Sudbury Complex Research Articles

Genesis of sulfide vein mineralization at the Sakatti Ni-Cu-PGE deposit, Finland

ABSTRACT The Sakatti Ni-Cu-platinum-group element deposit is situated in northern Finland and comprises massive, disseminated, and vein sulfide mineralization. A stockwork is formed by chalcopyrite-rich sulfide veins, which contain exceptionally high platinum-group elements and Au grades. The mineralogy and geochemistry of this stockwork zone ore is documented in this investigation. The results are used to develop the first robust genetic concept and its relationship to massive and disseminated mineralization of the Sakatti deposit. This model is similar to that proposed for many Cu-rich magmatic sulfide ores, most importantly the Cu-rich footwall veins described from the Sudbury Complex in Canada and the Cu-rich ore at Noril'sk-Talnakh in Russia. Detailed petrographic studies using a sample suite from exploration drill core intersecting vein-style mineralization revealed a classic magmatic sulfide assemblage of chalcopyrite ± pyrrhotite, pentlandite, and pyrite. More than 1000 platinum-group mineral grains belonging almost exclusively to the moncheite (PtTe2) – merenskyite (PdTe2) – melonite (NiTe2) solid solution series were identified in the studied samples. Notably, almost two thirds of the platinum-group element-bearing minerals consist of melonite. Some of the platinum-group minerals contain inclusions of Ag-rich gold (AgAu2) and muthmannite (AuAgTe2). Most of the platinum-group minerals occur as inclusions in chalcopyrite, although a few grains are located at base-metal sulfide grain boundaries and in fractures in base-metal sulfides. The whole-rock compositions of the stockwork veins are Cu-rich and are interpreted to represent a fractionated Cu-rich sulfide liquid enriched in Pt, Pd, Au, Ag, As, Bi, Pb, Se, Te, Zn, which separated from a monosulfide solid solution (mss). An intermediate solid solution (iss) solidified from the Cu-rich sulfide liquid, recrystallizing chalcopyrite at <550 °C. Simultaneously, small volumes of intercumulus residual melt contained mainly the precious metals, Bi, and Te due to their incompatibility in iss. Solitary and composite platinum-group minerals as well as Au-minerals crystallized first from the residual melt (<600 °C), followed by a succession of various Bi-, Ag-, and Pb-tellurides (∼540 °C), and finally sphalerite and galena. Melonite crystallized as mostly large, solitary grains exsolved directly from Ni-bearing intermediate solid solution (∼600 °C), shortly after the formation of moncheite and merenskyite from the residual melt. Finally, remobilization of the platinum-group minerals occurred at temperatures of <300 °C, as suggested by the presence of minor amounts of Cl-bearing minerals and ragged grain shapes.

High-Grade Magmatic Platinum Group Element-Cu(-Ni) Sulfide Mineralization Associated with the Rathbun Offset Dike of the Sudbury Igneous Complex (Ontario, Canada)

