Southern Abitibi Subprovince Research Articles

Variable Modes of Formation for Tonalite–Trondhjemite–Granodiorite–Diorite (TTG)-related Porphyry-type Cu ± Au Deposits in the Neoarchean Southern Abitibi Subprovince (Canada): Evidence from Petrochronology and Oxybarometry

Abstract Most known porphyry Cu ± Au deposits are associated with moderately oxidized and sulfur-rich, calc-alkaline to mildly alkalic arc-related magmas in the Phanerozoic. In contrast, sodium-enriched tonalite–trondhjemite–granodiorite–diorite (TTG) magmas predominant in the Archean are hypothesized to be unoxidized and sulfur-poor, which together preclude porphyry Cu deposit formation. Here, we test this hypothesis by interrogating the causative magmas for the ~2·7 Ga TTG-related Côté Gold, St-Jude, and Clifford porphyry-type Cu ± Au deposit settings in the Neoarchean southern Abitibi subprovince. New and previously published geochronological results constrain the age of emplacement of the causative magmas at ~2·74 Ga, ~2·70 Ga, and ~ 2·69 Ga, respectively. The dioritic and trondhjemitic magmas associated with Côté Gold and St-Jude evolved along a plagioclase-dominated fractionation trend, in contrast to amphibole-dominated fractionation for tonalitic magma at Clifford. Analyses of zircon grains from the Côté Gold, St-Jude, and Clifford igneous rocks yielded εHf(t) ± SD values of 4·5 ± 0·3, 4·2 ± 0·6, and 4·3 ± 0·4, and δ18O ± SD values of 5·40 ± 0·11 ‰, 3·91 ± 0·13 ‰, and 4·83 ± 0·12 ‰, respectively. These isotopic signatures indicate that, although these magmas are mantle-sourced with minimal crustal contamination, for the St-Jude and Clifford settings the magmas or their sources may have undergone variable alteration by heated seawater or meteoric fluids. Primary barometric minerals (i.e. zircon, amphibole, apatite, and magnetite–ilmenite) that survived variable alteration and metamorphism (up to greenschist facies) were used for estimating fO2 of the causative magmas. Estimation of magmatic fO2 values, reported relative to the fayalite–magnetite–quartz buffer as ΔFMQ, using zircon geochemistry indicates that the fO2 values of the St-Jude, Côté Gold, and Clifford magmas increase from ΔFMQ –0·3 ± 0·6 to ΔFMQ +0·8 ± 0·4 and to ΔFMQ +1·2 ± 0·4, respectively. In contrast, amphibole chemistry yielded systematically higher fO2 values of ΔFMQ +1·6 ± 0·3 and ΔFMQ +2·6 ± 0·1 for Côté Gold and Clifford, respectively, which are consistent with previous studies that indicate that amphibole may overestimate the fO2 of intrusive rocks by up to 1 log unit. Micro X-ray absorption near edge structure (μ-XANES) spectrometric determination of sulfur (i.e. S6+/ΣS) in primary apatite yielded ≥ΔFMQ −0·3 and ΔFMQ +1·4–1·8 for St-Jude and Clifford, respectively. The magnetite–ilmenite mineral pairs from the Clifford tonalite yielded ΔFMQ +3·3 ± 1·3 at equilibrium temperatures of 634 ± 21 °C, recording the redox state of the late stage of magma crystallization. Electron probe microanalyses revealed that apatite grains from Clifford are enriched in S (up to 0·1 wt%) relative to those of Côté Gold and St-Jude (below the detection limit), which is attributed to either relatively oxidized or sulfur-rich features of the Clifford tonalite. We interpret these results to indicate that the deposits at Côté Gold and Clifford formed from mildly (~ΔFMQ +0·8 ± 0·4) to moderately (~ΔFMQ +1·5) oxidized magmas where voluminous early sulfide saturation was probably limited, whereas the St-Jude deposit represents a rare case whereby the ingress of externally derived hydrothermal fluids facilitated metal fertility in a relatively reduced magma chamber (~ΔFMQ +0). Furthermore, we conclude that variable modes of formation for these deposits and, in addition, the apparent rarity of porphyry-type Cu–Au deposits in the Archean may be attributed to either local restriction of favorable metallogenic conditions, and/or preservation, or an exploration bias.

