Huronian Supergroup Research Articles

New Feldspar Lead Isotope and Trace Element Evidence from the Sudbury Igneous Complex Indicate a Complex Origin of Associated Ni-Cu-PGE Mineralization Involving Underlying Country Rocks

The Sudbury Igneous Complex is the preserved remnant of a 1.85 Ga impact-induced crustal melt sheet that eventually crystallized into a coherent magmatic stratigraphy. At its base, the complex hosts world-class Ni-Cu- platinum group element (PGE) sulfide mineralization, distributed around the basin. Previous studies have shown strong Pb and Sr isotope contrasts between the North and South ranges of the Sudbury Igneous Complex. This study presents an extensive dataset of Pb isotopes and trace elements of hand-picked feldspars and sulfides from the complex, Ni-Cu-PGE mineralization, and underlying country rocks that reveals the finer details of isotopic decoupling between the North and South ranges. In terms of mean Pb isotope compositions, the new data confirm the previously established picture that North Range Main Mass rocks and associated Ni-Cu-PGE sulfide deposits are more juvenile than South Range Main Mass and associated mineralization, but there are several findings that point to a more complex evolution, with considerable implications for the source of sulfide in the Sudbury Igneous Complex. First, North Range sulfide ores and Main Mass silicates are more radiogenic than local footwall rocks. When expressed as per mil deviation in Pb-207/Pb-204 from a model contemporaneous mantle isochron (Delta Pb-207/Pb-204), the North Range Main Mass ores range from + 130 to + 421 and North Range ores range from + 160 to + 518 (primarily + 160 to + 260). This indicates that the Huronian Supergroup cover constituted a significant target rock type also in the North Range of the Sudbury Igneous Complex. Second, there is significant variability in the Pb isotope compositions between Ni-Cu-PGE sulfide deposits in both the North and South ranges and systematic differences between the average silicate compositions and sulfide compositions of Ni-Cu-PGE deposits. In the South Range Main Mass rocks, the Delta Pb-207/Pb-204 values range from + 360 to + 494, and associated ores range from + 150 to + 481 (primarily + 400 to + 480). Third, North Range mafic norite is much less radiogenic than North Range Main Mass, indicating that it may be contaminated by or derived from less radiogenic Ni-Cu-PGE-bearing mafic-ultramafic rocks. Finally, the Pb isotope variations between Sudbury Igneous Complex ores are corroborated by similar systematic variations in rare earth element patterns likely inherited by assimilation of local footwall rocks. Importantly, the considerable variations in the Pb isotope compositions of the ores between different deposits and the intermediate compositions between overlying Main Mass and footwall rocks suggest that the sulfides originally equilibrated with Main Mass, but acquired significant amounts of Pb (and therefore likely S and other metals) from local footwall rocks, most likely during thermomechanical erosion of the footwall rocks by dense molten sulfides. Potential S and metal sources are the Nipissing gabbros and the East Bull Lake intrusions, both of which are mineralized and whose Pb isotope compositions help to explain the variability in Sudbury Igneous Complex Ni-Cu-PGE sulfide ores. Small-scale lateral variations in Pb isotopes previously identified in the South Range of the Sudbury Igneous Complex are also present in the North Range. This indicates that the laterally extensive (200-km wide x 2.5-km thick) superheated Sudbury Igneous Complex melt sheet developed multiple chemically and isotopically isolated convection cells that continued to assimilate isotopically distinctive footwall rocks after impact. Lateral variations are much greater at lower levels in the magmatic stratigraphy than in the upper part of the stratigraphy. This resulted from a combination of three factors: (1) the overall depletion in Pb of the proto-Sudbury Igneous Complex (likely due to volatile element loss upon impact) made it more susceptible to contamination in this element than in more refractory elements; (2) assimilation of heterogeneous footwall into the base vs. incorporation of more homogeneous fallback breccias into the roof of the Sudbury Igneous Complex; and (3) possible hydrothermal roof alteration. Lead isotopes and trace element data for samples of the footwall collected and analyzed as part of this study used in tandem with data from previous work confirm that the Sudbury Igneous Complex can be reproduced by mixing the currently exposed target lithologies and indicate that no mantle or lower crustal component is required.

Climatic cycles recorded in glacially influenced rhythmites of the Gowganda Formation, Huronian Supergroup

The Gowganda Formation of the 2.45–2.2Ga Huronian Supergroup contains glacially-induced, varve-like rhythmites that potentially preserve a detailed record of climatic conditions during the Paleoproterozoic Era. Four rhythmic couplet thickness records were measured at two outcrops near Wharncliffe, Ontario for the purpose of time-series analysis. The couplets, which range from 1 to 32mm thick, are composed of alternating layers of siltstone and claystone. Time-series analysis of the couplet thickness records using the MTM Toolkit of Mann and Lees (1996) consistently revealed periodicities in the range of 2.2–2.9 couplets per cycle, which is consistent with climatic cycles such as the quasi-biennial oscillation (QBO) and the El Niño Southern Oscillation (ENSO) observed in modern times. This periodicity suggests that the rhythmic couplets represent annual deposits (i.e. varves). Evidence for the presence of cycles at 3.0–4.9 couplets, 6.6–6.9 couplets, 8.8–9.2 couplets, 22.8 couplets, and 30.1–31.0 couplets were also observed in some couplet thickness records; however, the presence of these longer term cycles was inconsistent from site to site.