Abstract Quartz dioritic impact melt dikes around the 1.85 Ga Sudbury Igneous Complex, locally referred to as offset dikes, are well endowed with respect to Ni-Cu-platinum group elements (PGE). However, only those dikes proximal (<6 km) to the main mass of the Sudbury Complex are mineralized at an economic grade and, in places, host world-class deposits. We report on a new discovery of such heavily mineralized offset dike at Rathbun Lake, about 15 km east of the currently known extent of the Sudbury Igneous Complex. There, a segment of amphibole quartz diorite is exposed at the contact between Huronian metasedimentary rocks and gabbro of the 2.22 Ga Nipissing Suite, xenoliths of which are abundant throughout the diorite and record textural evidence of partial melting. The mafic inclusion-bearing quartz diorite is the host of the Rathbun Lake showing, a small but high-grade PGE-Cu(-Ni) sulfide occurrence of hitherto controversial origin. A detailed petrographic and mineralogical characterization of this occurrence revealed a two-stage mineralization history. Disseminated to semimassive (net-textured) chalcopyrite ± loop-textured pentlandite ± magnetite containing Pd-bismuthotellurides and, more rarely, sperrylite and native gold—all of which are closely associated with base metal sulfides—are interpreted as magmatic. The semimassive sulfide averages ~40 g/t Pd + Pt + Au at a Cu/(Cu + Ni) of >0.9 and a Pd/Ir of >100,000. Mineralogy, ore textures, and mantle-normalized PGE + Au patterns match a specific type of Cu-rich mineralization in the Sudbury Igneous Complex known as footwall mineralization. By analogy with these footwall deposits, the occurrence is interpreted as having formed by downward percolation of a highly fractionated sulfide melt toward the bottom of a now largely eroded offset dike. The magmatic paragenesis was hydrothermally overprinted at lower greenschist-facies conditions to pyrite-chalcopyrite-violarite ± covellite ± millerite. This involved also local remobilization into pyrite-chalcopyrite veinlets and the liberation of precious metal minerals from their sulfide hosts. In contrast to base metal sulfides, most precious metal minerals were resistant to hydrothermal alteration, although corrosion of some grains is noted as well as their truncation by chlorite and epidote. Micron-scale X-ray mapping revealed a progressive replacement of magmatic Pd-Bi-Te minerals, where in contact with hydrous silicates, by Sb- and Hg-bearing Pd minerals such as temagamite, Pd3HgTe3. The timing and nature of this hydrothermal overprint remains uncertain, but a connection to later regional metamorphism and faulting seems most plausible. Our finding of magmatic PGE-base metal sulfide at Rathbun Lake suggests a new subtype of distal offset dike-hosted mineralization in an area so far not known for offset dikes. It opens up new opportunities in the search for unconventional ore deposits around the Sudbury impact structure and improves our understanding on the distribution of impact melt-derived dikes around complex craters.

Vitric Compositions in the Onaping Formation and Their Relationship to the Sudbury Igneous Complex, Sudbury Structure

The 1.85 Ga Sudbury structure is the largest, well-preserved, exposed example of a terrestrial impact crater; it contains world-class Ni-Cu-platinum-group element (PGE) ores associated with the igneous component. A new understanding of the geology and geochemistry of the vitric crater-fill sequence, the Onaping Formation, provides constraints on the evolution of 1.85 Ga igneous rocks in the Sudbury structure. The Onaping Formation has a well-defined mappable stratigraphy composed of three members, seven stratigraphic units, and two types of intraformational dikes that are overprinted by an impact-generated hydrothermal system. Stratigraphic and alteration mapping provided a basis for well-constrained sampling and allowed the definition of the geochemical characteristics of the less altered rocks from each unit, including vitric bombs and blocks and intraformational vitric dikes. Significantly, the identification of a least altered composition of the vitric Onaping Formation enabled an investigation of the link between the early formed vitric melt phase in the impact crater-fill sequence and the Sudbury Igneous Complex and offset dikes. The least altered vitric Onaping Formation, represented by the massive, xenolith-poor cores of vitric bombs, blocks, and intraformational dikes, is andesitic in composition, consistent with a crustally derived melt. The melt composition (61.6 wt % SiO 2 , 4.28 MgO, 60 ppm Ni) is remarkably uniform throughout the Onaping Formation with a typical enrichment of large ion lithophile and light rare earth elements and pronounced negative Nb and Ti anomalies. Incompatible trace element ratios and bulk concentrations of the Onaping Formation coincide with the main mass norite and offset quartz diorite compositions of the underlying Sudbury Igneous Complex but differ distinctly from the granophyre and quartz gabbro. Root mean squared differences in composition of the vitric Onaping Formation with the units of the Sudbury Igneous Complex and quartz diorite offset dikes show that the vitric Onaping Formation does not directly match any of these igneous units but more closely resembles the composition of the Ministic and Manchester offset dikes. The quartz diorite offset dikes have been proposed as representative of the bulk composition of the original melt of the Sudbury Igneous Complex. In detail, there are small compositional differences between North and South Range offset dikes and between these offset dikes and the concentric Manchester offset dike; however, such differences are lacking in the Onaping Formation. We propose that the least altered vitric composition of the Onaping Formation represents the best estimate of the bulk composition of an initial quenched melt, or shock melt, formed early in the evolution of the Sudbury structure and is a logical starting liquid to model the evolution of the Sudbury main mass and offset dikes. The offset dike compositions can be derived from Onaping Formation vitric shock melt by assimilation of progressively greater amounts of mafic material at deeper levels in the crater. This mafic-contaminated melt, or impact melt, developed at the base of the melt sheet and was injected into the offset dikes both prior to and contemporaneously with the earliest stages of sulfide precipitation from the Sudbury Complex. Fractionation of the mafic-contaminated melt by plagioclase and pyroxene accumulation produced the North Range felsic and mafic norite and the South Range norites. The chalcophile metal content of the Onaping Formation vitric phase closely reflects the overall metal ratios of the magmatic sulfide ores. The vitric Onaping Formation and unmineralized offset dikes have low liquidus temperatures (1,115°–1,125°C) and, if these units represent the source of metals in the sulfide deposits, unusually high mass distribution coefficients for Cu and Ni and high R factors are necessary to account for the metal endowment.

