deeper seated rocks containing subeconomic Cu in skarns north of the fault. Faulting as well as ore formation may be related to orogenic processes in the central and northern part of the Mount Woods domain. Iron oxide introduction was decoupled from, and at least partly preceded, hydrothermal deposition of high-grade Cu. Geochemical and petrographic data indicate that economic Cu mineralization occurred together with mildly acidic hematitechlorite- sericite ± siderite alteration of originally carbonate-, illite-, and feldspar-bearing sedimentary rocks. The presence of copper enrichment with an intermediate degree of cation leaching from the host rocks indicates that pH neutralization of initially highly acidic metal-transporting fluids was an essential factor causing Cu sulfide deposition. Distinct ranges in Na-K-Al ratios and low HMSI values offer potential as exploration indicators pointing toward higher ore grades. These results from Prominent Hill are consistent with recently published mineralogical studies at the giant Olympic Dam deposit, indicating similar ore depositional controls despite lithologically different host rocks.
In a reconstructed supercontinent assembly at ~0.9 Ga, the Grenville orogen extends from Scandinavia through North America and Antarctica to Australia. Part of it, the 2000 km long Grenville Province, exposed in the southeastern Canadian Shield, is large enough to allow a comprehensive view of its tectonic character. It has an orogen-parallel zonation: older, reworked crust is restricted to its northwest side; supracrustal and plutonic rocks of Grenvillian age are limited to the southeastern half. A pre-Grenvillian period of quiescence at ~1.5 Ga may have followed an earlier continental assembly. Grenvillian calc-alkaline igneous rocks represent arc accretion that terminated with ocean closure by ~1.2 Ga. New crust was added after continent-continent collision and attendant crustal thickening by emplacement of large gabbro-anorthosite massifs of mantle origin, associated with, and in part responsible for, granitoid magma derived from the lower crust. This magmatism, beginning at ~1.18 Ga, was accompanied or followed by high-grade metamorphism, except in parts of the Grenvillian supracrustal terranes, and by low-angle, thrust-sense, ductile deformation directed toward the north and northwest. -from Author
The Challenger gold deposit in South Australia is hosted by pelitic migmatites that underwent peak metamorphism at ∼7.5 ± 1.5 kbars and at least 800°C. Nd model ages suggest a protolith age of ∼2900 Ma. Zircon U-Pb and garnet Sm-Nd dating indicates that peak metamorphism occurred at ∼2440 Ma and that garnet was present during this event. At this time in situ vapor-absent melting affected the pelitic host rock, indicating that peak metamorphism occurred without introduction of a foreign fluid or melt. Thus, evidence indicating the presence of ore minerals during peak metamorphism implies a prepeak mineralization event. This evidence includes the occurrence of invisible gold in löllingite but not in adjacent arsenopyrite, and the presence of spherical gold sulfide inclusions in peak metamorphic garnet and other silicates. Textures developed between pyrrhotite, löllingite, arsenopyrite, and gold; the lack of silicate alteration minerals supports this conclusion. Experimental evidence is presented to support our interpretation that a gold-rich polymetallic melt was mobilized into leucosomes synchronously with silicate melt during peak metamorphism. Visible gold is restricted to migmatitic leucosomes and, to a lesser extent, melanosomes. Large inclusions of gold with coexisting arsenopyrite, pyrrhotite, and bismuth are hosted within peak metamorphic silicate minerals and at grain boundaries. Trails of micrometric spherical inclusions of predominantly gold and bismuth propagate along annealed fractures from these larger inclusions. We show that these gold sulfide inclusions represent the crystallized products of an immiscible polymetallic melt. Metamorphism of the Challenger deposit resulted in partial melting of the pelitic host rock and formation of this gold-rich melt. These two melts were synchronously redistributed to accumulate as leucosomes during development of a stromatic migmatite. Formation of a polymetallic melt thereby enabled extensive mobilization of gold, producing leucosomes enriched in gold. This occurred because the immiscible metallic melt was physically entrained rather than chemically dissolved within the silicate melt. Gold-rich shoots at Challenger parallel the plunge of ptygmatically folded leucosomes that are shown to be parasitic to a large scale fold geometry which appears to be structurally related to the ore shoots. It is interpreted that concurrent migration of polymetallic and silicate melt allowed concentration of gold into a series of dilational structures which developed within the larger scale fold geometry. Challenger represents a new deposite type. Other deposits around the world, such as Renco and Griffins Find, are hosted in granulite facies rocks, but Challenger is the first reported example of leucosome-hosted gold in migmatites.