Geology of the orogenic Cheminis gold deposit along the Larder Lake – Cadillac deformation zone, Ontario

The Larder Lake – Cadillac deformation zone (LLCDZ) is one of two major, auriferous, deformation zones in the southern Abitibi subprovince of the Archean Superior Province. It hosts the Cheminis and the giant Kerr Addison – Chesterville deposits within a strongly deformed band of Fe-rich tholeiitic basalt and komatiite of the Larder Lake Group (ca. 2705 Ma). The latter is bounded on both sides by younger, less deformed, Timiskaming turbidites (2674–2670 Ma). The earliest deformation features are F1 folds affecting the Timiskaming rocks, which formed either during D1 extensional faulting or during early D2 north–south shortening related to the opening and closure, respectively, of the Timiskaming basin. Continued shortening during D2 imbricated the older volcanic rocks and turbidites and produced regional F2 folds with an axial planar S2 cleavage. D2 deformation was partitioned into the weaker band of volcanic rocks, producing the strong S2 foliation, L2 stretching lineation, and south-side-up shear sense indicators, which characterize the LLCDZ. Gold is present in quartz–carbonate veins in deformed fuchsitic komatiites (carbonate ore) and turbiditic sandstone (sandstone-hosted ore), and in association with disseminated pyrite in altered Fe-rich tholeiitic basalts (flow ore). All host rocks underwent strong mass gains in CO2, S, K2O, Ba, As, and W, during sericitization, carbonatization, and sulphidation of the host rocks, suggesting that they interacted with the same hydrothermal fluids. Textural relationships between alteration minerals and S2 cleavage indicate that mineralization is syn-cleavage. Thus, gold was deposited as hydrothermal fluids migrated upward along the LLCDZ during contractional, D2 south-side-up shearing. The gold zones were subsequently modified during D3 reactivation of the LLCDZ as a dextral transcurrent fault zone.

Paleoproterozoic hydrothermal reactivation in a neoarchean orogenic lode-gold deposit of the southern Abitibi subprovince: U-Pb monazite geochronological evidence from the Young-Davidson mine, Ontario

An important but poorly constrained issue for neoarchean orogenic lode-gold deposits of the southern Abitibi subprovince is whether they were reactivated, reworked or even enriched by younger hydrothermal fluids. The Young-Davidson mine is a lode-gold deposit that at least in part was structurally controlled. It experienced polyphase deformation and contains multiple generations of auriferous veins. Four stages of veining (V1–V4) occurred after the emplacement of the host Neoarchean syenite. V1–V3 veins formed during Neoarchean time and were responsible for the major phase of gold mineralization of the Young-Davidson deposit. Crosscutting V1–V3 veins is a distinct group of V4 planar to en echelon carbonate-quartz±hematite±epidote veins, which also cut both of the Neoarchean syenite and Paleoproterozoic Matachewan diabase dyke. Hydrothermal monazite extracted from the V4 veins gives a 235U-207Pb age of 1730±5Ma. This represents the first Paleoproterozoic age reported from Neoarchean gold deposits of the southern Abitibi subprovince. The hydrothermal activity was most likely related to the far-field 1750–1700Ma Killarnean or Yavapai magmatism and metamorphism event that ultimately reactivated the pre-existing Neoarchean structures. This Paleoproterozoic activity provides a new perspective for understanding Neoarchean lode-gold deposits of the Abitibi.