Microbially induced sedimentary structures in the Paleoproterozoic, upper Huronian Supergroup, Canada

The Paleoproterozoic Gordon Lake and Bar River formations, Huronian Supergroup, contain a variety of sedimentary structures in the Flack Lake area of Ontario, Canada, that have been considered of debatable origin. We identify these structures as microbially induced sedimentary structures (MISS). The preserved MISS are related to microbial mat destruction and decay, and include sand cracks, mat chips, remnant gas domes, pyrite patches, and iron laminae. A biological origin for the fossil structures is supported by their similarities to modern and ancient documented examples of MISS, the sand-dominated nature of the substrate in which they are preserved, and key microtextures identified in thin section. Microtextures include curled, frayed and layered mat chips, carbonaceous laminae, oriented grains, and concentrated heavy minerals. On outcrop scale, the presence of desiccation cracks, flaser and lenticular bedding, and ripples in association with the types of MISS identified in the Gordon Lake Formation support the interpretation of a tidal flat depositional environment. The Gordon Lake Formation contains a greater number and diversity of MISS than the overlying Bar River Formation, as a result of lower energy deposition in the former. The quartz arenite of the Bar River Formation contains fine-grained to pebbly granulestone characterized mainly by tangential and planar cross beds, which is consistent with a tidal channel or sand shoal setting. Although fossil evidence of life is rare in the rocks of the Huronian Supergroup, identification of MISS in the Flack Lake area provides a significant and convincing indication of microbial colonization at the time of deposition.

A 2169 Ma U–Pb baddeleyite age for the Otish Gabbro, Quebec: implications for correlation of Proterozoic magmatic events and sedimentary sequences in the eastern Superior Province

Otish Gabbro sills intrude sedimentary rocks in the Otish Basin of the southeastern Superior Province. Here, deposition of Otish Supergroup sediments had previously been thought to be older than K–Ar and Sm–Nd ages of ca. 1750–1710 Ma for Otish Gabbro sills, and younger than ca. 2515–2500 Ma U–Pb ages of underlying Mistassini dykes. However, a much older U–Pb baddeleyite age of 2169.0 ± 1.4 Ma is presented here for an Otish sill, indicating that they are coeval with, and likely genetically related to, the giant 2172–2167 Ma Biscotasing dyke swarm to the southwest and (or) the Cramolet sills and Payne River dykes to the north. The new date also indicates that the age of the Otish Supergroup falls between ca. 2515 Ma and ca. 2169 Ma, only a little different from the ca. 2450–2217 Ma bracket for the Huronian Supergroup of the Southern Province, and is consistent with both supergroups spanning the oxy-atmo inversion. The Otish Supergroup could also be coeval with the Sakami Formation to the north, but is likely older than the Richmond Gulf Group on the east coast of Hudson Bay. Early paleomagnetic study of Otish sills yielded a remanence ∼20° from that expected for Biscotasing-aged intrusions. This may indicate that too few distinct sills were studied to average out paleosecular variation, that demagnetization techniques failed to fully remove unstable magnetization components, or that the remanence is a stable secondary overprint, perhaps acquired during a fluid event related to uranium mineralization at ca. 1720 Ma.

Chapter 11 Palaeoproterozoic supracrustals of the Bastar Craton: Dongargarh Supergroup and Sausar Group

Abstract The Bastar Craton of India is composed of Archaean nuclei of tonalite–trondhjemite–granodiorite gneisses, enveloped by an older granite–greenstone belt (>3000 Ma) with banded iron formation (BIF), and an auriferous younger granite–greenstone belt with BIF. Available geological, geochemical and geochronological data indicate multiple episodes of orogeny with high-grade metamorphism at 3200–3300, 2600–2700, 2100–2200, 1900–2000, 1800–1850, 1500–1600 and 1400–1450 Ma, and continental rifting and basin development marked by emplacement of mafic dyke swarms at c. 2900 (subalkaline mafic dykes; BD-1A), 2480 (high-Mg mafic dykes; BD-1B), 2100 (Fe–tholeiite dykes; BD-2A), 1880 (Fe–tholeiites dykes; BD-2B), 1776 and 1422 Ma. Associations of extensive bimodal volcanics and riftogenic sediments are found in the Neoarchaean and Palaeoproterozoic basins of the craton. Evidence of Palaeoproterozoic (Huronian) glaciation and associated ‘cap carbonate’ followed by deposition of fine clastics with manganese ore is found in the Palaeoproterozoic Sausar Group. The lithological association of the Sausar Group is comparable to the carbonate–tillite association of the Huronian Supergroup, Snowy Pass Supergroup, Transvaal Supergroup and Turee Creek Group. The geological evolution of the Bastar Craton matches that of Western Australia and South Africa. Such similarities can be analysed to develop a unified Palaeoproterozoic assembly for these provinces.

