Riding the supercontinent cycle: Paleoproterozoic basins and their metal endowment
The Paleoproterozoic period of Earth history was a time of major change in numerous factors that influence basin development and mineralizing systems, including atmosphere-hydrosphere composition, lithospheric architecture, and critically, tectonic processes. The latter were profoundly influenced by the supercontinent cycle, dominated in this time period by the assembly of Earth's first true supercontinent, Nuna. Nuna involved the amalgamation of nearly 35 cratons, that themselves were the diaspora of at least two latest Neoarchean supercratons: Superia and Nunavutia. Superia amalgamated by ca. 2.7-2.65 Ga, and was relatively long lived, with initial cover and rift sequences forming by the earliest Paleoproterozoic, ca. 2.5 Ga and lasting to nearly 2.1 Ga. Superia-related cratonic cover and passive margin sequences were deposited before world-wide oxygenation of the atmosphere and are world class repositories of Paleoplacer gold, uranium and Iron. Nunavutia by contrast was assembled over a more protracted time period, between 2.6 and 2.45-2.3 Ga and began to break up by ca 2.2 Ga. Its resulting cover and passive margin sequences formed around the time of major oxygenation and host significant unconformity uranium districts and sediment-hosted copper deposits. The global ocean Manikewan reached its zenith and began to contract by 2.1 Ga with progressive amalgamation of the early Paleoproterozoic cratons up to 1.7 Ga. Crustal growth was limited between 2.1 and 1.9 Ga, but increased dramatically thereafter, with time period 1.9-1.8 Ga seeing accretion of numerous arcs and microcontinents and attendant preservation of sequences hosting major volcanic-hosted massive sulphide districts. The major global magmatic nickel metallotect at ca. 1.88 Ga formed where mantle-derived magmatism invaded sulphidic marine shales of the older passive margin sequences and late stage syncollisional transtensional basins. As amalgamation of Nuna proceeded Sedex, stratiform copper and MVT-type deposits formed globally typically just preceding or overlapping orogenesis and just at or before major basin inversion phases. A period of period of relative supercontinent stability followed until the end of the Paleoproterozoic at ca.1.6 Ga. This phase is associated with world class metallotects in both interior and marginal basin settings. Gentle transpression and subsidence over the core of Nuna, deep paleo-weathering, and reactivation of aggregation structures favoured development of major basins and associated unconformity-hosted U districts above sites of mantle downwelling that subtly controlled the relatively long-lived stable interior of the supercontinent. Continued subduction on the periphery of the drifting supercontinent influenced phases of intracontinental backarc spreading that host contemporaneous Pb-Zn districts.
Age and Correlation of Late Paleoproterozoic Sedimentary Successions in Northwestern Canada and Their Bearing on the Paleogeography of Laurentia
Nearly 40 years ago, Fraser et al. (1970) proposed that thick basinal deposits of late Paleoproterozoic age along the western paleo-continental margin of Laurentia might represent the marine, deep-water complement of thinner but broadly correlative terrestrial sandstone deposits preserved today in intracontinental basins of the Canadian shield such as the Hornby Bay, Athabasca and Thelon. These basins exhibit comparable geometry, lithology, stratigraphy and overall paleocurrent patterns, which suggested they were initially co- extensive. Regional paleocurrents derived from crossbedded sandstone units interpreted as braided river deposits are dominantly west-directed in the mid-upper parts of all basins but are variable in the lower parts, supporting distinct initial basins that were later joined by broad fluvial braidplains originating from sources along active orogenic uplands located to the east (e.g. Trans-Hudson orogen). The sediment from these rivers was shunted westward across the craton and ultimately deposited along Laurentia's western margin. Geophysical data suggest that the distal parts of these river systems are preserved in the subsurface of northwestern Canada and are contiguous with fine-grained siliclastic and carbonate rocks of the Wernecke Supergroup and Muskwa Assemblage. One way to test this paleogeographic model is to compare the provenance of different parts of this sedimentary system using detrital zircon geochronology. Previous studies of the Muskwa assemblage (Ross et al. 2001) were compared with data for the Athabasca Group of the Athabasca Basin (Rainbird et al. 2007). A prominent peak of ages between 1.9-1.8 Ga is present in both successions and suggests common provenance from the Trans-Hudson orogen and delivery of detritus to the western margin of Laurentia by a >1000 km long drainage system. Based on correlation of seismic sections, MacLean and Cook (2004) proposed that the Wernecke Supergroup is equivalent to the lower part of the Hornby Bay Group (Sequence A1) in the Hornby Bay Basin. Our recent detrital zircon studies of the Wernecke Supergroup indicate that it correlates with the upper part of the Hornby Bay Group (Sequence A2) based on comparable detrital zircon age profiles, including the presence of a suite of zircons ranging in age from 1660-1620 Ma. Sources for zircons of this age are found in Narakay Volcanic Complex and the East River Formation of Sequence A2. The East River Formation is a carbonate platform succession that thickens and deepens toward the west. Our recent detrital zircon studies permit that it could be a shallow-water correlative of the Gillespie Lake Group (Wernecke Supergroup). Further tests of this correlation will be made by comparing stable isotope stratigraphy of carbonate units and Nd isotopes of mudstone units. A proximal-distal connection between the Hornby Bay-Athabasca-Thelon and the Muskwa-Wernecke would however, require unimpeded delivery of detritus, across several roughly N-S trending orogenic belts, including the ca. 2.0-1.9 Ga Taltson-Thelon orogen, the ca. 1.84 Ga Wopmay orogen and 1.88-1.84 Ga Great Bear and Hottah magmatic zones.