Archean subqueous high-silica rhyolite coulées: Examples from the Kidd-Munro Assemblage in the Abitibi Subprovince

Subaqueous high-silica rhyolite in Prosser Township, southern Abitibi Subprovince, Ontario, consists of intercalated, thin (<15 m; low aspect ratio < 0.3) aphyric lavas with brown felsic tuffs, overlain by a pyroclastic pumice- and spatter-rich tuff breccia and are capped by quartz–porphyritic rhyolite. The quartz–porphyritic flows contain ∼10% phenocrysts whereas ‘aphyric’ flows have only sparse phenocrysts, typically 1%. The aphyric flows are characterized by features that are typical of subaqueous rhyolite flows, including well developed flow-top breccia, basal peperite, flow-banding, and hyaloclastite dominated breccia. Mass balance calculations and textural analysis demonstrate that the high-silica aphyric rhyolites were originally glassy having been recrystallized to a pseudo-spherulitic quartz–albite mixture which was subsequently sericitized and chloritized. The REE were moderately mobile (10–40%) whereas the high field strength elements were less mobile (<10%). The major elements within the flow interior are shown to have been relatively immobile and represent a reasonable estimate of the protolith composition. The viscosity of the aphyric lava protolith was estimated to be 7.8 × 10 6 Pa s based on an eruption temperature of ∼850 °C calculated from measurements using the Ti-in-zircon saturation thermometer. This viscosity estimate is similar to the calculated viscosity (∼4 × 10 7 Pa s) of the subaerial Námshraun rhyolite flow, Iceland. The Prosser and Námshraun rhyolites have comparable dimensions and eruption temperature. It is concluded that the low aspect ratio of the Prosser Township flows may have been controlled by a number of environmental factors including paleo-slope, magma rheology, eruption rate and the insulative effects of the flow breccia.

Pb ISOTOPE COMPOSITIONS OF PYRITE FROM THE C QUARTZ-TOURMALINE VEIN OF THE SISCOE GOLD DEPOSIT, VAL D'OR, QUEBEC: CONSTRAINTS ON THE ORIGIN AND AGE OF THE GOLD MINERALIZATION

The Siscoe mine was one of the richest in the Val d’Or mining camp, producing 27.5 tonnes (t) of gold and 9.5 t of silver from 1929 to 1949. The C quartz-tourmaline vein at the Siscoe mine, which is the youngest auriferous vein on the property, is an excellent example of a high-grade Archean lode gold deposit; it contains an average of 45 g/t Au and locally up to 221 g/t Au. This vein cuts all dikes, the main regional foliation (S2), and several auriferous quartz-carbonate veins, and is interpreted to be a shear structure that formed during the development of a late- to post-D2 reverse fault. The vein comprises mainly alternating tourmaline- and quartz-rich layers, and gold occurs commonly in late fractures cutting the vein. The visible wall-rock alteration is characterized by the occurrence of tourmaline, pyrite, and calcite. In this study, we report the Pb isotope compositions of hydrothermal pyrite separates taken from the C quartz-tourmaline vein and its wall rock. Pyrite is abundant in the wall rock, where it occurs as grains with inclusions of chalcopyrite, gold, tetradymite, pyrrhotite, rutile, silicates, and carbonates. In the vein, it forms euhedra generally devoid of inclusions. Locally, the euhedra are fractured, and the fractures are filled with native gold, calcite, and tetradymite. Textural relationships suggest that pyrite precipitated synchronously with and/or after gold in the wall rock, but predated gold in the vein. Pyrite separates from the vein and its wall rock yielded similar 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios (13.63–13.83, 14.58–14.62, and 33.42–33.76, respectively), which are more radiogenic than those reported for volcanic supracrustal and syntectonic plutonic rocks of the southern Abitibi subprovince, but are very similar to those of the late- to posttectonic S-type granites in the Pontiac subprovince and Lacorne block. This suggests that the Pb originated from a reservoir similar to that of the magmas which produced these granites, or alternatively, that it was leached from the latter. The Pb ratios yield a model age of 2.57 ± 0.07 Ga, which is interpreted to date the cooling of the hydrothermal system that introduced and/or remobilized gold into the C quartz-tourmaline vein. These results, when considered in the context of the evolution of the southern Abitibi, indicate that widespread hydrothermal circulation occurred between 2.64 and 2.55 Ga, after the collision of the Pontiac and Abitibi subprovinces, and was caused by dewatering and S-type magmatism in the thickened unstable continental crust. This long-lived hydrothermal system promoted remobilization of gold from earlier veins and possibly introduction of gold from deeper zones, enhancing gold resources in the Siscoe deposit.