Carbon and oxygen isotope systematics of a Paleoproterozoic cap-carbonate sequence from the Sausar Group, Central India

Well-preserved sedimentological features of the Paleoproterozoic age Sausar Group of central India indicate glaciogenic origin for the diamictite in the lower part of the unit. Deglaciation was responsible for a marine transgression and the deposition of proximal glaciomarine facies and a cap-carbonate, followed by fine clastics with manganese ore. Analyses of δ13C and δ18O contents in the carbonate unit overlying the diamictite, together with Sr and Ba contents and REE data, indicate the preservation of primary geochemical signatures. The average δ13Ccarb content ranges between −3.1‰ and +0.1‰ V-PDB, having peak negative δ13C excursions of up to −7.4‰ V-PDB, similar to Paleoproterozoic and Neoproterozoic cap-carbonates elsewhere. High Sr and Ba contents indicate the presence of aragonite and barite precursors, similar to well-studied Neoproterozoic cap-carbonates. A single continuous carbonate unit shows δ13C excursions from −4.4 to +2.6‰ V-PDB. The lithological association and chemostratigraphy are comparable with the carbonate–tillite association of the Huronian Supergroup of Canada, the Snowy Pass Supergroup of the USA, the Transvaal Supergroup of South Africa, and the Turee Creek Group of Australia.

New structural, metamorphic, and U–Pb geochronological constraints on the Blezardian Orogeny and Yavapai Orogeny in the Southern Province, Sudbury, Canada

New structural and geochronological data are presented for two orogenic events, the Blezardian and Yavapai orogenies, which affected the Paleoproterozoic Southern Province near Sudbury, Ontario, Canada. The Southern Province comprises ca. 2452 Ma metavolcanic rocks and metasedimentary rocks of the Huronian Supergroup, which were deposited along the southern margin of the Archean Superior craton during its evolution from a rifted to passive continental margin. Emplacement of the ca. 2415 Ma Creighton pluton during rifting was followed by its deformation and the development of a penetrative gneissic fabric during the ca. 2415 − ca. 2219 Ma Blezardian Orogeny. New laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) U–Pb zircon ages of 2343 ± 17 and 2344 ± 47 Ma on two granitic dikes that cut this fabric provide a new minimum age of ca. 2.34 Ga for the Blezardian Orogeny. The Sudbury area was then impacted by a large extraterrestrial bolide at ca. 1.85 Ga and deformed during the Penokean Orogeny. The southern part of the Southern Province was later reworked by regional folding and north-directed thrusting during the younger 1.7 Ga Yavapai Orogeny. The 1744 ± 29 Ma Eden Lake Complex was emplaced and deformed during this event, which produced a strong foliation overprinting the complex. The foliation formed at pressures of 2.8–4 kbar (1 kbar = 100 MPa) and temperatures of 540–565 °C and was intruded by a weakly deformed 1704 ± 13 Ma old granitic dike, bracketing the Yavapai event between 1744 ± 29 and 1704 ± 13 Ma in the Sudbury segment of the Southern Province. Crustal thickening associated with the Yavapai event resulted, locally, in minor pressure increases before or during regional metamorphism as revealed by phase equilibria modeling in the Raft Lake area; this evolution may be recorded elsewhere in the Ontario segment of the Southern Province.

Did prolonged two-stage fragmentation of the supercontinent Kenorland lead to arrested orogenesis on the southern margin of the Superior province?

Recent geochronological investigations reinforce the early suggestion that the upper part of the Paleoproterozoic Huronian Supergroup of Ontario, Canada is present in the Animikie Basin on the south shore of Lake Superior. These rocks, beginning with the glaciogenic Gowganda Formation, are interpreted as passive margin deposits. The absence of the lower Huronian (rift succession) from the Animikie Basin may be explained by attributing the oldest Paleoroterozoic rocks in the Animikie Basin (Chocolay Group) to deposition on the upper plate of a north-dipping detachment fault, which lacks sediments of the rift phase. Following thermal uplift that led to opening of the Huronian Ocean on the south side of what is now the Superior province, renewed uplift (plume activity) caused large-scale gravitational folding of the Huronian Supergroup accompanied by intrusion of the Nipissing diabase suite and Senneterre dikes at about 2.2 Ga. Termination of passive margin sedimentation is normally followed by ocean closure but in the Huronian and Animikie basins there was a long hiatus -- the Great Stratigraphic Gap -- which lasted for about 350 Ma. This hiatus is attributed to a second prolonged thermal uplift of part of Kenorland that culminated in complete dismemberment of the supercontinent shortly before 2.0 Ga by opening of the Circum-Superior Ocean. These events caused regional uplift (the Great Stratigraphic Gap) and delayed completion of the Huronian Wilson Cycle until a regional compressional tectonic episode, including the Penokean orogeny, belatedly flooded the southern margin of the Superior province with foreland basin deposits, established the limits of the Superior structural province and played an important role in constructing Laurentia.