The Sudbury Igneous Complex: A Differentiated Impact Melt Sheet

The Sudbury structure, Ontario, is the remnant of a 1.85 Ga old impact crater, which originally had a diameter of 200 to 250 km. The Sudbury Igneous Complex occurs within the Sudbury structure. The Sudbury Igneous Complex is a 2.5- to 3.0-km-thick, ~60- × 27-km elliptical igneous-rock body, which consists of four major lithologies (from top to bottom) traditionally termed granophyre, gabbro, norite, and sublayer (sulfide- and inclusion-bearing noritic rock). With the exception of the latter, all these lithologies are continuous across the structure. Modal analyses reveal that, following the IUGS system of nomenclature, quartz gabbro samples are in fact quartz monzogabbros, a few of the norite samples are quartz gabbros, and most norite samples are quartz monzogabbros. In view of these observations, and in order to clarify the nomenclature, an updated terminology is proposed (from top to bottom): upper unit, middle unit, lower unit, and contact sublayer. The bulk composition of the Sudbury Igneous Complex, from North Range data, is granodioritic. Continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the Complex behaved as a single melt system. All the Sudbury Igneous Complex lithologies have the same light to heavy rare earth element (REE) ratio and an overall pattern of increased light REE and depleted heavy REE. The occurrence of primary hydrous minerals (hornblende and biotite), deuteric alteration, and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in H2O. Moreover, the granophyric and other far-from-equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase. The Sudbury Igneous Complex differs from traditional layered mafic complexes in the following aspects. It has an overall intermediate composition, a hydrous nature, a crustal isotopic signature, normative corundum, and an unusually large volume of granophyre. The Sudbury Complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical, and therefore, mineralogical layering. Reported here for the first time, and similar to those found in impact melt rocks elsewhere, are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts. The well-known ore deposits of the Sudbury region are directly related to the genesis of the Sudbury Igneous Complex. Some ores precipitated from the Sudbury melt, whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex. It is concluded that the Sudbury Igneous Complex is the best exposed and only well-documented, to date, terrestrial impact melt sheet to have differentiated.

