Because the concentrations of uranium and thorium in the crust must be determined precisely for the future geoneutrino observations planned at the Sudbury Neutrino Observatory, we investigate whether ...airborne radiometric surveys can be used to constrain crustal radioactivity. The regional airborne surveys cover a wide area with high spatial resolution (<250m), but are only sensitive to a very thin (25cm) surficial layer. We calculate crustal heat production in the Sudbury region from airborne radiometric surveys and compare with measurements on outcrop and core samples, and with heat flow data. The concentrations of uranium, thorium, and potassium from radiometric surveys are correlated with geology, but heat production estimates are lower than values from rock samples. The radiometric surveys give a mean heat production of 0.8±0.6 (σ) μWm−3 for more than 176,000 values. The outcrop samples collected along a transect in the Superior Province yield an average heat production of 2.9±2.4 (σ) μWm−3 and core samples from drill holes yield an average of 2.5±0.8 (σ) μWm−3. The high heat production in the rock samples is consistent with surface heat flux measurements near Sudbury with a mean value that is 12mWm−2 higher than the average Canadian Shield. The study shows that airborne aeromagnetic surveys give useful information on lateral variations in surface heat production but are unlikely to provide the reliable values of heat production needed to calculate the crustal geoneutrino flux. Crustal heat production will be best calculated from heat flux data complemented by heat production measurements on rock samples. The high mean heat production in Sudbury Igneous Complex samples (≈1.5μWm−3) suggests that the main source of the melt sheet was the very radioactive upper crust of the Superior Province or that the melt sheet was extremely enriched relative to a lower crustal source.
•New insight on crustal heat production near the Sudbury Neutrino Observatory.•Airborne radiometric surveys underestimate surface heat production.•Heat flux data provide robust constraints on crustal radioactivity.
Magmatic sulfide deposits consist of pyrrhotite, pentlandite, chalcopyrite (± pyrite), and platinum-group minerals (PGM). Understanding the distribution of the chalcophile and platinum-group element ...(PGE) concentrations among the base metal sulfide phases and PGM is important both for the petrogenetic models of the ores and for the efficient extraction of the PGE. Typically, pyrrhotite and pentlandite host much of the PGE, except Pt which forms Pt minerals. Chalcopyrite does not host PGE and the role of pyrite has not been closely investigated. The Ni–Cu–PGE ores from the South Range of Sudbury are unusual in that sulfarsenide PGM, rather than pyrrhotite and pentlandite, are the main carrier of PGE, probably as the result of arsenic contribution to the sulfide liquid by the As-bearing metasedimentary footwall rocks. In comparison, the North Range deposits of Sudbury, such as the McCreedy East deposit, have As-poor granites in the footwall, and the ores commonly contain pyrite. Our results show that in the pyrrhotite-rich ores of the McCreedy East deposit Os, Ir, Ru, Rh (IPGE), and Re are concentrated in pyrrhotite, pentlandite, and surprisingly in pyrite. This indicates that sulfarsenides, which are not present in the ores, were not important in concentrating PGE in the North Range of Sudbury. Palladium is present in pentlandite and, together with Pt, form PGM such as (PtPd)(TeBi)
2
. Platinum is also found in pyrite. Two generations of pyrite are present. One pyrite is primary and locally exsolved from monosulfide solid solution (MSS) in small amounts (<2 wt.%) together with pyrrhotite and pentlandite. This pyrite is unexpectedly enriched in IPGE, As (± Pt) and the concentrations of these elements are oscillatory zoned. The other pyrite is secondary and formed by alteration of the MSS cumulates by late magmatic/hydrothermal fluids. This pyrite is unzoned and has inherited the low concentrations of IPGE and Re from the pyrrhotite and pentlandite that it has replaced.
The basal contact of the Sudbury Igneous Complex (SIC) on the North Range is interpreted as the outer edge of a meteorite impact crater. Yet, the base of the SIC, and contacts within the SIC, and the ...overlying Onaping are not circular. Their outline is elliptical. This and other details of the geology of the North Range which have not been fully explained include variations in the width of the metamorphic contact aureole, lateral discontinuous variations in the thickness of the norite and granophyre units, paleomagnetic evidence that the North Range contact of the SIC originally had a dip of around 20degrees, and differing magnetic fabrics in the norite/gabbro versus the granophyre. Several metrics are used to determine how much of the current outline of the North Range is the result of post-impact deformation and how much is a primary feature related to a meteorite impact. Uplift, rotation, and translation experienced by different segments of the North Range of the SIC are established using dyke azimuth and petrographic analysis of Matachewan diabase dykes, and paleomagnetic and magnetic fabric data analysis. These analyses show (a) the elliptical form of the North Range is a primary feature associated with a near-circular impact crater, (b) some of the original crater wall must have been preserved, and (c) deformation of the North Range is limited to regional scale block rotation producing a southwest dip modified by minor block rotation tilting and vertical displacement associated with north--northwest-trending faults.