Contradictory correlations of Paleoproterozoic glacial deposits: Local, regional or global controls?

There is little agreement among recently proposed correlation schemes for Paleoproterozoic glaciogenic rocks scattered thinly around the globe. Correlations are hindered by the dearth of tight geochronological control on the ages of glacial deposits, which are notoriously difficult to date. Most attempts at global correlation are based on comparison to the Huronian Supergroup in Canada, which contains three glaciogenic formations. Although the two lower glacial units were deposited in a rift setting and have a restricted distribution even in North America, they have been used in attempts to effect international correlations. The Gowganda Formation, together with correlatives, comprises the thickest and by far the most widespread Paleoproterozoic glacial deposits in North America. Glacial deposits are also reported from South Africa, Western Australia and elsewhere, where they appear to fall within the same time period as the Huronian Supergroup (2.45–2.2Ga) but attempts to correlate individual diamictite-bearing units to the three Huronian glaciogenic formations have proved difficult. None of the Huronian glacial formations has been precisely dated. Until such geochronological data are available it is practical and prudent to recognize that, during the early Paleoproterozoic, as in the Cryogenian, the Earth was susceptible to glaciations that could be triggered by a variety of local events including uplift, related to rifting and compressional orogeny. Within the 250 million years of the ‘Huronian Glacial Event’ (2.45–2.2Ga) it is commonly assumed that ice sheet fluctuations were globally synchronous but this has not been demonstrated.

Dating Palaeoproterozoic glacial deposits of the Fennoscandian Shield using detrital zircons from the Kola Peninsula, Russia

New age constraints from detrital zircon U-Pb geochronology from clastic sediments and diamict deposits obtained maximum depositional ages for the Seidorechka and Polisarka Sedimentary formations of the Imandra-Varzuga Greenstone Belt of the Kola Province, NW-Russia. These delimit the deposition of Palaeoproterozoic Huronian-age glacial deposits of the Fennoscandian Shield from c. 2430Ma to <2411Ma. The time constraint allows comparison to the first two of three possible glacial intervals recognised in the Huronian Supergroup, Canada, and South African diamict deposits. Intercontinental comparisons support these constraints, the assumption of a glacial origin of the diamictites and, further, the eventually global extent of the event. Similar age distributions and depositional ages of c. 2400Ma within the Neverskrukk Formation of the Pechenga Greenstone Belt support a correlation with the Imandra-Varzuga Greenstone Belt succession, which was traditionally based on lithostratigraphic observations.The detrital zircon age spectra of eight analysed samples show an Archaean dominance and reoccurring peaks around 2800–2750Ma,which indicate similar origins for the zircons. Such peaks are corroborating with a major global crustal generation event being recorded in detrital zircon spectra worldwide. Additionally, certain age populations around 2440Ma and 2500Ma are present and indicate a correlation with intrusive magmatism that affected the region at that time and support a correlation based on overlapping age distributions.

Geochemistry of Deformed and Hydrothermally Mobilized Magmatic Ni-Cu-PGE Ores at the Garson Mine, Sudbury