Geology, Mineralization, and Emplacement of the Foy Offset Dike, Sudbury Impact Structure*

The Foy offset dike is the longest of the known radial dikes associated with the 1.85 Ga Sudbury Igneous Complex. The Foy dike extends for at least 30 km from the complex into the North Range Archean and Proterozoic footwall rocks of the Sudbury impact structure. The dike ranges in width from 400 m, where it connects to the Sudbury Igneous Complex, to 50 m at its most distal known exposure. We divide the dike into a proximal segment and a distal segment, based on where it intersects the concentric Hess offset dike. Three main dike lithologies are distinguished, all referred to as varieties of quartz diorite. Inclusion-rich pyroxene-quartz diorite occurs in the embayment, adjacent to the base of the main mass of the Sudbury Igneous Complex. This constitutes the sublayer. Northward, the sublayer gives way to amphibole-biotite inclusion-bearing quartz diorite and amphibole-biotite inclusion-poor quartz diorite. The dike is mineralized and deposits in the proximal segment have been mined. Mineralization is associated with the inclusion-rich dike phase. At the Nickel offset mine the dike contains inclusion- bearing massive Ni-Cu sulfides that also host platinum-group minerals. The orebodies occurred as strike-parallel sheets within the central parts of the dike in association with inclusion-bearing quartz diorite. The geochemistry of the dike lithologies indicates that both the inclusion-poor and inclusion-bearing quartz diorite varieties are close in composition to the postulated bulk composition of the Sudbury Igneous Complex. This implies that the dike was emplaced prior to any significant fractionation of the main melt body and, hence, very early in the evolution of the Sudbury Complex. We envisage dike emplacement rapidly following rebound and central uplift formation as part of the impact process. Early dike emplacement implies that the ore was also introduced early in the evolution of the melt sheet when the inclusion-poor quartz diorite phases were still unconsolidated. Uplift would have caused extension in the target rocks and so dilated the footwall fractures and faults that then became hosts, primarily via gravitationally driven intrusion, to the primitive polymict impact melt breccias and sulfide ores. The chilled margin compositions of the Foy inclusion-poor quartz diorite are considered good indicators of the original bulk composition of the Sudbury Igneous Complex; this idea is supported by comparison with calculated estimations of the bulk composition. A shift in geochemical trends, a grain size increase, and a decrease in inclusion content occurs ~22 km along the length of the dike, which we believe corresponds to a fault system that downfaults to the north. Given that the dense ore would have preferentially settled toward the base of the Foy dike-fracture system, postimpact downfaulting of the distal segment of the dike may have displaced any mineralized zones present to deeper levels relative to the proximal segment.

Geochemical relationships in the Sudbury igneous complex; origin of the main mass and offset dikes

Petrological models relating the different rock types constituting the 1.85 Ga Sudbury Igneous Complex are constrained with extensive new geochemical data. We show that the main mass felsic norite, transition zone quartz gabbro, and granophyre have similar ratios of the highly incompatible trace elements (e.g., La/Sm = 4.5-7, La/Nb = 2.8-4.2, Th/Zr = 0.04-0.05) and that these variations are consistent with the crystallization and differentiation of magma types largely (>80%) derived from the upper crust, with a smaller contribution from a mantle source. Although there is presently no conclusive proof that magma was generated in situ as a melt sheet produced by meteorite impact, we find no principal reason why this model should be rejected. However, we propose that a small contribution of mantle-derived picritic magma is required to explain the abundant Ni, Cu, and platinum-group elements (PGE) in the Sudbury deposits, as well as the compositions of the ultramafic inclusions (MgO = 12-36 wt %; Fo (sub 68-87) olivines with 450-3,700 ppm Ni, and abundant chrome-rich spinel), and the magnesian composition of the mafic norite (8-14 wt % MgO) and the sublayer (6-12 wt % MgO). We believe that the main mass of the Sudbury Complex achieved its present composition through incorporation of up to 20 percent mantle-derived picritic magma emplaced along crustal fractures produced by the impact event. These picritic magmas entered the melt sheet as a dense plume, vigorously mixing with it, and due to the marked compositional shift, the mixed magma formed magmatic sulfides which sank through the magma column, depleting the melt in Ni, Cu, and PGE. Since both the felsic norite and granophyre have indistinguishable ratios of the incompatible trace elements, we see no requirement to derive these units of rock by the crystallization of magmas derived from different sources. Rather, the compositional difference between the felsic norite and granophyre is attributed to the in situ differentiation of the magma. We show that the main mass has many compositional traits similar to those of most of the offset dike quartz diorites (e.g., the Parkin offset dike: La/Sm = 6.3, La/Nb = 4.5; Th/Zr = 0.05) and of embayment-related leucocratic norites from the Whistle mine (La/Sm = 6.2, La/Nb = 5.0, Th/Zr = 0.02). These rocks have compositions intermediate between the felsic norite and the granophyre, and therefore crystallized from the same magma type; arguably, the unmineralized quartz diorites provide the best possible estimate of the original magma from which the Sudbury Complex crystallized. In detail, there are subtle variations in composition within and between offset dikes, with the largest difference being between the North and South Range offsets; the North Range offset dikes cut Archean granitoids and gneisses and have elevated Sr, La/Yb, La/Sm, and Gd/Yb and low TiO 2 whereas the South Range dikes cut Early Proterozoic sediments, mafic volcanics, and intrusions, and have low Sr, La/Yb, Gd/Yb, La/Sm, and high TiO 2 . These differences may be caused by the assimilation of different country rocks during emplacement of the dike. A strongly mineralized offset dike at the Creighton mine has geochemical variations that are different when compared to the main mass, and in the case of Creighton, are more similar to the local mineralized sublayer. These data suggest that mineralized and barren quartz diorites have different geochemical compositions, and that these traits may be of value in mineral exploration.