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•Melt movement and decoupling of silicate magma and sulfide melt revealed by field relationships.•Downward melt migration into the Copper Cliff Offset (CCO) dyke, Sudbury Igneous ...Complex (SIC).•Deformation facilitated through re-activation of E-W-striking pre-impact faults.
The Main Mass of the Sudbury Igneous Complex (SIC) is a 1.5–5 km thick, layered sheet of impact melt rocks, intensely studied because of the magmatic Ni-Cu-PGE sulfide deposits associated with the base of the Main Mass and connected dykes, known as the Sublayer and Offset dykes, respectively. The mode of emplacement of the mineralized Offset dykes that connect to the Main Mass through morphologic crater-floor irregularities (embayments) and the style of post-cratering deformation that affected the Offset dykes is not fully understood. This field-based study of the Copper Cliff Embayment (CCE) and Offset (CCO) dyke contributes to unraveling the mode of melt emplacement and the role of pre-impact faults in the deformation of the southern SIC. Field relationships indicate that the CCO dyke formed before the CCE and Sublayer were chemically fully evolved. Respective melts were injected into footwall rocks weakened by pre-impact deformation and cratering as a protracted event, with barren quartz diorite (QD) emplaced prior to mineralized, inclusion-bearing quartz diorite (IQD). Massive sulfide ore bodies appear to have formed late in the evolution of the dyke and physical separation (decoupling) of silicate magma and sulfide melt is required. NW-SE-shortening folded and faulted the strata hosting the CCO dyke and deformation was facilitated through re-activated E-W-striking, pre-impact faults. Restoring the initial geometry of the dyke and embayment, using 3D modelling and field constraints, helped to refine total slip estimates along major faults and confirmed that melts migrated gravitationally downward into the CCO dyke.
Laser ablation ICP-MS analysis has been applied to many accessory minerals in order to understand better the process by which the rock formed and for provenance discrimination. We have determined ...trace element concentrations of Fe-oxides in massive sulfides that form Ni–Cu–PGE deposits at the base of the Sudbury Igneous Complex in Canada. The samples represent the crystallization products of fractionating sulfide liquids and consist of early-forming Fe-rich monosulfide solution (MSS) cumulates and residual Cu-rich intermediate solid solution (ISS). This study shows that Fe-oxide geochemistry is a sensitive petrogenetic indicator for the degree of fractionation of the sulfide liquid and provides an insight into the partitioning of elements between sulfide and Fe-oxide phases. In addition, it is useful in determining the provenance of detrital Fe-oxide.
In a sulfide melt, all lithophile elements (Cr, Ti, V, Al, Mn, Sc, Nb, Ga, Ge, Ta, Hf, W and Zr) are compatible into Fe-oxide. The concentrations of these elements are highest in the early-forming Fe-oxide (titanomagnetite) which crystallized with Fe-rich MSS. Upon the continual crystallization of Fe-oxide from the sulfide liquid, the lithophile elements gradually decrease so that late-forming Fe-oxide (magnetite), which crystallized from the residual Cu-rich liquid, is depleted in these elements. This behavior is in contrast with Fe-oxides that crystallized from a fractionating silicate melt, whereby the concentration of incompatible elements, such as Ti, increases rather than decreases. The behavior of the chalcophile elements in magnetite is largely controlled by the crystallization of the sulfide minerals with only Ni, Co, Zn, Mo, Sn and Pb present above detection limit in magnetite. Nickel, Mo and Co are compatible in Fe-rich MSS and thus the co-crystallizing Fe-oxide is depleted in these elements. In contrast, magnetite that crystallized later from the fractionated liquid with Cu-rich ISS is enriched in Ni, Mo and Co because Fe-rich MSS is absent. The concentrations of Sn and Pb, which are incompatible with Fe-rich MSS, are highest in magnetite that formed from the fractionated Cu-rich liquid. At subsolidus temperatures, ilmenite exsolved from titanomagnetite whereas Al-spinel exsolved from the cores of some magnetite, locally redistributing the trace elements. However, during laser ablation ICP-MS analysis of these Fe-oxides both the magnetite and its exsolution products are ablated so that the analysis represents the original magmatic composition of the Fe-oxide that crystallized from the sulfide melt.