Research Article| March 01, 2014 Geochemistry of Deformed and Hydrothermally Mobilized Magmatic Ni-Cu-PGE Ores at the Garson Mine, Sudbury* Joshua Mukwakwami; Joshua Mukwakwami † Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, 933 Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada †Corresponding author: e-mail, jx_mukwakwami@laurentian.ca Search for other works by this author on: GSW Google Scholar C. Michael Lesher; C. Michael Lesher Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, 933 Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada Search for other works by this author on: GSW Google Scholar Bruno Lafrance Bruno Lafrance Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, 933 Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Joshua Mukwakwami † Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, 933 Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada C. Michael Lesher Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, 933 Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada Bruno Lafrance Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, 933 Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada †Corresponding author: e-mail, jx_mukwakwami@laurentian.ca Publisher: Society of Economic Geologists Received: 17 Sep 2012 Accepted: 23 May 2013 First Online: 09 Mar 2017 Online ISSN: 1554-0774 Print ISSN: 0361-0128 © 2014 Society of Economic Geologists. Economic Geology (2014) 109 (2): 367–386. https://doi.org/10.2113/econgeo.109.2.367 Article history Received: 17 Sep 2012 Accepted: 23 May 2013 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Joshua Mukwakwami, C. Michael Lesher, Bruno Lafrance; Geochemistry of Deformed and Hydrothermally Mobilized Magmatic Ni-Cu-PGE Ores at the Garson Mine, Sudbury. Economic Geology 2014;; 109 (2): 367–386. doi: https://doi.org/10.2113/econgeo.109.2.367 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The Garson Ni-Cu-platinum group element (PGE) deposit is a deformed, overturned, contact-type deposit along the contact between the Sudbury Igneous Complex and underlying rocks of the Huronian Supergroup in the South Range of the 1850 Ma Sudbury structure. The main Garson orebodies occur along steeply S dipping ductile shear zones, which formed as D1 north-over-south amphibolite facies thrust zones that were rotated and overturned during buckling of the Sudbury Igneous Complex. The shear zones were reactivated as south-over-north greenschist facies reverse shear zones during D2. The mineralization is primarily composed of pyrrhotite-pentlandite-chalcopyrite-magnetite, occurring as four main types: (1) disseminated to matrix-textured sulfides in Main Mass norite and slivers of norite within shear zones, (2) semimassive breccia ore within the shear zones, (3) quartz-calcite-sulfide veins spatially associated with breccia ores, and (4) Cu-Pd-Pt-Au–rich “footwall-type” mineralization in the nearby Garson Ramp deposit. Garson disseminated sulfides and breccia ores have relatively low Pd/Ir ratios and restricted ranges of Ni tenors and Fe/Ni ratios, very similar to those of undeformed disseminated sulfides and contact-type massive ores at the less deformed Creighton deposit. This suggests that the mobilization of the breccia ores by ductile plastic flow during D1 and D2 did not significantly alter their original magmatic geochemical signatures. Garson breccia ores are slightly depleted in Cu-Pt-Pd-Au and slightly enriched in Rh-Ru-Ir relative to disseminated sulfides, similar to other contact-type deposits at Sudbury. In contrast, quartz-calcite-sulfide veins at Garson are characterized by very low Rh-Ru-Ir abundances and very high Pd/Ir ratios, consistent with their mobilization and deposition from metamorphic-hydrothermal fluids. Despite being strongly enriched in Cu-Pd-Pt-Au, there are too few of these veins to account for the broad depletion in Cu-Pd-Pt-Au from the main mass of breccia ores, suggesting that the breccia ores are monosulfide solid solution cumulates that left a residual sulfide melt represented by the Cu-Pd-Pt-Au–rich ores in the Garson Ramp deposit. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geochemistry of pyrite from diamictites of the Boolgeeda Iron Formation, Western Australia with implications for the GOE and Paleoproterozoic ice ages

Sediments of the ca. 2400Ma Turee Creek Group of Western Australia are coincident with the oxygenation of Earth's surface associated with the ‘Great Oxidation Event’ (GOE). Diamictite within the Boolgeeda Iron Formation from the Boundary Ridge section at the Duck Creek Syncline was previously correlated to the glaciogenic sediments of the Meteorite Bore Member of the Turee Creek Group at the Hardey Syncline (Martin, 1999). The Meteorite Bore Member is considered correlative and time-equivalent with the Paleoproterozoic glacial diamictites of the 2.45–2.22Ga Huronian Supergroup in North America. If the diamictite units at Boundary Ridge correspond to worldwide Paleoproterozoic glaciations, they should also record the disappearance of mass independently fractionated (MIF) sulfur. Triple S-isotope compositions for pyrites from the Boundary Ridge sections were measured in situ by a multi-collector ion microprobe and yielded both mass dependently fractionated (MDF) and MIF sulfur isotope values (Δ33S values from −0.44 to +6.27‰). Trace element heterogeneities are indicated by measurements at multiple spatial scales within rounded pyrites in the Boundary Ridge section, consistent with multiple generations of pyrite that incorporated sulfur processed in an anoxic atmosphere. A compilation of S-isotope data from pyrite in the Boundary Ridge diamictites analyzed in this study and previous work (Williford et al., 2011) defines multiple δ34S vs. δ33S arrays, consistent with an origin of detrital pyrite from the underlying Hamersley and Fortescue groups. Sulfides in a shale unit from the Boundary Ridge section plot along the terrestrial fractionation line but still retain positive MIF-S. This and detrital pyrite suggest low atmospheric oxygen immediately before the initiation of Paleoproterozoic Ice Ages. This data is inconsistent with a correlation to glacially-influenced successions within the North American Huronian Supergroup, in which the MIF-S signal permanently disappears. The diamictites at the Duck Creek Syncline are older than the Meteorite Bore Member because their stratigraphic position is within the Boolgeeda Iron Formation that underlies the Turee Creek Group, which is separated from the Meteorite Bore Member by nearly 1000m of the Kungarra shale at the Hardey Syncline.

Deformation, metamorphism, and mobilization of Ni–Cu–PGE sulfide ores at Garson Mine, Sudbury

