Abitibi Subprovince Research Articles

Gravity driven and in situ fractional crystallization processes in the Centre Hill complex, Abitibi Subprovince, Canada: Evidence from bilaterally-paired cyclic units

The formation of layers in mafic intrusions has been explained by various processes, making it the subject of much controversy. The concept that layering originates from gravitational settling of crystals has been superseded in recent years by models involving in situ fractional crystallization. Here we present evidence from the Centre Hill complex that both processes may be operative simultaneously within the same intrusion. The Centre Hill complex is part of the Munro Lake sill, an Archean layered mafic intrusion emplaced in volcanic rocks of the Abitibi Subprovince. The Centre Hill complex comprises the following lithostratigraphic units: six lower cyclic units of peridotite and clinopyroxenite; a middle unit of leucogabbro; six upper cyclic units of branching-textured gabbro (BTG) and clotted-textured gabbro (CTG), the uppermost of these units being overlain by a marginal zone of fine-grained gabbro. The cyclic units of peridotite/clinopyroxenite and BTG/CTG are interpreted to have formed concurrently through fractional crystallization, associated with periodic replenishment of magma to the chamber. The units of peridotite and clinopyroxenite formed by gravitational accumulation of crystals that grew under the roof. The cyclic units of BTG and CTG formed along the upper margin of the sill by two different mechanisms: (1) layers of BTG crystallized in situ along an inward-growing roof and (2) layers of CTG formed by accumulation of buoyant plagioclase crystals. The layers of BTG are characterized by branching pseudomorphs after fayalite up to 50 cm in length that extend away from the upper margin. The original branching crystals are interpreted to have grown from stagnant intercumulus melt in a high thermal gradient resulting from the injection of new magma to the chamber.

Pb isotope constraints on the provenance and diagenesis of detrital feldspars from the Sudbury Basin, Canada

Residues after HF leaching of detrital feldspar grains from the Early Proterozoic Chelmsford Formation of the Sudbury Basin, Ontario record isotopic compositions very close to the initial Pb isotopic composition of their Late Archean, Superior Province source. These grains have been recrystallized to albite and K-feldspar. Residues of Chelmsford feldspar grains lie within the range of reported 206 Pb 204 Pb compositions of feldspars from plutons of the adjacent Abitibi subprovince of the Superior Province, with a steep 207 Pb 206 Pb slope of 0.35 ± 0.15 (3.7 ± 0.7 Ga). Leaches are more radiogenic than residues, but leach-residue pairs do not record in situ U (and Th) decay since either the Late Archean time of feldspar crystallization in the Superior Province or the Early Proterozoic time of recrystallization in the Chelmsford Formation. Instead, leach-residue pairs lie along an average 207 Pb 206 Pb slope of 0.250 ± 0.036, representing mixing trends. Most whole-rock samples from the Chelmsford Formation lie close to a 2.7 Ga slope, collinear with detrital feldspar compositions. We interpret the Pb data from the leaches as recording the passing of altering fluids. These fluids likely carried Pb derived from rocks or minerals with relatively high U Pb , Th Pb and Th U within the sedimentary basin itself. The residues appear to record very close to initial values because they lie along the very steep trend defined by feldspars of the Superior Province.

The economic significance of the Bell River Complex, Abitibi subprovince, Quebec

The exposed part of the Bell River Complex on the northern edge of the Abitibi subprovince represents the upper portion of a layered complex, with a massive gabbroic main zone being overlain by a magnetite-rich gabbroic–anorthositic layered zone and a granophyre. The cumulate rocks display cryptic variation in mineral and whole-rock compositions towards a more evolved signature with stratigraphic height. There is a decrease in An content of plagioclase (from An73 to An51), and in Mg# of augite (from Mg# 75 to Mg# 56), an increase in La/Yb ratio of whole rocks (from 0.5 to 2.9), and a decrease in Cr (from 770 to 5 ppm) and Ni (from 315 to 45 ppm) contents of whole rocks. Concentrations of platinum-group elements (PGE) are mostly below detection limit. Compositional comparison with the South African Bushveld Complex shows that the lowest exposed levels of the complex are equivalent to the critical–main zone boundary, that is, in the immediate hanging wall of the PGE reefs. Based on recent geophysical studies showing that the Bell River Complex is approximately 5 km thick, we consider the presence of a hidden ultramafic zone a distinct possibility and propose a site for an exploration drilling programme. A comparison of compositional data from the Bell River Complex and associated basalts and rhyolites of the Watson Lake and Wabassee groups suggests that the three suites are cogenetic. Based on the fact that the numerous important volcanogenic massive sulphide (VMS) deposits in the region are located around the intrusion, and the similar age of the intrusion and the volcanic rocks, we support earlier workers in suggesting that the intrusion and the VMS deposits are genetically linked, that is, that the Bell River Complex provided the heat source that drove the hydrothermal fluids responsible for forming the VMS deposits.