This chapter focuses on the distribution and tectonic setting of Precambrian crust. The bulk of Earth's Precambrian crust is located in nine Precambrian cratons—large, subcircular to oblong, tectonically stable continental entities composed of Precambrian rocks of diverse types and ages that dominate the main continents including Asia, Europe, Greenland, North America, South America, Africa, India, Australia, and Antarctica. The nine Precambrian cratons, together with rare neighboring island microcontinents, comprise both exposed shields (also called craton, block, uplift, rise, belt, nucleus, ridge), and buried basement and cover. Additional Precambrian crust lies in numerous median massifs (inliers), scattered within long, linear, pericratonic Phanerozoic mobile belts, and in certain peripheral and isolated oceanic environments. The chapter also describes the characteristic features of various cratons of different continents of the world. A global Precambrian sketch-map shows the distribution of exposed and buried (subPhanerozoic) Precambrian crust within conventionally defined continents. Areas of exposed and buried Precambrian crust are also calculated by planimeter surveys of sub-Phanerozoic geologic maps.
The early Eocene Skaergaard intrusion of Greenland includes enormous numbers of rocks of both exotic and cognate origins. The lower parts of the Marginal Border Series contain abundant fragments of feldspathic peridotite that are possibly autoliths, intermixed with occasional xenoliths of Precambrian gneiss and metasomatized Cretaceous-Paleocene sediments derived from adjoining country rocks. The Upper Border Series includes one exceptionally large block of gneiss (several hundred meters across), and numerous smaller fragments, these originating from the intrusion's footwalls, plus a few pieces of peridotite. The Layered Series contains countless autoliths of troctolite, gabbroic anorthosite, and oxide (magnetite-ilmenite) gabbro, broken from parts of the Upper Border Series that have otherwise been lost to erosion; at the upper midlevel of its western half, it contains a few xenoliths of basalt, derived probably from the now-eroded (Eocene) roof of the intrusion. A distinctive postintrusion composite basaltic dike at one place contains 40 or more xenoliths of troctolite, olivine gabbro, and gabbroic anorthosite that may represent parts of the Layered Series still hidden at depth. The Layered Series autoliths range from fragments a few centimeters on a side to blocks more than 400 m across, and they typically are coarser grained than their host cumulates, being in this respect more like Upper Border Series rocks. The autoliths are spread stratigraphically through the lower 70% of the exposed 2500 m thickness of the Layered Series and are generally concentrated in three broad stratigraphic zones. Their physical relationships to their host rocks-particularly the way they indent older layers beneath and are covered by younger layers above-provide abundant evidence that there was generally a sharp, well-defined interface between the top of the cumulate pile and the main body of magma in the intrusion while the Layered Series was forming. The distribution of the autoliths between and through the well-known, rhythmic, thin, modally graded layers shows that these layers were spread by magmatic currents; and their relations to the more extensive macrorhythmic layering suggest that it too was significantly shaped by currents. Many of the larger autoliths are crudely layered internally, and in places it is evident that their stratification existed before they broke loose; therefore, it must have formed in the Upper Border Series. One particularly large block of oxide gabbro exhibits extraordinarily well-developed modal and textural layering and includes small troctolitic autoliths of an earlier generation, and it provides evidence that currents also spread crystalline materials across the top of the magma body. Many of the very small autoliths in the Layered Series are highly anorthositic in composition, apparently because they were leached of mafic minerals, and some of the larger blocks show local patchy internal replacement by anorthosite. Most large blocks show little sign of postaccumulation modification, and some have thin, fine-grained augite-rich rims or rinds, demonstrating that even though they were out of thermal and chemical equilibrium with their host cumulates, they still were effectively armored against extensive chemical change. Also documented is a large block that was cut by several early basaltic dikes before it broke free from the top of the intrusion; these early dikes transgress small anorthositic replacement pipes in the block, showing that the replacement process also occurred in the upper border environment. Two mechanisms are described whereby graded cumulate layers can be sorted and deposited by magmatic crystal-liquid suspension currents. One, involving density surge currents, has been advocated previously; the other is a new concept based on boundary flow separation and reattachment vortex cells. The two mechanisms are used in complementary ways to illustrate the formation of (1) some of the principal Skaergaard structures involving blocks and layers; (2) modally graded layers in the Layered Series that rhythmically alternate with uniform layers; and (3) modally sorted layers in the Upper Border Series featuring 'underside draping' beneath small included blocks. Explanations are provided for (1) why plagioclase did not float away from the tops of graded layers even though it was less dense than the liquid, and (2) how the liquid part of a current was fractionated away from the crystalline materials. Modal and grain-size data from Skaergaard intrusion graded layers are shown to be in excellent accord with characteristics predicted for layers sorted by currents; a synthesis diagram is presented illustrating how all the above processes may have functioned in concert in the intrusion.