Albitization at 1700 ± 2 Ma in the Sudbury – Wanapitei Lake area, Ontario: implications for deep-seated alkalic magmatism in the Southern province

U–Pb geochronology of hydrothermal monazite in albitized rocks from two gold deposits east of the Sudbury complex indicates that albitization in the Sudbury – Wanapitei Lake area occurred at 1700 ± 2 Ma and was coeval with a period of granitic plutonism in the Southern structural province between 1750 and 1700 Ma.A variety of rare earth element (REE) minerals, such as two generations of hydrothermal monazite, bastnäsite, synchysite, and gadolinite were identified in the albitized Huronian sediments in the Espanola – Sudbury – Wanapitei Lake areas. The presence of these REE minerals, the extraordinary light rare earth element enrichment in rocks from the Sheppard gold property east of the Sudbury igneous complex and the elevated REE concentrations in some albitized rocks suggests that sodium-rich fluids may have been generated by carbonatitic or alkalic intrusions at depth.Gold mineralization occurs in rocks that have been altered by at least two different types of fluids: (1) peralkaline; Na–REE bearing and (2) low pH, Co bearing. The high Co content of most mineralized samples and the relatively weak correlation between Au and Na2O suggests that gold was probably concentrated to economic grade by the low pH, Co-bearing fluids. The spatial association of albite and gold suggests that the albitized rocks may represent earlier fluid conduits that were subsequently refractured and invaded by the mineralizing solutions.

Osmium isotope evidence for a crustal origin of platinum group elements in the Sudbury nickel ore, Ontario, Canada

Sulphide ores from the International Nickel Company's (INCO) Creighton Mine, Sudbury, were analysed for osmium isotope ratios by ICP-MS. Rhenium and osmium abundances were determined by isotope dilution using a mixed spike in solid solution in a nickel sulphide matrix. Calculated initial 187Os 188Os ratios at 1.85 Ga (the emplacement age of the Sudbury complex) cluster around 0.60. The occurrence of less radiogenic compositions is attributed to post-emplacement, open-system behaviour of the Re Os system. The Creighton results strongly overlap the initial ratios of published osmium data from two other Sudbury mines, suggesting a narrow range of isotope ratio in the original ore, within the range of estimated osmium isotope compositions in the country rock at 1.85 Ga, using published and new data. Therefore, the sulphide ores of the Sudbury complex can have an entirely crustal source without a mantle-derived contribution. This conclusion is consistent with published neodymium isotope data for the complex and supports the meteorite impact hypothesis, since this is the most effective means of fusing the large quantity of crustal rocks necessary to generate the complex. The extraction of platinum group elements (PGE) from this silicate melt by a nickel sulphide liquid was probably analogous to the laboratory fire assay procedure.