Geochemistry and provenance of metasedimentary rocks from the Archean Golden Pond sequence (Casa Berardi mining district, Abitibi subprovince)

The Archean Golden Pond sequence is made up of deformed and metamorphosed conglomerates, greywackes, and mafic volcanic rocks, and is overlain by ferrugineous metasedimentary rocks of the North iron formation. The clastic rocks were derived mainly from a volcanic source that had undergone weak chemical weathering. Their source area was dominated by the presence of 60–80% high-Al2O3 felsic volcanics having strongly fractionated [La/Sm]N (= 3.7 ± 0.3) and very low Ta/Th ratios (= 0.09 ± 0.02), with lesser proportions of basaltic (10–30%) and ultramafic volcanic rocks (1–10%). The ferrugineous metasedimentary rocks can be modelled by mixing 20–40% siliciclastic material, of the composition of the average Golden Pond greywacke, with an Fe- and Si-rich precipitate (molecular Fe/Si = 0.6 ± 0.2). The high-Al2O3 felsic source rocks were most likely produced by subduction processes within an oceanic arc environment, but the mafic and ultramafic volcanic rocks were derived by different processes from an asthenospheric mantle source, possibly in an oceanic rift environment. Therefore, it is suggested that the ultramafic, mafic, and felsic volcanic rocks were brought to the same erosional level by dissection of the arc system and rapid exhumation of the felsic arc lithologies and the deeper ocean floor. Intrabasinal hydrothermal activity associated with contemporaneous mafic volcanism and (or) graben development may have also been responsible for the local production of the Fe-rich precipitates of the North iron formation.

Late Archean regional deformation and structural controls on gold-quartz vein mineralization in the northwestern Slave Province, N.W.T., Canada

Late Archean crustal accretion in the northwestern Slave Province is suggested to have involved approximately west-northwest–east-southeast directed horizontal compression that produced three episodes of deformation recognizable in the northwestern Anialik River igneous complex (ARIC) and Anialik River greenstone belt (ARGB). Observations show that the ARIC was probably emplaced as a series of synvolcanic sills prior to the earliest deformational event. Regional shortening produced a pervasive foliation, downdip lineations and folding in the ARGB, and an early, subsequently folded, foliation in the northwestern ARIC. Postfold ductile and brittle–ductile deformation produced regional- and outcrop-scale shear zones including the Sheeted Zone, which defines the ARIC–ARGB contact. Younger rocks of the northwestern ARGB appear to have been tectonically juxtaposed along the Sheeted Zone against the older rocks of the northwestern ARIC. Greater brittle response, enhanced permeability, and cyclical increases in fluid pressure led to the development and concentration of an anastamosing network of gold-quartz vein bearing shear zones in the ARIC. Steep to subvertical shear-related linear fabrics show that regional-scale and mineralized shear zones have a large component of vertical shear with predominantly east-side-up movement. The age relationships, proximity, and similarity of deformational structures in the Kangguyak gneiss belt, containing a craton-scale ductile deformation zone, and shear zones within the Arcadia Bay area, suggest contemporaneous development and regional late Archean structural relationships similar to those of shear zone hosted gold-quartz vein mineralization seen in the Abitibi Subprovince (Canada) and Yilgarn Craton (Australia).

A pre-2686 Ma intrusion-related gold deposit at the Kiena Mine, Val d'Or, Quebec, southern Abitibi Subprovince

A pre-2686 Ma intrusion-related gold deposit at the Kiena Mine, Val d'Or, Quebec, southern Abitibi Subprovince

Temporal relationships of lode gold mineralization to accretion, magmatism, metamorphism and deformation — Archean to present: a review: R. Kerrich & K. F. Cassidy, Ore Geology Reviews, 9(4), 1994, pp 263–310