In recent years, a rapidly expanding database, especially in sensitive high-resolution ion microprobe (SHRIMP) geochronology, has led to significant advances in under standing of the Pre cambrian tectonic evolution of the Grenville Province, including its Adirondack outlier, and the Mesoproterozoic inliers of the Appalachians. Based upon this information, we review the geochronology and tectonic evolution of these regions and signifi cant similarities and differences between them. Isotopic data, including Pb isotopic mapping, suggest that a complex belt of marginal arcs and orogens existed from Labrador through the Adirondacks, the midcontinent, and into the southwest during the interval ca. 1.8-1.3Ga. Other data indicate that Mesoproterozoic inliers of the Appalachians, extending from Vermont to at least as far south as the New Jersey Highlands, are, in part, similar in composition and age to rocks in the south western Grenville Province. Mesoproterozoic inliers of the Appalachian Blue Ridge likewise contain some lithologies similar to northern terranes but exhibit Nd and Pb isotopic characteristics suggesting non-Laurentian, and perhaps Amazonian, affinities. Models invoking an oblique collision of eastern Laurentia withAmazonia are consistent with paleomagnetic results, and collision is inferred to have begun at ca. 1.2 Ga. The collision resulted in both the ca. 1190-1140 Ma Shawinigan orogenyand the ca. 1090-980 Ma Grenvillian orogeny, which are well represented in the Appalachians. Several investigators have proposed that some Amazonian Mesoproterozoic crust may havebeen tectonically transferred to Laurentia at ca. 1.2 Ga. Data that potentially support or contradict this model are presented.
The local metamorphic history is an essential topic in the study of high-grade gneiss terrains. This chapter only deals with those aspects of the metamorphic history that can be studied in the field. We will briefly outline terminology, the methods which can be applied, and the difficulties involved. For more information we refer to textbooks such as Miyashiro (1975), Winkler (1976), Mason (1981), Best (1982), Vernon (1983), Spry (1986), Yardley (1989) and to the references given in the text.
Understanding the magmatic evolution of the rocks once comprising the hinterland of the Grenville Orogen through their Mesoproterozoic formation is a key to understanding the Grenvillian Orogeny as whole. In this contribution we present high precision ID-TIMS U-Pb and coupled S-MC-ICP-MS Lu-Hf zircon data from magmatic rocks occurring in the allochthonous belt of the Grenville Orogen in the central part of the Grenville Province. We document the presence of a large tract of Pinwarian crust represented by a 1497 ± 5 Ma granitic gneiss, as well as large late Geon 14 to early Geon 13 (1434 +7/-11 Ma, 1413 ± 12 Ma, 1393 ± 8 Ma, 1383 ± 1 Ma) magmatic complexes. One Grenvillian plutonic suite of 1015 ± 2 Ma that cross-cut the host rock metamorphic fabric has also been dated. This age provides a minimum age of Ottawan metamorphism in the region. The Hf-isotopic data show that the magmatic rocks of Geon 14 and 13 had mixed mantle and crustal sources compatible with intrusion in a supra-subduction setting as is also supported by the whole rock geochemical data presented. Emplacement of the magmatic rocks occurred in settings varying from a distal margin arc to a contractional and extensional continental arc. Grenvillian aged magmatism is more ambiguous, but our data indicate that rocks as young as ca. 1015 Ma may have formed in an ensialic setting.