U–Pb geochronology of Archean metasedimentary rocks in the Pontiac and Abitibi subprovinces, Quebec, constraints on timing, provenance and regional tectonics

U–Pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the Pontiac subprovince and from the Cadillac, Kewagama and Lac Caste Groups within the southern Abitibi subprovince. Youngest detrital zircon ages from these sequences are 2685, 2686, 2687, and 2693 Ma, respectively. The Lac Fournière pluton, which intrudes Pontiac Group metasedimentary rocks, is dated at 2682±1 Ma. A similar age was previously measured from a pluton that cuts the Kewagama Group. These data bracket deposition of greywacke in the Pontiac and Abitibi subprovinces to 2685±3 Ma. In contrast, the youngest zircon found in a volcaniclastic unit interbedded with Timiskaming Group metawacke is 2673±2 Ma, while the age of a pluton that intrudes the Timiskaming is 2672±1 Ma. Thus, the Timiskaming Group is distinctly younger than the other metasedimentary rocks. The normalized probability distributions of detrital zircon ages from the Pontiac and the Abitibi metasedimentary rocks, as well as of dated igneous rocks from the southern Abitibi subprovince, all show that their major zircon populations formed in the age range 2685–2710 Ma. The distributions for the Abitibi metaturbidites and dated igneous rocks are very similar, suggesting that metasedimentary rocks in the Abitibi belt were mostly derived from the surrounding granite–greenstone terrain. The largest peak of the Pontiac distribution is a few million years younger than that from the Abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older, pre-Abitibi components (2750–2850 and 3020–3030 Ma). Deposition of the ca. 2685 Ma sediments was virtually syn-deformational. Based on previous field, geochemical and isotopic studies, as well as the detrital zircon age data, the Pontiac Group may represent a foreland basin that may have developed from an accretionary prism. The source of much of the sediment may have been an uplifted extinct continental arc to the north. The arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin, now partly preserved in the southern volcanic zone of the Abitibi subprovince. Metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones. These were brought up by high angle faults, which also localized deposition of non-marine (Timiskaming-type) sediments. It is tentatively suggested that Pontiac-type metasedimentary rocks extend most of the way beneath the Abitibi subprovince, the oldest part of the basin emerging as the Quetico Group in the western Superior province.

Lode Gold Deposits and Archean Mantle Plume-Island Arc Interaction, Abitibi Subprovince, Canada.

In combination with seismic interpretations and geochronological constraints, the association of juvenile arc-type low-Ti tholeiitic basalts with komatiites in the southeastern Abitibi subprovince, Canada, supports a history of subduction step back following Late Archean mantle plume-island arc interaction. The resulting paired collision zones preserved abundant komatiites and numerous massive sulphide deposits and established the critical metallogenic features to concentrate the majority of Canada's Precambrian gold resources in a small area of the southern Abitibi subprovince.

Age and isotopic composition of late Archean leucogranites: implications for continental collision in the western Superior Province

U-Pb and Sm-Nd isotopic data are presented for leucogranites and pegmatites from the boundary zones of the English River, Winnipeg River, and Wabigoon subprovinces in the late Archean Superior Province of northwestern Ontario. The Ghost Lake batholith and the Separation Rapids pluton are posttectonic, rare-metal-bearing, S-type leucogranites that were generated during the final stages of the amalgamation of the Superior Province. U-Pb dating of monazites yields ages of 2650 ± 3 Ma for a pegmatite from the Dryden area and 2646 ± 2 Ma for the Separation Rapids pluton. Sm-Nd data from these granitoids are compared with similar data from late Archean intrusions and adjacent rocks from the same regions. Values of εNd range from 0 to +2 for pretectonic tonalites, from -2 to +2 for both the Ghost Lake batholith and the Separation Rapids pluton, from +1 to +3.5 for metavolcanic rocks, and from -0.5 to -1.5 for metasedimentary rocks. There is an overall trend of decreasing εNd values from pretectonic tonalites to the latest leucocratic pegmatites. This reflects the origin of more and more granitoids as a result of anatexis as the crust grew and thickened through accretion. The ranges of εNd values found among leucogranites from the two regions overlap with the isotopic values of the basalts and sediments. This suggests that the leucogranites were generated by similar processes involving both a crustal and a juvenile mantle component. Furthermore, the ages from this study and from pegmatites (2652-2643 Ma) in the Bird River greenstone belt provide the best estimate of the age of rare metal mineralization in the western Superior Province. Similar ages (2651-2639 Ma) for rare-metal-bearing leucogranites in southern Abitibi subprovince suggest a specific period of emplacement over a wide area.