Quantitative comparisons of the Paleoproterozoic Thelon and Athabasca basins
Quantitative maximum grain size, conglomerate, clay intraclast, mudstone, siltstone and gamma ray data provide objective and graphically clear multiparameter plots on which to hang sedimentary attributes for basin analysis and sequence stratigraphy of siliciclastic basins. Coupled with paleocurrents, structure, and geochronology such plots provide a robust framework to define 3D geometry and interpret basin development. Quantitative graphical data are more readily absorbed and transferrable between researchers than qualitative pictorial logs, but do not end controversy over correlations - however they do facilitate open discussion and provide additional tools to resolve differences. For example, the Athabasca Group's eleven formations were recently formalized with type and representative sections characterized by 5 parameters (EXTECH IV Athabasca Basin Multidisciplinary Study), and these formations were grouped into four upward-fining siliciclastic sequences separated by three regional unconformities, with the third sequence (1644+/-13 Ma on igneous zircon in tuff) being roughly coeval with 1640-1620 Ma basin-wide fluorapatite cement. The uppermost sequence is capped by tuff-bearing oil shale dated at 1541+/-13 Ma (Re-Os) and stromatolitic carbonate. Questions between areas of dense data separated by sparse data are now being resolved by new quantitative logs of recent drill campaigns, leading to revised and clarified subdivisions of the Manitoba Falls Formation, and better constraints on the underlying Read and Smart formations. However these revisions also raise the possibility that the two middle sequences may be reorganized into one comprising geographically distinct deposystems that entered the basin from the east, northeast and south. In the northeastern Thelon Basin, previous workers had proposed three upward fining siliciclastic sequences based on qualitative sedimentology but were unable to create a geometrically viable 3-D model of either the present basin shape or its past development. New quantitative data from the same drill cores, coupled with surface mapping, reinforce and refine the three sequences, highlighting syn-depositional faulting and distinct lateral grain-size changes. The lowest sequence is discontinuous and quartz dominated. The middle sequence has polymict conglomerate including a major channel that entered from the east, interlayered with local sand sheets derived from the south. The third siliciclastic sequence is the top unit of the northeastern sub-basin. It is apparently capped by the Kuungmi lavas and Lookout Point carbonates in the main basin. A new age of 1540+/-30 Ma (U-Pb) on baddeleyite from the Kuungmi lavas supports correlation of the uppermost Thelon Basin sequence with the uppermost sequences in the Athabasca and Hornby basins. However unlike the Athabasca Basin there is no known depositional unit corresponding to the basin-wide 1667± 6 Ma fluorapatite cement.
A New Model of Terrane Accretion in Northwestern Laurentia required by U-Pb SHRIMP Analysis of Detrital Zircons from the Paleoproterozoic Wernecke Supergroup, Wernecke Mountains, Yukon
The late Paleoproterozoic history of the Wernecke Basin of northwestern Laurentia was previously thought to involve deposition of a thick clastic-carbonate succession (Wernecke Supergroup), intrusion of small gabbroic to syenitic stocks and dykes at ∼1.71 Ga (Bonnet Plume River Intrusions), deformation and greenschist- grade metamorphism (Racklan Orogeny), and formation of voluminous hydrothermal breccias (Wernecke Breccia) at ∼1.6 Ga. Recent U-Pb SHRIMP geochronology of detrital zircon from the Wernecke Supergroup requires a re-ordering of these events and is the impetus for a new model of terrane accretion. The revision is required because the zircons include a population of six young grains ranging in age from 1.67 +/- 0.02 to 1.61 +/- 0.03 Ga. The Wernecke Supergroup must be younger than the youngest of these grains, i.e., <1.64 Ga, which is significantly younger than the age of the Bonnet Plume River intrusions (which are well- dated by four U-Pb ID-TIMS ages that average ∼1.71 Ga). Thus, the Bonnet Plume River Intrusions cannot have intruded the Wernecke Supergroup. Since all of the dated Bonnet Plume River Intrusions occur as megaclasts within zones of Wernecke Breccia, the intrusions are now considered to be fragments from an overlying allochtonous terrane that were dropped down into the breccia zones. (Only a few of the many occurrences of the Bonnet Plume River Intrusions have been mapped as actual intrusions within the Wernecke Supergroup, but these are now tentatively regarded as younger intrusions, such as the 1.38 Ga Hart River or 1.27 Ga Bear River intrusions.) When field relations and other information are considered along with the new zircon ages, the most plausible sequence of events is: (1) growth of a volcano-plutonic terrane outboard from the continental margin of northwest Laurentia at ca. 1.71 Ga; (2) deposition of the Wernecke Supergroup in an extensional basin on or near the continental margin after 1.64 Ga; (3) folding, metamorphism and exhumation of the Wernecke Supergroup; (4) obduction of the volcano-plutonic terrane; and (5) widespread hydrothermal brecciation and engulfment of foundered clasts derived from the terrane at ∼1.6 Ga. Erosion of the terrane from currently exposed parts of the Proterozoic inliers of Yukon appears to be complete, although not all parts of the inliers have been mapped and investigated in detail.