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 km(2). 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
Mafic lavas can undergo extreme alteration and chemical reconstitution when exposed to the low-temperature, bydrous conditions of late-magmatic and diagenetic environments. Of the various alteration products found some have the compositions (apart from extra H2O) of cordierite-anthophyllite rocks. Such altered mafic lavas occur at Yalwal, N.S.W., and have been converted locally to cordierite-anthophyllite hornfelses by essentially isochemical thermal metamorphism.
The Bondy gneiss complex in the Grenville Province of Southwest Quebec hosts a mineralized iron oxide- and copper-rich hydrothermal system. The northern part of the complex overlies the lithospheric-scale Mont-Laurier lineament and is cut by the regional Mont-Laurier South shear zone interpreted from Bouguer gravity. A sinistral 6 km wide strike-slip corridor defined by several second-order shears (the Mont-Laurier South shear zone) in the complex was identified from geophysical data, including a new high-resolution airborne magnetic survey, and field observations. The spatial association of a metamorphosed alteration system, several pre- to post-metamorphic mineralized zones and mafic intrusions within the Mont-Laurier South shear zone suggests that (i) underlying basement structures controlled hydrothermal fluid migration during the formation of epithermal-IOCG mineralization and associated alteration system before ca. 1.2 Ga high-grade metamorphism and penetrative ductile deformation in the complex; (ii) post-metamorphic reactivation allowed magma ascent and pluton emplacement in the complex and adjacent supracrustal rocks within dilatational sites; and (iii) brittle-ductile shears that postdate high-grade metamorphism provided channel ways for fluid migration associated with magnetite-related mineralization. Although the complex does not host an economic mineral deposit, the role between structures at different levels and the combination of gravity and aeromagnetics at different scales provides an example of an approach for mineral exploration in similar high grade gneiss terrains.
Detrital zircons from quartzites constrain the timing of deposition, provenance, and basin evolution of the Grenville supergroup in the Adirondack Lowlands. Three samples from the stratigraphic succession yield maximum depositional ages of 1284 ± 16 Ma to 1257.6 ± 16 Ma. Shifts in provenance from rifting (unimodal volcanic), carbonate platform (Laurentian interior), deepwater clastics (southern Adirondack arc), to final basin fill (Laurentian interior) are documented. The Trans-Adirondack backarc basin is one of several that opened at ca. 1.30 Ga, filled with thick carbonate-dominated sequences, and closed during the Elzevirian orogeny (ca. 1245-1220 Ma), terminating widespread extension across the leading edge of southeast Laurentia.
Uranium and polymetallic U mineralization hosted within brecciated albitites occurs one kilometer south of the magnetite-rich Au–Co–Bi–Cu NICO deposit in the southern Great Bear magmatic zone (GBMZ), Canada. Concentrations up to 1 wt% U are distributed throughout a 3 by 0.5 km albitization corridor defined as the Southern Breccia zone. Two distinct U mineralization events are observed. Primary uraninite precipitated with or without pyrite–chalcopyrite ± molybdenite within magnetite–ilmenite–biotite–K-feldspar-altered breccias during high-temperature potassic–iron alteration. Subsequently, pitchblende precipitated in earthy hematite–specular hematite–chlorite veins associated with a low-temperature iron–magnesium alteration. The uraninite-bearing mineralization postdates sodic (albite) and more localized high-temperature potassic–iron (biotite–magnetite ± K-feldspar) alteration yet predates potassic (K-feldspar), boron (tourmaline) and potassic–iron–magnesium (hematite ± K-feldspar ± chlorite) alteration. The Southern Breccia zone shares attributes of the Valhalla (Australia) and Lagoa Real (Brazil) albitite-hosted U deposits but contains greater iron oxide contents and lower contents of riebeckite and carbonates. Potassium, Ni, and Th are also enriched whereas Zr and Sr are depleted with respect to the aforementioned albitite-hosted U deposits. Field relationships, geochemical signatures and available U–Pb dates on pre-, syn- and post-mineralization intrusions place the development of the Southern Breccia and the NICO deposit as part of a single iron oxide alkali-altered (IOAA) system. In addition, this case example illustrates that albitite-hosted U deposits can form in albitization zones that predate base and precious metal ore zones in a single IOAA system and become traps for U and multiple metals once the tectonic regime favors fluid mixing and oxidation-reduction reactions.