Abstract Late Archean (ca. 2.7 Ga) lode gold mineralization in the Superior Province of Canada and Yilgarn Craton of Australia was late kinematic, and syn to late peak metamorphic, and intimately related in space and time to accretionary tectonic processes. Gold mineralization in the Superior Province was coeval with late-kinematic diachronous accretion of allochthonous subprovinces from north to south over ca. 2710 to 2670 Ma. Timing of gold genesis is constrained by field relationships, and rigorous lower ages of cross-cutting dikes. Gold vein formation is an integral part of the thermal and rheological history of deposit host rocks, such that, in general, deposits in greenschist facies rocks have greenschist facies assemblages and occur in brittle-ductile structures, whereas counterparts in amphibolite facies rocks feature amphibolite facies alteration assemblages and generally occur in ductile structures. Gold mineralization in the southeastern Yilgarn Craton is also temporally related to craton-scale compressional events. There is abundant evidence for thermal, structural and hydrothermal disturbance post orogeny in the Yilgarn Craton, with most ArAr, RbSr and reset UPb zircon ages corresponding to known periods of tectonism in adjacent terranes. In addition, gold-related sulphides typically have reset model Pb ages recorded by secondary isochrons. Accordingly, anomalously young UPb ages on rutile and titanite and ArAr ages on hydrothermal silicates in deposits that post-date peak metamorphism by 100 to 240 m.y. are interpreted to reflect multiple secondary hydrothermal events that also locally remobilized primary ore metals. These variably young, reset ages are also recorded in lithological units of undisputed primary age, including VMS deposits, syn-tectonic granitoids and komatiites. These variably young ages provide an exact parallel for reset secondary ages in gold deposits. Mineralization interpreted to have formed 100 to 240 m.y. after terrane accretion based on scattered ages from non-robust isotopic systems or minerals is not compatible with the above relationships, but rather reflects intermittent secondary reactivation of the primary ore hosting structures. The southern Superior Province has experienced thermal, structural and hydrothermal disturbances since termination of S-type magmatism at ca. 2645–2611 Ma, reflected in KAr, RbSr and SmNd ages 100 to 240 m.y. younger than UPb zircon ages on the same lithological units, and reversed apparent blocking sequences. Eight different models, with distinct time connotations, exist for lode gold deposits in the late Archean terranes. The TTG model linking mineralization in the southern Abitibi Subprovince to synvolcanic tonalite-trondhjemite granodiorite batholiths (2695–2685 Ma) can be ruled out on geochronological grounds, as the deposits are most closely associated in space and time to late-kinematic, calc-alkaline and shoshonitic rocks (2685–2670 Ma). Two “late gold” models based on the non-concordant young ages, involving either delayed thermal rebound or young events in Kapuskasing-like mid-crust can be reinterpreted as late kinematic primary mineralization with multistage isotopic resetting. Both young gold models are based on geochronology alone, and ignore tectonic, structural and metamorphic relationships that provide evidence of deposits forming in thermal and rheological equilibrium with their host terranes. Five models (“early gold” models) for lode gold deposits in late-Archean terranes, namely granulization, cratonization, two gold-shoshonite schemes and the crustal continuum model, explicitly relate mineralization to processes resulting from terrane accretion. All five are consistent with rigorously constrained ages of some gold deposits, including primary mineralization at Val d'Or as given by hydrothermal zircons. Lode gold deposits in the Proterozoic Trans-Hudson orogenic belt (ca. 2.0 to 1.7 Ga) also closely follow terrane collision and metamorphism. Similarly, lode gold deposits in late Paleozoic orogenic belts of Appalachia and the Lachlan fold belt of southeastern Australia show temporal relationships of mineralization to collisional tectonics, metamorphism, magmatism and deformation. The temporal relationships of mineralization to collisional tectonics is also repeated in the Mesozoic-Cenozoic Cordilleran belt of the North American continent and the Cenozoic Alpine-Himalayan chain. Giant lode gold metallogenic provinces formed at three times in Earth history, in the late-Archean (2.7-2.6 Ga), late Paleozoic (450–340 Ma) and Mesozoic-Cenozoic. These times correspond to accretionary tectonic assembly of a supercontinent, in external orogens. Minor gold deposits also formed in space and time with collisional tectonics in internal orogenic belts of the supercontinent cycle. Consequently, the local temporal relationships of lode gold mineralization to collisional tectonics and post-peak metamorphism is also part of the secular development of a larger global geodynamic cycle.