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.

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.

Plutonism in the southern Abitibi Subprovince; a tectonic and petrogenetic framework

Studies of plutonic rocks in Archean terranes such as the southern Abitibi subprovince have been an important component in the development of an accretionary tectonic model for the Superior province. In the southern Abitibi subprovince, pretectonic ( approximately 2.75-2.70 Ga), low K, tholeiitic calc-alkaline, dioritic to tonalitic plutons (e.g., Bourlamaque batholith; Flavrian pluton) are associated with volcanism and are interpreted to be derived from both partial melting of hydrated mantle during oceanic arc formation and fractionation of tholeiitic to calc-alkaline basalt-andesite. Pretectonic, synvolcanic tholeiitic intrusions (e.g., Dundonald sill, Kamiskotia gabbroic complex: 2.72-2.70 Ga) show in situ fractionation trends, and locally, have mixed magma textures with contemporaneous calc-alkaline granitoids. They are believed to be derived from a midocean ridge basalt (MORB)-like depleted mantle, with little evidence for contamination by an older crustal component. Pre- to syntectonic ( approximately 2.72-2.69 Ga) porphyry and tonalite plutons (e.g., Timmins porphyries, Round Lake batholith) have geochemical characteristics typical of Archean tonalite-trondhjemite suites considered to be derived from melting of basaltic rocks at high metamorphic grades. Late to post-tectonic (<2.69 Ga) diorite-monzonite and syenite plutons (e.g., Watabeag batholith, Lamaque stocks, Otto stock) have large ion lithophile element-enriched calc-alkaline to sho-shonitic compositions. These intrusions and associated Timiskaming-type volcanic rocks are attributed to melting of large ion lithophile element-enriched mantle in the late stages of subduction. Late to post-tectonic biotite-muscovite granite intrusions contain a significant contribution of evolved crust (e.g., S-type characteristics of the Preissac-Lacorne batholith).In the southern Abitibi subprovince the tectonic setting of mantle- and crust-derived intrusions is broadly comparable to magmatic processes in Phanerozoic arcs and accreted terranes. The plutonic rocks record a transition from pre- to synaccretionary subduction-related plutonic suites to late to postaccretionary suites derived from enriched mantle and crustal sources. This framework provides a basis for understanding mineral deposits associated with intrusive rocks of the southern Abitibi subprovince which range from Ni-Cu sulfides in layered synvolcanic tholeiitic intrusions (e.g., Kamiskotia gabbroic complex, Ontario) to volcanogenic sulfides in early tholeiitic and calc-alkaline sequences (e.g., Flavrian pluton, Noranda district, Quebec) to the spatial association of gold with late large ion lithophile element-enriched intrusions (e.g., Lamaque plugs, Val d'Or district, Quebec).

U [sbnd]Pb zircon and rutile chronology of Archean greenstone formation and gold mineralization in the Val d'Or region, Quebec

U Pb geochronology provides an absolute time framework for the evolution of the Sigma gold deposit and surrounding rocks at Val d'Or, southern Abitibi subprovince. The Bourlamaque batholith, the largest pluton in the area, gives a 2699.8 ± 1.0 Ma U Pb zircon age. This pluton cuts the Val d'Or Formation which hosts the mineralization. A U Pb zircon age of 2704.9 ± 1.1 Ma on a felsic volcanic rock, the Colombière “rhyolite”, 13 km east of the mine dates that formation. The gold-bearing quartz vein system at Sigma is hosted by andesites and two generations of porphyry intrusions, all metamorphosed to the greenschist facies. The oldest porphyry (“porphyritic diorite”) shows the same deformation as the volcanic rocks, and has a 2703.7 ± 2.5 Ma U Pb zircon age. The porphyritic diorite and volcanic rocks are cut by feldspar-porphyry dykes which post-date regional folding and have a 2694.0 ± 2.2 Ma U Pb zircon age. Regional greenschist metamorphism has been dated directly, with a U Pb date of 2684 ± 7 Ma on rutile in the Colombière “rhyolite”. The mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals. Hydrothermal rutile, from an alteration halo around the veins in andesite, has a 2599 ± 9 Ma U Pb age. Textural evidence clearly indicates that the wall-rock alteration and vein filling are contemporaneous, and hence the vein system and gold mineralization appear to have developed at least 80 m.y. after the formation and metamorphism of host greenstones.