The NICO Au-Co-Bi(±Cu±W) deposit is located in the Great Bear magmatic zone, NWT, Canada, where numerous polymetallic, iron oxide-dominated mineralized systems have been recognized. Petrographic, electron microprobe analysis (EMPA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) studies of host-rock alteration and ore mineralogy, together with sulfarsenide geothermometry, have been carried out to constrain the nature of alteration and/or mineralization assemblages in this deposit. Metasedimentary rocks of the Treasure Lake Group host NICO and are pervasively altered to an assemblage of ferrohornblende I + actinolite I + biotite I + magnetite I ± orthoclase, which is cut by barren veins composed of quartz ± ferrohornblende-orthoclase-calcite (Set 1). These alteration events are overprinted by metasomatic prograde and retrograde mineralized assemblages and both brittle and ductile deformation accompanied the metasomatism. The prograde assemblage (>400°C) consists of cobaltite, Co-rich loellingite, and Co-rich arsenopyrite (stage I), magnetite II, ferrohornblende II, actinolite II, biotite II, pyrite, and minor scheelite and orthoclase. The earliest retrograde mineralization consists of arsenopyrite (stages II and III), which contains variable amounts of Co, together with native Bi (±bismuthinite) and Au, with lesser magnetite, marcasite, pyrite, hastingsite, and minor quartz. The preservation of solidified native Bi droplets suggests a temperature range of 270° to <400°C for precipitation of this assemblage. The final stage of retrograde mineralization consists of a chalcopyrite-bismuthinite-hematite-chlorite assemblage, together with hastingsite ± emplectite, which formed at temperatures of less than 270°C. Textural and trace element evidence indicates that the Au and Bi present within arsenides and sulfarsenides in the NICO system resulted from the initial partitioning of structurally bound Au and/or "invisible" (nanometer-sized particles) of Au and Bi into the prograde sulfarsenide and arsenide phases, which contain up to 81 ppm Au. The Au and Bi were remobilized following retrograde alteration of those minerals to arsenopyrite II. Molten Bi droplets are interpreted to have scavenged Au insitu, resulting in the formation of the Bi-Au inclusions observed in arsenopyrite II. The second mechanism of gold refining is explained by the occurrence of contemporaneous Bi (±Te) melt and hydrothermal fluids that also could have fractionated gold during transport in solution and deposited it in fractures, interstitially to earlier mineral grains, and as disseminations within Ca-Fe-amphibole-magnetite-biotite-altered rocks. Overall, the gold upgrading at NICO is consistent with the liquid bismuth collector model, suggesting that this process was an important control on gold concentration in this and potentially other Au-Bi-Te-Fe-As-S-rich iron oxide-copper-gold (IOCG) deposits.
Marble occurs abundantly in a 31,000 km2 segment of the southern Grenville Province of the Canadian Precambrian Shield, where it is associated with quartzite, biotite-garnet gneiss, and amphibolite to form the Grenville Group. An 1800 km2 area on the western margin of this segment, north of the Ottawa river, displays a great variety of carbonate rocks, which may be divided into two groups:(I) major marble, with calcite, dolomite, graphite, phlogopite, Ca amphibole, Ca pyroxene, forsterite, humite group minerals,. (II) minor marble, with pink calcite, phlogopite, Ca amphibole, Ca pyroxene, K feldspar, scapolite, sphene.Rocks of the first group are associated with plagioclase gneiss and amphibolite, and are metamorphosed limestone, little affected by metasomatism; rocks of the second group, which are less common, are associated with potassium feldspar gneiss and heterogeneous granitic and syenitic rocks, and are inferred to be metasomatic rocks.Numerous mineral reactions have taken place in the carbonate rocks during metamorphism. The calcite-dolomite reaction, which governs the Mg content of calcite, indicates a metamorphic temperature of about 650 °C. Forsterite was possibly produced from low-Al amphibole, and forsterite + spinel from high-Al amphibole. The crystallization of some silicate minerals in the minor marble units, and the enrichment in the contained calcite in Fe and Sr are attributed to metasomatic reactions. Metamorphic ion-exchange reactions involving carbonates produced the following distribution coefficients:. Sr in calcite/Sr in dolomite = 2.5. Mn in calcite/Mn in dolomite = 0.89. Fe in calcite/Fe in dolomite = 0.29. from which inferences may be drawn concerning the distribution of these elements between the Ca and Mg sites within dolomite during metamorphic crystallization. Ion-exchange reactions involving silicates produced the following distribution of Mn:. humite group Ca pyroxene.Ca amphibole phlogopite. where the numbers are distribution coefficients. An equilibrium distribution of Fe between silicates and calcite in the minor marble was evidently not attained during metasomatic crystallization. Numerous retrograde reactions have taken place, including the alteration of pyroxene to amphibole, forsterite to serpentine, and the exsolution of dolomite from calcite.Forsterite in marble, and orthopyroxene in the associated gneisses and amphibolites crystallized sporadically in the Laurentian highlands, but not in the lowlands of the Ottawa rift valley, where peak metamorphic temperatures may have been slightly lower. In the highlands, reactions to produce forsterite and orthopyroxene were initiated in response to a local increase in temperature, local peculiarities in the chemical composition of amphibole, which produced these minerals, or a local decrease in the activity of CO2 and H2O in the grain-boundary phase.