The Garson Ni–Cu–platinum group element deposit is a deformed, overturned, low Ni tenor contact-type deposit along the contact between the Sudbury Igneous Complex (SIC) and stratigraphically underlying rocks of the Huronian Supergroup in the South Range of the 1.85-Ga Sudbury structure. The ore bodies are coincident with steeply south-dipping, north-over-south D1 shear zones, which imbricated the SIC, its ore zones, and underlying Huronian rocks during mid-amphibolite facies metamorphism. The shear zones were reactivated as south-over-north, reverse shear zones during D2 at mid-greenschist facies metamorphism. Syn-D2 metamorphic titanite yields an age of 1,849 ± 6 Ma, suggesting that D1 and D2 occurred immediately after crystallization of the SIC during the Penokean Orogeny. The ore bodies plunge steeply to the south parallel to colinear L1 and L2 mineral lineations, indicating that the geometry of the ore bodies are strongly controlled by D1 and D2. Sulfide mineralization consists of breccia ores, with minor disseminated sulfides hosted in norite, and syn-D2 quartz–calcite–sulfide veins. Mobilization by ductile plastic flow was the dominant mechanism of sulfide/metal mobilization during D1 and D2, with additional minor hydrothermal mobilization of Cu, Fe, and Ni by hydrothermal fluids during D2. Metamorphic pentlandite overgrows a S1 ferrotschermakite foliation in D1 deformed ore zones. Pentlandite was exsolved from recrystallized polygonal pyrrhotite grains after cessation of D1, which resulted in randomly distributed large pentlandite grains and randomly oriented pentlandite loops along the grain boundaries of polygonal pyrrhotite within the breccia ore. It also overgrows a S2 chlorite foliation in D2 shear zones. Pyrrhotite recrystallized and was flattened during D2 deformation of breccia ore along narrow shear zones. Exsolution of pentlandite loops along the grain boundaries of these flattened grains produced a pyrrhotite–pentlandite layering that is not observed in D1 deformed ore zones. The overprinting of the two foliations by pentlandite and exsolution of pentlandite along the grain boundaries of flattened pyrrhotite grains suggest that the Garson ores reverted to a metamorphic monosulfide solid solution at temperatures ranging between 550 and 600 °C during D1 and continued to deform as a monosulfide solid solution during D2.

Redox conditions in the atmosphere and shallow-marine environments during the first Huronian deglaciation: Insights from Os isotopes and redox-sensitive elements

The Paleoproterozoic (2.5–2.0 Ga) is one of the most important periods in Earthʼs history, and was characterized by a rise in atmospheric oxygen levels and repeated (at least three) severe glaciations (the Huronian glaciations). In this study, we investigate redox conditions in the atmosphere and in shallow-marine environments immediately after the first Huronian glaciation based on the isotopic composition of Os, and the abundance of redox-sensitive elements (Os, Re, and Mo) in sedimentary rocks from the Huronian Supergroup, Canada. We found no significant authigenic enrichment of Os in the sedimentary rocks deposited during the first Huronian deglaciation. The initial isotopic composition of Os in the sediments was close to that of chondrite at the time of deposition (Os187/188Os=∼0.11). These results suggest that atmospheric O2 levels were insufficient to mobilize radiogenic Os through continental weathering (pO2<10−5–10−3 present atmospheric level (PAL)). In contrast, we found enrichment of Re in the sedimentary rocks, which suggests the occurrence of oxidative weathering of Re under mildly oxidizing conditions (>10−8–10−5 PAL). Despite the Re enrichment, low abundances of Mo imply possible non-sulfidic conditions in shallow-marine environments at the time of deposition. Together with the results of organic carbon and sulfur analyses, we suggest that atmospheric O2 remained at relatively low levels of around 10−8–10−5 PAL after the first Huronian deglaciation, which contrasts with proposed dramatic increases in O2 after the second and third Huronian deglaciations. These results imply that the second and third Huronian glaciations may have been global events, associated with climatic jumps from severe glaciations to super-greenhouse conditions and the subsequent blooming of photosynthetic cyanobacteria in the glacial aftermath.

Age, petrogenesis and tectonic setting of the Thessalon volcanic rocks, Huronian Supergroup, Canada

The ∼2.5Ga Huronian Supergroup volcanic rocks in northern Ontario represent one of the better preserved Paleoproterozoic continental flood basalt sequences on Earth. The youngest age of Huronian Supergroup volcanism is refined to 2452.5±6.2Ma based on new U–Pb zircon analyses of the Copper Cliff rhyolite. Recent mapping and geochemical analyses of Thessalon Formation volcanic rocks define a well-developed chemostratigraphy; a lower series consisting of four geochemically distinct mafic lava sequences and an upper basalt–andesite volcanic series. These two mafic volcanic series are separated by felsic volcanic rocks (rhyolite and dacite). The mafic lavas are low-Ti tholeiites, dominated by basalt and andesite, and are characterized by enriched light rare earth element profiles and high Th contents (up to 12ppm). Most basalt–andesite sequences show some internal geochemical variation reflecting a combination of fractional crystallization and, in some cases, limited crustal contamination but each sequence represents a discrete magma batch derived by different degrees of partial melting in the subcontinental lithospheric mantle. All magma batches display prominent negative Nb–Ta and Ti anomalies on primitive mantle normalized multi-element diagrams, interpreted to reflect melting of a heterogeneous and previously metasomatized garnet lherzolite that acquired some chemical characteristics of a source typical of subduction zone magmatism. At least three distinct mantle source compositions are required to explain the range in the observed lava compositions. The most unusual of these reservoirs has a number of geochemical features akin to upper continental crust and is characterized by high Th/Yb (∼10) and low Zr/Nb (∼11). This reservoir is interpreted to be peridotite modified through metasomatism and entrapment of arc magmas during northward Neoarchean subduction along the southern Superior craton margin.