Paleoproterozoic (2050-1680 Ma) Divergent and Convergent Sedimentary Basins of the Capricorn Orogen of Western Australia
The Capricorn Orogen is a 300-400 km-wide belt of Palaeoproterozoic basin formation, plutonic magmatism and deformation in Western Australia that, in part, records Late Palaeoproterozoic collision between the Archaean Pilbara and Yilgarn Cratons. Sequence-stratigraphic analyses indicate that supracrustal sequences of the Ashburton Province on the Pilbara Carton (to the north) and of the Nabberu Province on the Yilgarn Craton (to the south) evolved on separate continents until united during collision. The traditional perspective of the Capricorn Orogen is that it records north-south orthogonal collision between the cratons, but basin studies that indicate that the Pilbara-Yilgarn suture, and the fold belts of the Capricorn Orogen, was an ESE-trending transcurrent megashear, such that the Pilbara and Yilgarn Cratons were involved in east-west transcurrent collision. The age of collision is poorly constrained, but estimated to have been at about 1770 Ma. The Ashburton Province comprises an older (2050-1865 Ma) divergent margin megasequence corresponding to the opening of an ocean basin to the northwest, and not to the south as most previous reconstructions maintain. The younger (1865-1680 Ma) megasequence records ocean closure and transcurrent convergence. The basal rift sequence of the divergent margin times the end of the Carbon-Isotope excursion, at 2035 Ma, whereas both megasequences contain oxide-facies banded iron formations that were deposited during the time in Earth history when oxidation of the atmosphere appears to contraindicate their deposition.
Nd Isotopic Constraints on the Provenance of Cover Sequences in the Southern Australian Palaeoproterozoic
Within Australia, the late Palaeoproterozoic (c. 1790-1620 Ma) was a period of widespread sedimentation and basin development. These sequences host world class mineral deposits, including Broken Hill Pb-Zn-Ag, Mt Isa Pb-Zn-Cu and Century Pb-Zn. Provenance data from Palaeoproterozoic basins from the South Australian Craton provide constraints on the types of crust that were exposed before, during and after the 1730-1690 Ma Kimban Orogeny, a major orogenic event with age correlatives in Antarctica, the North Australian Craton and Laurentia. In the eastern Gawler Craton, siliciclastic sediments belonging to the c. 1790-1740 Ma Hutchison Group record a significant change in provenance as basin development proceeded. Basal units are derived from evolved crust (ϵNd (1790Ma) -11), corresponding to c. 2000-3150 Ma components of the underlying Gawler Craton. In contrast, the upper sequences were derived from significantly more juvenile crust (ϵNd (1790Ma) -4 to -2). In the c. 1720-1640 Ma Willyama Supergroup in the adjacent Curnamona Province, detrital zircon ages suggest derivation from the Gawler Craton. However, Nd isotopic data from the upper parts of the package record increasingly juvenile input (ϵNd (1650Ma) -4 to 0), requiring sources that are not preserved in the Gawler Craton. In the northwestern Gawler Craton poorly exposed metasedimentary units have maximum depositional ages constrained by detrital zircons of 1770 - 1710 Ma and minimum depositional ages constrained by metamorphic monazite at 1720 - 1660 Ma. These packages are similar in age to those in the eastern Gawler Craton and Curnamona Province. Nd isotopic data (ϵNd (1740Ma) -4 to -2) suggest they source comparatively juvenile material. This juvenile signal is also recorded by younger sequences in the central Gawler craton (ϵNd (1656Ma) around -3), which were deposited at c. 1655 Ma. In general the Nd isotopic compositions of c. 1790-1620 Ma metasedimentary packages in the southern Australian Proterozoic are too juvenile to have been derived from erosion of the presently exposed underlying crust. The ages of detrital zircons suggest they eroded crust from between c. 1780 and c. 1650 Ma. At present the location of a juvenile system is not well constrained, but the Nd isotopic data suggest that the Late Palaeoproterozoic basins in southern Australia were in sedimentary connection with a long-lived (> 100 Ma) system that was producing comparatively juvenile crust.