The PALMEIRÓPOLIS Cu-Zn (Pb) volcanogenic massive sulfide deposit, Brazil, consists of three ore bodies enclosed by hydrothermal alteration zones. The ore bodies and the alteration zones were metamorphosed under amphibolite fades conditions. The Palmeirdpolis alteration zones are characterized by a great diversity of bulk rock composition that originated a wide variety of low variance mineral assemblages. These assemblages are composed of orthoamphiboles (anthophyllite and gedrite), hornblende, biotite, garnet, staurolite, sillimanite, gahnite and, rarer, cordierite. Based on analyses of mineral chemistry aad mineral assemblages, temperatures are estimated to have been 550 -625°C and pressures 2 -5.5 kbar. The temperature of metamorphism that prevailed at the Palmeirdpolis deposit is comparable to other amphibolite fades massive sulfide deposits, such as Geco and Linda, Canada; Falun, Sweden; and Bleikvassli, Norway. The mineralogy of the alteration zones is similar in all these deposits even though they were metamorphosed at different pressure conditions, reflected by the crystallization of one of Al 2 SiO 5 phase (andalusite, sillimanite or kyanite). depósito de sulfeto MACIÇO a Zn e Cu vulcanogenico de Palmeirdpolis, Brasil, consiste de tres corpos de minério associados a zonas de alteração hidrotermal. Os corpos de minério e as zonas de alteração foram metamorfisados no fades anfibolito. As zonas de alteração do depósito são caracterizadas por uma grande diversidade química que originou uma grande variedade de assembléias mineralógicas. Essas assembléias são compostas por ortoanfibólios (antofilita e gedrita), homblenda, biotita, granada, estaurolita, silimanita, gahnita e, mais raramente, cordierita. Baseado na composição química de diferentes minerals e na associacdes mineralógicas, a temperatura de metamorfismo foi estimada entre 550 e 625°C e a pressão entre 2 e 5,5 kbar. A temperatura de metamorfismo a qual o depósito de Palmeirópolis foi submetido é comparável a de outros depósitos de sulfeto maciço também metamorfisados no fades anfibolito, tais como Geco e Linda, no Canadá; Falun, na Suécia; e Bleikvassli, na Noruega. A mineralogia das zonas de alteração é similar em todos esses depósitos apesar de terem sido submetidos a diferentes condições de pressão, refletidas apenas pela cristalização de um ou outro polimorfo de A1 2 SiO 5 (andalusita, silimanita ou cianita). Palavras-chaves: deposito vulcanogênico, zonas de alteração hidrotermal, facies anfibolito.