Zircon thermometry of high-temperature rhyolites near volcanic-associated massive sulfide deposits, Abitibi subprovince, Canada

Research Article| February 01, 1995 Zircon thermometry of high-temperature rhyolites near volcanic-associated massive sulfide deposits, Abitibi subprovince, Canada C. Tucker Barrie C. Tucker Barrie 1Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar Geology (1995) 23 (2): 169–172. https://doi.org/10.1130/0091-7613(1995)023<0169:ZTOHTR>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation C. Tucker Barrie; Zircon thermometry of high-temperature rhyolites near volcanic-associated massive sulfide deposits, Abitibi subprovince, Canada. Geology 1995;; 23 (2): 169–172. doi: https://doi.org/10.1130/0091-7613(1995)023<0169:ZTOHTR>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract High-temperature, high-silica rhyolites are commonly found near Cu-Zn volcanic-associated massive sulfide (VMS) deposits in the Archean Abitibi subprovince. They are characterized by anhydrous phenocrysts (feldspar, quartz) in concentrations of generally <15 modal percent and have >73% SiO(2), high incompatible element contents, Rb/Sr > 1, Zr/Y < 5, La(N)/Yb(N) < 3.5, and negative Eu anomalies. They have zircon saturation temperatures of 840–940 ° C, as calculated by using the zircon solubility model, which are higher than most other Abitibi rhyolites. Considering their high zircon saturation temperatures, their composition, and their spatial and temporal associations with tholeiitic mafic and ultramafic rocks, Abitibi high-temperature rhyolites are similar to silicic volcanic rocks found in hotspot-related oceanic settings with anomalously high heat flow, such as in eastern Iceland. The association of high-temperature rhyolites with VMS deposits may provide a useful exploration guide where zircon inheritance is minimal, such as in many of the primitive Archean and Early Proterozoic greenstone belts. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

Formation of the Abitibi greenstone belt by arc-trench migration

Volcanic and plutonic rocks of Archean greenstone belts in the Abitibi subprovince become younger southward; plutonic rocks are ∼10–30 m.y. younger than volcanic rocks. The Abitibi greenstone belt has an upper crust of ∼2.75–2.70 Ga oceanic supracrustal assemblages and a middle crust of ∼2.70–2.68 Ga granitic and gneissic rocks. The greenstone belt upper crust is greenschist facies with upright structures and low reflectivity, and the middle crust is amphibolite facies with moderate to shallowly dipping structures and strong reflectivity. We propose that the age progression and crustal layering resulted from accretion of oceanic assemblages followed by extensive mid-crustal magmatism linked to southward migration of an arc-trench system. Thermal softening of the middle crust during magmatism caused ductile flow now represented by strong, mid-crustal reflectivity. Upward heat flow following magmatism caused late-tectonic (2.67–2.65 Ga) low-pressure metamorphism observed in the upper crust.

Harrisitic textures in the Centre Hill complex, Munro Township, Ontario: product of diffusion limited growth

A spectacular occurrence of harrisitic textures has been observed near the upper margin of the Centre Hill complex, a cyclically layered mafic intrusion emplaced within komatiitic and tholeiitic volcanic rocks of the Archean Abitibi Subprovince in the Canadian Shield. The harrisitic textures, interpreted as pseudomorphs after fayalite, reach up to 50 cm in length and consistently extend away from the upper sill margin. They poikilitically enclose crystals of the surrounding matrix, indicating that they originally formed within a crystal-liquid mush. The harrisitic fayalite crystals are interpreted to have grown in situ under far from equilibrium conditions, where growth was limited by the supply of nutrients to the crystal boundaries. Mineral growth conditions were simulated using a diffusion limited aggregation (DLA) algorithm to produce branching clusters qualitatively similar to the observed textures. Based on this model, it is proposed that the original fayalite crystals formed in the late stages of differentiation cycles, where diffusivity was low due to increasing viscosity and Fe-Si enrichment of the interstitial melt. Mineral growth was initiated by an imposed thermal gradient established through the injection of primitive magma into the chamber.