Back-thrusting and overturning of the southern margin of the 1.85 Ga Sudbury Igneous Complex at the Garson mine, Sudbury, Ontario

The Garson deposit is one of several deformed magmatic Ni–Cu–platinum group element (PGE) deposits in the South Range of the 1.85Ga Sudbury structure. The deposits occur along the southeast limb of the folded Sudbury Igneous Complex (SIC), at the contact between the SIC basal norite and underlying Paleoproterozoic metabasalt and metasedimentary rocks of the Huronian Supergroup. At the Garson deposit inclusion-rich breccia and disseminated Ni–Cu–PGE sulfide ores are hosted by steeply south-dipping shear zones and splays that underwent two major ductile deformation events (D1 and D2). D1 is characterized by a steeply, south-dipping, S1 foliation and a down-dip L1 mineral stretching lineation defined by ferrotschermakite in metabasalt and by magnesiohornblende in norite. Coexisting ferrotschermakite and oligoclase in metabasalt indicate amphibolite facies conditions during D1. The shear zones formed along or near the SIC-Huronian contact during the 1.7–1.6Ga Mazatzal–Labradorian Orogeny. They formed as layer-parallel, north-dipping, north-over-south thrusts in response to flexural slip during buckling of the SIC. As the general transport direction was from south to north during the Mazatzal–Labradorian Orogeny, the D1 shear zones are back-thrusts with opposite transport direction. The thrusts imbricated the SIC, underlying Huronian rocks, and ore zones, and emplaced slivers of Huronian rocks and metabreccia into the overlying norite. In contrast to the D1 shear zones at Garson, the Thayer Lindsley and the regional South Range shear zones, which transgress the SIC-Huronian contact at a high angle, formed as moderately SE-dipping reverse shear zones as a result of localization of folding-induced strain near the hinge zone in order to accommodate further flattening and tightening of the SIC with progressive D1 shortening. Together with the southeast limb of the SIC, the D1 shear zones were overturned into their present steep southerly dips and were reactivated as south-over-north shear zones either later during the Mazatzal–Labradorian Orogeny or during the 1.5–1.45Ga Chieflakian event, which was coeval with accretion of juvenile Andean-style, calc–alkaline magmatic arcs along the entire southeastern margin of Laurentia. During this D2 reactivation event, S1 was transposed and locally preserved in crenulations bounded by a S2 chloritic shear foliation, which has a strong down-dip L2 mineral chlorite lineation and parallel L2 ductile slickenlines.

Osmium evidence for synchronicity between a rise in atmospheric oxygen and Palaeoproterozoic deglaciation

Early Palaeoproterozoic (2.5-2.0 billion years ago) was a critical phase in Earth's history, characterized by multiple severe glaciations and a rise in atmospheric O(2) (the Great Oxidation Event). Although glaciations occurred at the time of O(2) increase, the relationship between climatic and atmospheric transitions remains poorly understood. Here we report high concentrations of the redox-sensitive element Os with high initial (187)Os/(188)Os values in a sandstone-siltstone interval that spans the transition from glacial diamictite to overlying carbonate in the Huronian Supergroup, Canada. Together with the results of Re, Mo and S analyses of the sediments, we suggest that immediately after the second Palaeoproterozoic glaciation, atmospheric O(2) levels became sufficiently high to deliver radiogenic continental Os to shallow-marine environments, indicating the synchronicity of an episode of increasing O(2) and deglaciation. This result supports the hypothesis that climatic recovery from the glaciations acted to accelerate the Great Oxidation Event.

Manganese enrichment in the Gowganda Formation of the Huronian Supergroup: A highly oxidizing shallow-marine environment after the last Huronian glaciation

Oxidative precipitation and authigenic enrichment of the redox sensitive element Mn in sedimentary rocks can serve as a proxy for the release of high levels of O 2 during the Great Oxidization Event (GOE). Here we investigate Mn abundance in sedimentary rocks of the 2.45–2.22 Ga Huronian Supergroup, Canada. We found authigenic Mn enrichments with high Mn/Fe ratios following the appearance of Fe oxides in the Firstbrook Member of the Gowganda Formation of the Huronian Supergroup, which was deposited immediately after the last Huronian glaciation. The Mn-bearing minerals in the Firstbrook Member are spessartine-rich almandine and Mn-bearing chlorite, which are likely to have been formed through post-depositional diagenesis and/or metamorphism using Mn oxides precipitated in the ocean at the time of deposition. When assuming the solution equilibrium between the atmosphere and shallow oceans, oxidative Mn precipitation requires that atmospheric O 2 be higher than ~ 10 − 2 times the present atmospheric level (PAL). The cumulative Mn amount per unit area in the Firstbrook Member is comparable in magnitude to that in the Mn deposits in the Hotazel Formation of the Transvaal Supergroup, South Africa. Our results suggest an appearance of highly active aerobic biosphere immediately after the last Huronian glaciation, supporting the hypothesis that climatic recovery from the Huronian glaciation accelerated the GOE.