RÉSUMÉ En 2003-2004, le Ministère des Ressources naturelles et de la Faune a entrepris un nouveau projet de synthèse du Front du Grenville comportant la réalisation annuelle de deux nouvelles cartes au 1 : 50 000, la caractérisation de tous les indices minéralisés et une synthèse tectono-métamorphique. L'objectif principal de cette synthèse multidisciplinaire est de documenter les équivalents métamorphisés des ceintures volcano-sédimentaires archéennes de la Sous-province de l'Abitibi dans le Parautochtone grenvillien. Ce document présente les premiers résultats de l'étude métamorphique réalisée dans le cadre de ce projet. Elle comporte deux volets complémentaires qui permettront de comprendre l'évolution spatio-temporelle des minéralisa-tions en parallèle avec l'évolution tectono-métamorphique des lithologies cartographiées. Cette étude métamorphique régionale a pour objectif de déterminer les conditions de pression et de température des ceintures de roches supracrustales de l'Abitibi métamorphisées dans le Parautochtone grenvillien. La compréhension de l'évolution thermodynamique régionale de ces ceintures est importante pour comprendre les minéralisations dans ces terrains de haut grade métamorphique (amphibolitiques à granulitiques). Historiquement, au Québec, l'industrie minière s'est surtout concentrée sur des terrains de bas grades métamorphiques (faciès des schistes verts et faciès inférieur des amphibolites). Le développement de nouveaux outils permettra de faciliter l'exploration dans des terrains métamorphiques de haut grade. Cette étude métamorphique régionale est basée sur l'échantillonnage des métabasites et des métasédiments dans la région de Chibougamau, le long de traverses d'orientation NO-SE qui passent de la Province du Supérieur au Parautochtone grenvillien. Les indices métamorphisés déjà reconnus, ainsi que les nouveaux indices mis au jour pendant les travaux de l'été 2003, sont aussi échantillonnés. La méthodologie à suivre s'organise en quatre étapes analytiques suivies de deux méthodes de traitement des données : 1) l'étude pétrographique détaillée, 2) la chimie minérale effectuée à la microsonde électronique, 3) les analyses de roches totales et partielles (métaux usuels et précieux), 4) l'utilisation du microscope électronique à balayage (MEB) pour l'analyse des sulfures, 5) l'estimation des conditions de pression-température (P-T) et 6) la datation des différents événements métamorphiques. Les résultats préliminaires présentés dans ce document sont basés sur les travaux de cartographie de l'été 2003 dans les régions du lac Lagacé et du lac Charron. Ils rassemblent l'interprétation des isogrades, la chimie minérale et les calculs des conditions de pression-température (P-T) subies par les métabasites. Les isogrades longent les grandes failles régionales et montrent que le gradient métamorphique augmente de la Province du Supérieur vers le Parautochtone grenvillien, passant du faciès inférieur des amphibolites, au faciès des granulites. La chimie minérale des grenats et des amphiboles présents dans les métabasites suit également cette tendance. La composition en grossulaire-almandin (Ca-Fe) des grenats indique une augmentation de la pression, tandis que la teneur en TiO2 des amphiboles montre une augmentation de la température. Les conditions P-T évoluent de la Province du Supérieur au Parautochtone grenvillien. Au nord-ouest, les métabasites montrent des conditions maximales de 5 ± 1 kbar et de 647 ± 50 o C et minimales de 4 ± 1 kbar et de 617 ± 50 o C. Au Front du Grenville, on obtient des conditions de pressions de l'ordre de 10,3 à 5.1 ± 1 kbar et de températures comprises entre 801 et 462 ± 50 o C. La zone transitionnelle, où l'on observe encore des reliques de structures primaires, montre des conditions P-T qui s'étalent de 13,3 à 4 ± 1 kbar et de 714 à 535 ± 50 o C. Le Parautochtone grenvillien présente les conditions P-T les plus élevées avec un maximum et un minimum de 17 ± 1 kbar et 874 ± 50 o C ainsi que de 8,5 ± 1 kbar et 690 ± 50 o C respectivement. Du point de vue économique, les résultats préliminaires de la chimie minérale des grenats et des amphiboles dans des zones d'altérations métamorphisées de type sulfures massifs volcanogènes (SMV) montrent un enrichissement marqué en manganèse dans les grenats situés à proximité des sulfures. La teneur en titane des amphiboles nous renseigne sur la nature syn ou tardi-métamorphique de la remobilisation des sulfures. Ces premiers résultats permettent d'émettre plusieurs hypothèses de travail sur la chronologie relative et la caractérisation des traceurs chimiques typiques des minéralisations de type SMV en terrains de haut grade métamorphique.