Nd- and Sr-isotope systematics for the Kamiskotia–Montcalm area: implications for the formation of late Archean crust in the western Abitibi Subprovince, Canada

Geochemistry and Nd isotopic compositions are used to characterize mantle and crustal sources and to provide constraints on petrogenetic models for tholeiitic, calc-alkalic, and lamprophyric suites in the Kamiskotia–Montcalm area. The Kamiskotia gabbroic complex (KGC) and cogenetic, bimodal volcanic rocks have εNd(t) = +2.2 to +2.6, consistent with a direct derivation from a long-term, light rare-earth element (LREE)-depleted mantle. The Montcalm gabbroic complex has decreasing εNd(t) upsection from +2.8 to +1.0, consistent with contamination by long-term, LREE-enriched (with respect to the long-term, LREE-depleted Abitibi mantle) crust during fractionation. Two calc-alkalic lamprophyre samples, characterized by large-ion lithophile element (LILE) and LREE enrichment and high MgO, Ni, and Cr contents, have εNd(t) of +2.5 and +2.8, indicating a derivation from a depleted mantle source that had undergone recent trace-element enrichment. A different lamprophyre suite is extremely LILE and LREE enriched and has an εNd(t) of +1.0, indicating a derivation from a slightly different source that had earlier LREE enrichment. Granitoid rocks internal and external to greenstone belt rocks have εNd(t) = +2.5 to +3.8 and +0.6 to −0.4, respectively. The lower values provide additional evidence for the existence of LREE-enriched crust in this area.Considering these data along with other radiogenic isotope studies, a petrogenetic and tectonic model is suggested for the crustal development of the southern Abitibi Subprovince. From &gt;2740 to 2698 Ma—the major period of volcanic activity—komatiitic and tholeiitic suites and one lamprophyre suite were derived from a uniformly LREE-depleted mantle reservoir with εNd(t) = +2 to +3. Calc-alkalic granitoids were emplaced generally after 2700 Ma. Their long-term, LREE-depleted and LREE-enriched Nd isotopic signatures are similar to signatures in continental-arc settings (e.g., the Coastal Batholith of Peru). Form 2690 to 2670 Ma, when transpressional tectonism prevailed, mantle-derived magmatism was represented by long-term, LREE-enriched (εNd(t) = +1 to +2) lamprophyric and alkalic volcanic suites.The Kamiskotia suite has a seven-point, whole-rock – mineral isochron Sm–Nd age of 2710 ± 30 Ma, identical to U–Pb zircon ages for the suite, indicating closed-system behavior. An Rb–Sr mineral – whole-rock isochron age from one KGC sample is 2450 ± 30 Ma, identical to U–Pb ages for the Hearst–Matachewan dike swarm, a prominent feature in the KGC area. Regression of whole-rock and mineral-isotope data for one granitoid sample with a U–Pb zircon age of 2696 ± 1.5 Ma gives identical ages of 2530 ± 30 Ma in the Sm–Nd and Rb–Sr systems. The latter data add to an increasing body of evidence for cryptic, late thermal events after granitoid–greenstone belt development in the southern Abitibi Subprovince.