Temporal relationships of lode gold mineralization to accretion, magmatism, metamorphism and deformation — Archean to present: A review

Late Archean (ca. 2.7 Ga) lode gold mineralization in the Superior Province of Canada and Yilgarn Craton of Australia was late kinematic, and syn to late peak metamorphic, and intimately related in space and time to accretionary tectonic processes. Gold mineralization in the Superior Province was coeval with late-kinematic diachronous accretion of allochthonous subprovinces from north to south over ca. 2710 to 2670 Ma. Timing of gold genesis is constrained by field relationships, and rigorous lower ages of cross-cutting dikes. Gold vein formation is an integral part of the thermal and rheological history of deposit host rocks, such that, in general, deposits in greenschist facies rocks have greenschist facies assemblages and occur in brittle-ductile structures, whereas counterparts in amphibolite facies rocks feature amphibolite facies alteration assemblages and generally occur in ductile structures. Gold mineralization in the southeastern Yilgarn Craton is also temporally related to craton-scale compressional events. There is abundant evidence for thermal, structural and hydrothermal disturbance post orogeny in the Yilgarn Craton, with most ArAr, RbSr and reset UPb zircon ages corresponding to known periods of tectonism in adjacent terranes. In addition, gold-related sulphides typically have reset model Pb ages recorded by secondary isochrons. Accordingly, anomalously young UPb ages on rutile and titanite and ArAr ages on hydrothermal silicates in deposits that post-date peak metamorphism by 100 to 240 m.y. are interpreted to reflect multiple secondary hydrothermal events that also locally remobilized primary ore metals. These variably young, reset ages are also recorded in lithological units of undisputed primary age, including VMS deposits, syn-tectonic granitoids and komatiites. These variably young ages provide an exact parallel for reset secondary ages in gold deposits. Mineralization interpreted to have formed 100 to 240 m.y. after terrane accretion based on scattered ages from non-robust isotopic systems or minerals is not compatible with the above relationships, but rather reflects intermittent secondary reactivation of the primary ore hosting structures. The southern Superior Province has experienced thermal, structural and hydrothermal disturbances since termination of S-type magmatism at ca. 2645–2611 Ma, reflected in KAr, RbSr and SmNd ages 100 to 240 m.y. younger than UPb zircon ages on the same lithological units, and reversed apparent blocking sequences. Eight different models, with distinct time connotations, exist for lode gold deposits in the late Archean terranes. The TTG model linking mineralization in the southern Abitibi Subprovince to synvolcanic tonalite-trondhjemite granodiorite batholiths (2695–2685 Ma) can be ruled out on geochronological grounds, as the deposits are most closely associated in space and time to late-kinematic, calc-alkaline and shoshonitic rocks (2685–2670 Ma). Two “late gold” models based on the non-concordant young ages, involving either delayed thermal rebound or young events in Kapuskasing-like mid-crust can be reinterpreted as late kinematic primary mineralization with multistage isotopic resetting. Both young gold models are based on geochronology alone, and ignore tectonic, structural and metamorphic relationships that provide evidence of deposits forming in thermal and rheological equilibrium with their host terranes. Five models (“early gold” models) for lode gold deposits in late-Archean terranes, namely granulization, cratonization, two gold-shoshonite schemes and the crustal continuum model, explicitly relate mineralization to processes resulting from terrane accretion. All five are consistent with rigorously constrained ages of some gold deposits, including primary mineralization at Val d'Or as given by hydrothermal zircons. Lode gold deposits in the Proterozoic Trans-Hudson orogenic belt (ca. 2.0 to 1.7 Ga) also closely follow terrane collision and metamorphism. Similarly, lode gold deposits in late Paleozoic orogenic belts of Appalachia and the Lachlan fold belt of southeastern Australia show temporal relationships of mineralization to collisional tectonics, metamorphism, magmatism and deformation. The temporal relationships of mineralization to collisional tectonics is also repeated in the Mesozoic-Cenozoic Cordilleran belt of the North American continent and the Cenozoic Alpine-Himalayan chain. Giant lode gold metallogenic provinces formed at three times in Earth history, in the late-Archean (2.7-2.6 Ga), late Paleozoic (450–340 Ma) and Mesozoic-Cenozoic. These times correspond to accretionary tectonic assembly of a supercontinent, in external orogens. Minor gold deposits also formed in space and time with collisional tectonics in internal orogenic belts of the supercontinent cycle. Consequently, the local temporal relationships of lode gold mineralization to collisional tectonics and post-peak metamorphism is also part of the secular development of a larger global geodynamic cycle.