NW-SE trending, transverse lineaments, including the lithospheric-scale Mont-Laurier lineament, are interpreted from regional Bouguer gravity of the Grenville orogen of SW Quebec and adjacent Superior Craton in southeastern Canada. These lineaments, transverse to the ENE trending Grenville orogen, are inferred to correspond to Palaeoproterozoic structures in Archaean basement that have played an important role: (i) in the development of volcano-sedimentary back-arc basins along this segment of the Laurentian margin; (ii) on the geometry of thrust sheets and folds formed during thrusting in the ca. 1.23-1.2 Ga Elzevirian orogeny and incorporation of the basins within the orogen; (iii) on reorientation of early-formed structures in the Central Metasedimentary Belt of Quebec (CMB-Q) during ca. 1.19-1.17 Ga post-Elzevirian orogenic collapse; and (iv) for development of syn-plutonic deformation corridors and shear zones at the onset of the emplacement of the Morin anorthosite-mangerite-charnockite-granite (AMCG) suite. In the CMB-Q, a 100 km wide megakink zone developed during ca. 1.19-1.17 Ga differential, post-Elzevirian orogenic collapse in the upper-most nappe above transverse sinistral shear corridors 10-20 km wide located in an underlying thrust sheet or “lower-deck”. Emplacement of 1.17 Ga Chevreuil intrusive suite preferentially occurred within the megakink zone, starting late in the post-Elzevirian collapse and culminating during a switch to local shortening early in (and in part as a consequence of) the emplacement of the voluminous Morin anorthosite and associated AMCG-suite plutons. The Labelle deformation zone separating the CMB-Q and Morin terrane is interpreted as a post-Shawinigan, reverse shear zone that truncates folded lithological layering in the eastern CMB-Q and western Morin terrane that is either subsequently folded above the Mont-Laurier lineament during its further reactivation, or developed as a curved shear zone stepping across the Mont-Laurier lineament. The Grenville Province of SW Quebec therefore provides an example of strain partitioning and distinct deformation responses at different crustal levels during reactivation of basement structures.
On the basis of geological, geophysical, and geochronological data, the
Grenville Province has been divided into three first-order longitudinal
belts, the Parautochthonous Belt (PB), Allochthonous Polycyclic Belt
(APB), and Allochthonous Monocyclic Belt (AMB). These are set apart by
three first-order tectonic boundaries, the Grenville Front (GF),
Allochthon Boundary Thrust (ABT), and Monocyclic Belt Boundary Zone
(MBBZ). The belts are subdivided into terranes based on internal
lithological character. The GF separates the Archean to Proterozoic
foreland northwest of the orogen from reworked equivalents to the
southeast. Continuous at the scale of the orogen, its main
characteristic is that of a crustal-scale contraction fault. The PB,
although less clearly identified along the length of the orogen, in most
places represents upgraded and tectonically reworked rocks of the
adjacent foreland. The boundary between the PB and the APB to the
southeast, the ABT, is most clearly delineated in the eastern half of
the province. It is the locus of major crustal delamination along which
high-grade, mostly middle Proterozoic, polycyclic terranes were
tectonically transported northwest toward and onto the PB. The AMB
comprises two separate areas underlain by the Wakeham Supergroup and
what is currently known as the Grenville Supergroup, respectively; its
basal contact, the MBBZ, is a décollement zone of variable
kinematic significance between older polycyclic rocks and tectonically
overlying monocyclic rocks. This first-order zonation implies a tectonic
polarity to the Grenville Province, superimposed on which are
second-order features evident from contrasting tectonic styles and
radiometric ages. These characteristics are consistent with a
diachronous or oblique collisional model for the Grenville orogen.
Zonal alteration is a common feature in volcanic rocks surrounding sea-floor massive sulfide deposits. Alteration indexes, such as the Ishikawa alteration index (AI) and the chlorite-carbonate-pyrite index (CCPI), have been developed to measure the intensity of sericite, chlorite, carbonate, and pyrite replacement of sodic
feldspars and glass associated with hydrothermal alteration proximal to the orebodies. In this paper a simple graphical representation of the Ishikawa AI plotted against the CCPI, termed the “alteration box plot,” is used to characterize the different alteration trends related to massive sulfide ores and to assist in the distinction of volcanic-hosted massive sulfide (VHMS)-related hydrothermal alteration from regional diagenetic alteration. Although there are some limitations with the technique, a series of case studies are used to demonstrate that the alteration box plot is a powerful means of understanding the relationship between mineralogy, lithogeochemistry, and intensity of alteration in zoned alteration systems related to VHMS deposits and should assist the exploration geologist in determining vectors to the center of the ore system.