Shallow impact: Isotopic insights into crustal contributions to the Sudbury impact melt sheet

The largest known terrestrial impact melt sheet occurs within the 1850 Ma Sudbury Structure, Ontario. In order to evaluate the relative contributions of different target lithologies to the melt sheet, we have investigated the Pb isotope compositions of feldspar separates from early-formed quartz diorite magmas within Offset Dykes from around the impact structure. The samples define a linear array on plots of age-corrected 206Pb/ 204Pb versus 207Pb/ 204Pb. Samples from Offset Dykes hosted by the Huronian Supergroup (South Range) have a range of 206Pb/ 204Pb 1850 from 15.424 to 17.255 and 207Pb/ 204Pb 1850 from 15.390 to 15.801, whilst those hosted by Archean gneisses of the Superior Province (North Range) cluster around 206Pb/ 204Pb 1850 ≈ 14.8 and 206Pb/ 204Pb 1850 ≈ 15.1. These values can be approximated by binary mixing between the two major groups of target lithologies. A mix of 60–70% of Superior Province gneisses with 30–40% of Huronian metasedimentary material closely matches the Pb isotope compositions of North Range Offset Dyke samples, whereas in the South Range the required Huronian component is up to ca. 80%. These mixing proportions are consistent with Sr, Nd and Os isotope and trace element constraints. A third minor component, either locally-exposed Paleoproterozoic mafic rocks or the lower crust is also required. However, the isotopic, trace element and Ni–Cu–platinum group element characteristics of the melt sheet can be accommodated without the involvement of an average lower crustal or meteoritic component. A major contribution of Huronian supracrustal material, which had a pre-impact thickness of up to 12 km, is required to explain the chemical characteristics of the impact melts, which also have a strong upper crustal affinity (e.g. Eu/Sm = 0.22, Rb/Sr = 0.2–0.35). As such, a shallower level of melting is apparent than that predicted by many previous impact models for the Sudbury event. This can be accommodated by considering approach trajectories for the impactor oblique to the Earth’s surface. In addition, the isotopic and trace element variability identified indicates that the melt sheet was heterogeneous at an early stage, and may not have been completely homogenised during crater formation. Our findings have significant implications for the nature of the Sudbury impact event, the evolution of the melt sheet and the crustal sources of metals contained in Sudbury’s world class Ni–Cu–PGE sulphide ores.

THE STABLE AND RADIOGENIC ISOTOPIC ATTRIBUTES OF PRECIOUS-METAL-BEARING POLYMETALLIC VEINS FROM THE COBALT EMBAYMENT, NORTHERN ONTARIO, CANADA: GENETIC AND EXPLORATION IMPLICATIONS

The Cobalt Embayment is a large (~30,000 km 2 ) domain of Paleoproterozoic (2.45–2.22 Ga) siliciclastic sedimentary rocks (the Huronian Supergroup) that unconformably overlies Archean basement rocks of the Abitibi Greenstone Belt. The sedimentary rocks were affected by subgreenschist-facies metamorphism and regionally distributed metasomatism related to waning stages of the Penokean Orogeny ( ca . 1.9–1.7 Ga). Precious-metal-bearing polymetallic calcite–quartz vein systems occur throughout the embayment and share many common features, including their morphological characteristics, ore and gangue mineralogy, paragenetic sequence, and spatial association with regional faults rooted in the Archean basement. The polymetallic veins are typically hosted in Nipissing Diabase, a regionally distributed suite of mafic sills and dykes, which intruded the Huronian sedimentary rocks at ca. 2.2 Ga. Integrated carbon, oxygen, and strontium isotopic data for calcite, the major mineral in the gangue, support a common hydrothermal origin for the regionally distributed systems of polymetallic veins, involving mixing of meteoric-water-dominated fluids with basin brines. Local variations in δ 13 C likely reflect the availability of, and degree of hydrothermal fluid interaction with, organic matter in different regions of the embayment. Significant intradeposit variation in δ 34 S values of vein sulfides (pyrite, chalcopyrite, galena) suggests that localized sources of sulfur existed in the Huronian and Archean rocks, with positive values (>0‰) in several of the mineralized vein systems likely reflecting the introduction of sulfur remobilized from the basement as a result of the conversion of pyrite to pyrrhotite in the underlying Archean rocks. Data and observations suggest that metals were transported in oxidized fluids, and precipitation occurred as a result of oxidation–reduction reactions along reactivated faults, which facilitated fluid mixing. Together with Pb isotope data for the vein systems, the isotopic data presented support a model for ore genesis in which the systems of precious-metal-bearing polymetallic calcite–quartz veins formed, and were reactivated, in response to two major episodes of hydrothermal fluid activity that occurred throughout the Cobalt Embayment, at ca. 2.2 and ca. 1.7 Ga. The proposed genetic model identifies the polymetallic veins as one manifestation of hydrothermal fluid activity during these basin-wide events and also recognizes the regional unconformity as a potential site for undiscovered, unconformity-associated hydrothermal polymetallic mineralization.