Rounded argillite clasts within the lower Gowganda Formation of the Huronian Supergroup near Whitefish Falls, Ontario, have been historically mapped as Sudbury Breccia, implying that their formation ...was initiated by the Sudbury meteorite impact event. Alternative genetic models proposed to explain the breccia at Whitefish Falls include formation through intrusion of diabase into wet sediment accompanied by soft-sediment deformation events. Outcrops in the Whitefish Falls area contain clear evidence for early post-depositional fracturing: flow of argillites into brittle fractured sandstones. Linking these geological processes suggests that the formation of the breccia at Whitefish Falls was generated by faulting of the Huronian sedimentary basin during the sedimentation of the Gowganda argillites. Using a GIS approach to compare the distribution of known breccia bodies with mapped lithology and structure, it is apparent that the term Sudbury Breccia has been applied to two types of breccias. First, true Sudbury Breccia, which is characterised by rounded heterogeneous clasts in an aphanitic matrix, is only found in proximity to the Sudbury Impact crater. The distribution of the second, primarily sediment derived, type of breccia, as seen at Whitefish Falls, is strongly associated with mapped faults and regional-scale basement discontinuities, as defined by gravity and magnetic data. Since this type of breccia is present throughout the entire Huronian sedimentary sequence, the term "Huronian Breccia" is more appropriate. This breccia is not the result of a single geological event but rather episodes of fault activity, as the geometry of the Huronian basin evolved over time.
Broken Hammer is a hybrid Cu-Ni-Platinum Group Element (PGE) footwall deposit located in Archean basement rocks below the impact-induced Sudbury Igneous Complex (SIC), Canada. The deposit consists of ...massive chalcopyrite veins surrounded by thin epidote, actinolite, and quartz selvedges and low-sulfide, high-PGE mineralization consisting of disseminated chalcopyrite (<5%) and platinum group minerals, associated with Ni-bearing chlorite overprinting alteration patches of epidote, actinolite, and quartz. The veins are grouped into five steeply-dipping sets, striking northeast-, southwest-, southeast-, south-, and east-west, which were emplaced along impact-related fractures that were reactivated multiple times during stabilization of the crater floor. Early reactivation of the fractures created pathways for the migration of hydrothermal fluids from which quartz and chlorite precipitated sealing the fractures. Renewed slip shattered the quartz-chlorite veins into fragments that were incorporated in massive sulfide veins that crystallized from fractionated sulfide melts or from high temperature (400-500°C) hydrothermal fluids, which migrated outward into the basement rocks from a cooling and crystallizing SIC melt sheet. Hydrothermal fluids syn-genetic with the epidote-actinolite-quartz alteration distributed the PGE into the footwall rocks, or late hydrothermal fluids associated with the Ni-bearing chlorite leached Ni and PGMs from the sulfide veins and redistributed them to form low-sulfide, high-PGE zones in the footwall rocks. During post-impact tectonic events, slip at temperatures below the brittle-ductile transition for chalcopyrite (<200°C to 250°C) produced striations along the vein margins. The Broken Hammer deposit exemplifies how Cu-Ni-PGE footwall deposits formed by the reactivation of impact-related fractures that provided conduits for the migration of melts and hydrothermal fluids.
We have compiled the trace element concentrations in pyrrhotite, pentlandite, chalcopyrite, and pyrite from magmatic Ni-Cu-PGE ore deposits with the aim of understanding their petrogenesis and ...whether these minerals can be used as indicator minerals. Among the samples, there are some of the most studied world-class Ni-Cu- (Aguablanca, Duluth, Jinchuan, Noril’sk-Talnakh-Kharaelakh, Sudbury, Voisey’s Bay, and others) and PGE-dominated (Bushveld, Lac des Iles, Stillwater, Great Dyke, and Penikat) deposits. Crustal assimilation may be constrained using As/Se and Sb/Se ratios in pentlandite. The degree of interaction between the silicate and sulfide liquids (R-factor) can be estimated by the content of highly chalcophile elements (D
sulf liq/sil liq
above 1000) in sulfide minerals. The fractional crystallization of the sulfide liquid can be traced using Se/Te ratios of pentlandite. Pyrite formed by exsolution from MSS has higher Rh, Ru, Ir, and Os than co-existing pyrrhotite, whereas pyrite formed by hydrothermal alteration of pyrrhotite inherits the Rh, Ru, Ir, and Os contents of the pyrrhotite it replaced. Sulfide minerals are preserved in transported glacial cover and their trace element chemistry can be used to discriminate their source. Pentlandite from Ni-Cu deposits has much lower Rh and Pd concentrations than those from PGE-dominated deposits, pyrite from magmatic deposits has higher Co/Sb and Se/As ratios relative to pyrite from hydrothermal deposits, and chalcopyrite from magmatic deposits has much higher Ni and lower Cd concentrations than those from hydrothermal deposits.