Thrusting, magmatic intraplating, and metamorphic core complex development in the Archaean Belleterre-Angliers Greenstone Belt, Superior Province, Quebec, Canada

Abstract The Belleterre-Angliers Greenstone Belt in the Pontiac Subprovince is the youngest and southernmost of the greenstone belts in the Archaean Superior Province of the Canadian shield. The greenstone belt has been disrupted into three fragments: from north to south the Baby, Belleterre and Lac des Bois groups, respectively. The Belleterre-Angliers Greenstone Belt has sheared contacts with younger Pontiac metasediments and older tonalites. The Pontiac metasediments dip beneath the Baby Group and are tectonically interlayered with the tonalites. Thus the Belleterre-Angliers belt is interpreted to be a thrust sheet (6 km thick in the north, thinner in the south) above a mid-crustal duplex consisting of metasedimentary and tonalite imbricates. The thrust sheet was transported from the north, but age and geochemical constraints rule out derivation from the Abitibi Subprovince. It probably roots in a band of ultramafic rocks that mark a major south-vergent thrust in the northern Pontiac Subprovince. After emplacement the Belleterre-Angliers sheet and the adjacent Pontiac metasediments were deformed by a system of linked, extensional shears and transfer faults that border dome or antiformal areas in a style similar to Phanerozoic metamorphic core complexes. The orientation of shear zones and foliations indicates that the regional far-field stresses were compressional both before and after extensional shearing. Since the system of extensional shears is intruded by a suite of monzodiorite, granodiorite, syenodiorite and syenite plutons (with high Mg number) it is concluded that crustal inflation caused by the injection (intraplating) of large volumes of magma perturbed the regional compressive stress field and facilitated extensional faulting.

Synopsis of paleomagnetic studies in the Kapuskasing structural zone

This paper summarizes results from paleomagnetic studies sponsored by Lithoprobe on the Kapuskasing structural zone (KSZ). Data from Archean rocks outside the KSZ indicate that the Wawa Subprovince has not been significantly rotated or translated (&lt; 5°) relative to the Abitibi Subprovince. Results from the granulites and amphibolites indicate that the KSZ underwent several kilometres of uplift at ca. 2.51 Ga and then 10 ± 5° west-northwest tilt with several kilometres of further uplift between 2.04 and 1.88 Ga from thrust faulting on the Ivanhoe Lake fault zone. Localized chemical remagnetization occurred at 1.1 Ga along the west side of the Shawmere anorthosite. Paleomagnetic data from the 2.45 Ga Matachewan diabase dike swarm indicate that it was emplaced within one reversed to normal polarity interval of less than 5 Ma. Their polarity pattern indicates major north-trending faults with several kilometres of dip-slip displacement. Their remanence confirms that the Superior Province was deformed around the KSZ into an oroclinal flexure with 40° changes in trend between 2.04 and 1.88 Ga. Results from eight 1.1 Ga alkali syenite–carbonatite complexes show that the KSZ and adjacent subprovinces have undergone only minor uplift (&lt; 6 ± 2 km) since emplacement. Also, these data refine the radiometric ages of some complexes, demonstrate that the use of superchrons to correlate Keweenawan units in the Midcontinental Rift is unsound, and show that Keweenawan magnetic field was symmetrical. Many specific conclusions that relate to a given unit or limited area were drawn in the KSZ paleomagnetic studies.

Heat flow, gravity and structure of the Abitibi belt, Superior Province, Canada: Implications for mantle heat flow

The results of new heat flow measurements are presented for eleven sites located in the Abitibi greenstone belt of the Archean Superior Province. These new and previously published heat flow data are used with gravity and seismic data to constrain the crustal structure and composition of the Abitibi subprovince, and to determine the mantle heat flow beneath the Canadian Shield. In order to analyze the long-wavelength variations in heat flow and Bouguer gravity anomaly, the crust was divided into three layers with distinctive composition and physical properties. Average density, heat production and thickness for each layer were varied within a range compatible with measurements and yielded crustal models consistent with gravity and heat flow data. The dominant contribution to crustal heat production is that of a tonalitic type layer which has the lowest density. The models require mantle heat flow to range between 10 and 14 mW m −2. The westward increase in heat flow requires the tonalitic layer to thicken from the Grenville Front toward the Kapuskasing uplift at the expense of the greenstone supracrustals. Long-wavelength gravity data are more affected by changes in crustal thickness than by composition, and the low Bouguer anomaly near the eastern Grenville Front is caused by 5–6 km of crustal thickening. In greenstone terranes, the long-wavelength variations in heat flow are more useful for determining average crustal composition and the thickness of supracrustals than the gravity data.