Timing of magmatism and deformation in the Kamiskotia-Kidd Creek area, Western Abitibi subprovince, Canada

The Kamiskotia-Kidd Creek area comprises well-preserved granitoid-greenstone belt lithologies of the western-most Abitibi Subprovince, similar in composition and structural style to other Late Archean granitoid-greenstone terranes (e.g., Kalgoorlie-Norseman, Western Australia; Belingwe, Zimbabwe; Slave Province, Canada). It contains the Kidd Creek volcanogenic massive sulfide deposit and lesser deposits of the Kamiskotia district, and lode gold deposits associated with the Destor-Porcupine Fault Zone near the prolific Porcupine Gold District. The magmatic and tectonic history for the area is provided through detailed structural mapping and high-precision U-Pb geochronology. Field observations support the following sequence of magmatic and structural events in the Kamiskotia area: (1) formation of older mafic and intermediate volcanic rocks, followed by the emplacement of the Kamiskotia gabbroic complex (KGC) and cogenetic, massive sulfide-bearing bimodal volcanic rocks (KV); also accompanied by coeval intrusion of granitoid A into the crystallizing KGC; (2) regional, bulk rotation about a sub-horizontal axis of this stratigraphy to form a NE-facing monocline; (3) emplacement of three large granitoid intrusions, imparting their respective contact strain aureoles on the stratigraphy; (4) regional sub-horizontal, bulk north-south shortening that formed a non-penetrative east-west flattening fabric; with an element of (5) late dextral transpression, that formed high strain zones related to western Destor-Porcupine Fault Zone (DPFZ) within less competent volcanic units; and (6) emplacement of alkalic dikes associated with the late stages of transpressive deformation. U-Pb ages help substantiate this chronology and aid in the correlation of Kamiskotia lithologies across the southern Abitibi Subprovince. Zircon ages for the KGC and a Kamiskotia rhyolite are 2707 ± 2 Ma and 2705 ± 2 Ma, respectively. These ages are coeval with the Reid rhyolite, with a zircon age of 2706±2 Ma, but are significantly younger than the Kidd Creek rhyolite and the Prosser rhyolite, with zircon ages of 2717±2 Ma (data refined by the addition of an abraded, concordant analysis) and 2716+6/−3 Ma, respectively. Two of the granitoid intrusions have zircon ages of 2696± 1.5 and 2694±4 Ma. The older granitoid intrusion has a titanite population with a 207 Pb/ 206 Pb minimum age of 2692±5 Ma, which may represent a protracted cooling history or a metamorphic event after the setting of the zircon U-Pb system. Two zircon fractions from the younger granitoid intrusion exhibit inheritance, with 207 Pb/ 206 Pb minimum ages up to 2926 Ma, indicating the presence of significantly older crust in this region. Timing of north-south compression and late transpression may be constrained by a U-Pb garnet-titanite age on a late- to post-tectonic alkalic dike, at 2687±3 Ma.

2704. Trace metals in young blood: Delves, H. T., Clayton, Barbara E. & Bicknell, Joan (1973). Concentration of trace metals in the blood of children. Br. J. prev. soc. Med.27, 100

UPb geochronology provides an absolute time framework for the evolution of the Sigma gold deposit and surrounding rocks at Val d'Or, southern Abitibi subprovince. The Bourlamaque batholith, the largest pluton in the area, gives a 2699.8 ± 1.0 Ma UPb zircon age. This pluton cuts the Val d'Or Formation which hosts the mineralization. A UPb zircon age of 2704.9 ± 1.1 Ma on a felsic volcanic rock, the Colombière “rhyolite”, 13 km east of the mine dates that formation. The gold-bearing quartz vein system at Sigma is hosted by andesites and two generations of porphyry intrusions, all metamorphosed to the greenschist facies. The oldest porphyry (“porphyritic diorite”) shows the same deformation as the volcanic rocks, and has a 2703.7 ± 2.5 Ma UPb zircon age. The porphyritic diorite and volcanic rocks are cut by feldspar-porphyry dykes which post-date regional folding and have a 2694.0 ± 2.2 Ma UPb zircon age.Regional greenschist metamorphism has been dated directly, with a UPb date of 2684 ± 7 Ma on rutile in the Colombière “rhyolite”. The mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals. Hydrothermal rutile, from an alteration halo around the veins in andesite, has a 2599 ± 9 Ma UPb age. Textural evidence clearly indicates that the wall-rock alteration and vein filling are contemporaneous, and hence the vein system and gold mineralization appear to have developed at least 80 m.y. after the formation and metamorphism of host greenstones.