Archean stromatolites from Abitibi greenstone belt, Quebec, Canada

The oldest fossils in eastern North America are Neoarchean stromatolites discovered near Joutel, in the northern part of the Abitibi Subprovince of the Superior Craton. They are found in allochthonous limestone blocks as much as 2 m across in 2730-2720 Ma volcanic breccia and turbiditic volcaniclastic rocks of the Joutel Volcanic Complex. The stromatolites, which include thyssagetacean and pseudocolumnar forms, represent an assemblage transported to greater depth from biohermal or biostromal complexes formed in shallow-water settings on a submerged volcanic edifice.

Pre-2.8 Ga crust in the Opatica gneiss belt:A potential source of detrital zircons in the Abitibi and Pontiac subprovinces, Superior Province, Canada

Ages of detrital zircons in post-2.7 Ga metasedimentary rocks of the southern Abitibi and northern Pontiac subprovinces require that pre-2.8 Ga crust was exposed during or following deformation of the 2.70 and 2.68 Ga, and the belt forms a high-grade crystalline core to a south-vergent mountain belt along the northern margin of the Abitibi subprovince. Erosion of the Opatica and northern Abitibi during thrusting would shed detritus southward in advance of a south-vergent thrust system. The Lac Rodayer complex may be the enigmatic source of ∼2.8 Ga detrital zircons within turbidites of the southern Abitibi and northern Pontiac subprovinces.

Structure of the high-grade Opatica Belt and adjacent low-grade Abitibi Subprovince, Canada: an Archaean mountain front

Plutonic gneisses in the Archaean Opatica Belt in Canada record crustal-scale, WSW-vergent (i.e. longitudinal), high-temperature D 1 ductile shearing. Similar structures occur in adjacent greenstone belts, but only in narrow amphibolite facies zones near the base; metavolcanics in the greenschist facies portions do not record penetrative D 1 structures. The greenstone belts are interpreted as allochthonous on the gneisses; they form the uppermost D 1 thrust sheets. SSE-vergent (i.e. approximately transverse) D 2 thrusting imbricated the Opatica Belt and formed a large antiform—either a culmination or an antiformal stack—in the southern Opatica. The imbricated Opatica Belt wedged under the northern part of the Abitibi Subprovince and created a tight syncline with NNW-vergent D 2 thrusts (interpreted as backthrusts) on its northern limb. The Opatica Belt-Abitibi Subprovince contact, therefore, resembles some Phanerozic mountain fronts. Further southwards-propagating imbrication of the crust beneath the Abitibi Subprovince created a S-vergent foreland fold and thrust belt in the overlying greenstones. High-grade Opatica gneisses and adjacent low-grade Abitibi greenstones contain complementary structural patterns and may therefore represent the internal and foreland parts of an Archaean mountain belt, similar in many respects to Phanerozoic examples.

Geochemistry of volcanic rocks associated with Cu-Zn and Ni-Cu deposits in the Abitibi Subprovince

Volcanogenic massive sulfide (VHMS) deposits in the Abitibi subprovince are preferentially associated with volcanic successions containing >150 m thicknesses of felsic volcanic rocks ( approximately 50% by area of volcanic terranes) and are found within volcanic sequences of at least three distinct affinities. Group I, which is host to greater than half of the volcanogenic massive sulfide deposits by tonnage and which comprises only approximately 10 percent by area of volcanic terranes, is composed of bimodal, tholeiitic basalt-basaltic andesite, and high silica rhyolite. The basaltic andesites and high silica rhyolites are characterized by high high field strength element and heavy rare earth element (REE) contents, low light to heavy REE ratios (most with La N /Yb N = 0.8-3), and strong negative Eu anomalies. The Kamiskotia, Matagami, and Chibougamau (Lower cycle) volcanogenic massive sulfide areas, all of which are also underlain by large, synvolcanic gabbroic complexes, are associated with group I volcanic sequences. The Kidd Creek, Potter, Normetal, and Horne deposits are also included in this category. Group II, which is host to one-third of the volcanogenic massive sulfide deposits by tonnage and which is also approximately 10 percent by area of volcanic terranes, is composed of bimodal, transitional tholeiitic to calc-alkalic andesitc and rhyolite, characterized by intermediate high field strength element contents and slightly higher REE ratios (La N /Yb N = 1-4: Noranda camp (excluding the Horne deposit) and Val d'Or camp). Group III is host to only one known deposit, the Selbaie mine, which is unusual in that much of its mineralization cuts stratigraphy. The Selbaie mine sequence contains calc-alkalic andesite-rhyolite with relatively low high field strength element and REE contents, and higher REE ratios (La N /Yb N = 3-9). The vast majority of volcanogenic massive sulfide-bearing mafic and felsic volcanic rocks in the Abitibi subprovince have La N /Yb N < 5.Barren volcanic sequences are group IV--calc-alkalic basaltic andesite to rhyodacite, with low high field strength element and relatively high REE ratios (La N /Yb N = 8-20), represented by the upper Skead Group, the Quebec Hunter Mine Group, and the Upper cycle Chibougamau rocks; and group V, mafic to felsic alkalic volcanic rocks, with high REE ratios (La N /Yb N = 12-62), represented by the Timiskaming, Opemisca, and Ridout series rocks.Group I is most similar to thickened oceanic rift suites (e.g., Galapagos spreading center, Iceland East rift), group II is similar to suites in rifled island arcs (e.g., Hokuroku district, Japan), and group III is comparable to continental arc suites (e.g., Southern Volcanic zone, central Chile). Groups IV and V are comparable to arc-related suites derived from metasomatized mantle, with variable amounts of crustal contamination (e.g., Setouchi area, Japan; Roman province, Italy).Known Ni-Cu deposits in volcanic sequences are hosted exclusively in komatiitic flows and hypabyssal sills, represented by chill compositions with high MgO contents (20-35 wt %, anhydrous), very low incompatible element contents, and depleted signatures (La N /Sm N = 0.5-0.8; Zr/Y < 2.5; Alexo, Marbridge, and Shaw Dome deposits), in comparison to most barren komatiites in the Abitibi subprovince. Their depleted trace element signatures indicate minimal crustal assimilation, which would appear contrary to the widely held view that crustal contamination (e.g., addition of silica, sulfur, oxygen) of a mantle-derived melt is necessary for sulfur saturation and magmatic sulfide segregation. This problem may be reconciled by considering the dynamics of komatiite flows (Lesher and Arndt, 1990) where contaminated* Present address: 23 Euclid Avenue, Ottawa, Ontario, Canada KiS 2W2. komatiite liquid segregated and deposited sulfide and was subsequently swept downstream and replenished by uncontaminated magma over the deposit as the flow ebbed and cooled. When considered together with geologic features such as evidence for thermal erosion in the footwall (Langmuir deposits) and the presence of sulfidic footwall rocks (Shaw Dome deposits), the distinctive, depleted geochemical signature of mineralized komatiites is useful in exploration for Ni-Cu deposits.

The Selbaie Zn-Cu-Ag deposits, Quebec, Canada; an example of evolution from subaqueous to subaerial volcanism and mineralization in an Archean caldera environment

The Selbaie deposits of western Quebec are hosted in calc-alkalic pyroclastic rocks of the Abitibi subprovince. Rocks range upward from subaqueous debris flows through pumiceous tuff to densely welded ignimbrite; they form an intracaldera sequence over 1,200 m thick which records evolution from subaqueous to subaerial deposition. Mineralization occurred within this sequence on the southwest flank of the Brouillan tonalite, a central intrusion and possibly a resurgent dome, and includes regionally extensive and temporally early exhalite, local massive sulfide, and later epithermal veins. These veins were emplaced by open-space filling along synvolcanic structures and make up the bulk of the economic resource.Mineralization and alteration show similarities to both massive sulfide and epithermal deposits. Cu/Zn increases with depth but also increases laterally toward veins and toward the intrusion. Ag/Cu in veins is higher in the upper levels of the system. Whole-rock mass balance calculations indicate enrichment in Si, Fe, Mg, and Mn with proximity to mineralization and depletion in Ca, K, and Na. Na metasomatism, common in host rocks to Archean massive sulfide deposits, is absent in subaerially emplaced tuff at Selbaie and K feldspar is preserved.Selbaie is unusual (1) as an Archean example of a subaerial caldera that evolved from subaqueous to subaerial volcanism, (2) in representing economically important, Archean epithermal mineralization, and (3) in demonstrating the link between subaqueous volcanogenic massive sulfide deposits and subaerial volcanogenic epithermal veins. It is, perhaps, unique in preserving so well all three of these aspects in a single mineralized system. The rarity of similar, mineralized subaerial volcanic settings in the Abitibi subprovince reflects its primitive crustal development, but may also be due to incomplete preservation of subaerial environments.

Pb isotope geochemistry of the Silidor and Launay gold deposits; implications for the source of Archean Au in the Abitibi Subprovince

Pb isotope geochemistry of the Silidor and Launay gold deposits; implications for the source of Archean Au in the Abitibi Subprovince