Proton-microprobe analyses of palladium and selenium in NiCu-sulfides

Samples from the NiCu-sulfide mineralization of the Acoje Ophiolite block (ancient oceanic crust), Zambales, Philippines were analyzed for Pd and Se using the Heidelberg proton microprobe. The background in the PIXE spectra around the Pd peaks was nearly zero, so that even small peaks were clearly revealed. Unexpected problems in the fitting of these peaks with the SESAMX program could only be solved by modification of this program. Experimental results allow the following conclusions: Selenium concentrations and the corresponding Se/S ratios of these sulfides are in comparable orders of magnitude to magmatic sulfides from the Stillwater and Sudbury complex; Palladium traces of secondary origin were found in secondary native copper. Pd of primary origin was found in magmatically formed pentlandite (Ni,Fe) 9S 8.

Eilectrochemical Investigations into the Flotation Chemistry of Pentlandite and Pyrrhotite: Process Water and Xanthate Interactions

Electrochemical studies were conducted to determine if ions present in the Copper Cliff process water at INCa's Sudbury complex were interactive on pentlandite and pyrrhotite surfaces. It was found that Ca2+, S2O32− and SO42− were significantly surface active at the normal process pH. From comparisons of other voltammetric traces obtained for xanthate interactions it was concluded that these ions would influence the extent of adsorption of X− by the sulphide minerals as well as control the onset of hydrophobicity. Simple hydrophobicity tests confirmed that S2O32− and Ca2+ competed with xanthate for adsorption on the surface of pentlandite whereas only Ca2+ increased the xanthate dosage required to render pyrrhotite hydrophobic. Mechanisms for pentlandite and pyrrhotite flotability arising from xanthate reactions were put forward. Xanthate chemisorption took place on the nickel sites in the case of pentlandite, which was oxidized at the surface to form dixanthogen which enhanced hydrophobicity. No xanthate chemisorption took place on pyrrhotite, the surface being rendered hydrophobic by dixanthogen which was formed as part of a mixed potential reaction involving with the normal pyrrhotite oxidation process.

Origin of the Sudbury Complex by Meteoritic Impact: Neodymium Isotopic Evidence

Samarium-neodymium isotopic data on whole rocks and minerals of the Sudbury Complex in Canada gave an igneous crystallization age of 1840 +/- 21 x 10(6) years. The initial epsilon neodymium values for 15 whole rocks are similar to those for average upper continental crust, falling on the crustal trend of neodymium isotopic evolution as defined by shales. The rare earth element concentration patterns of Sudbury rocks are also similar to upper crustal averages. These data suggest that the Sudbury Complex formed from melts generated in the upper crust and are consistent with a meteoritic impact.

PREDICTION OF THE ENVIRONMENTAL IMPACT OF MINING AND METALLURGICAL PRODUCTION IN THE TAYGA ZONE (with reference to the copper-nickel complexes of Mochegorsk and Sudbury)

The environmental impact of smelter gases of the nickel-copper complexes of Sudbury, Ont., and Monchegorsk, in the Soviet Union's Kola Peninsula, is compared. Despite the larger volume of production and of emissions from the Sudbury smelters, exceeding those of Monchegorsk by one order of magnitude, a relatively smaller area around the Sudbury smelter is affected by pollutants. The greater vulnerability of the biotic environment around Monchegorsk is related to the fact that the Soviet smelter complex lies in the northern tayga, with smaller biomass and biological productivity, while the Sudbury complex is situated in a richer forest region of mixed broadleaf and needle species, corresponding to the southern tayga in the Soviet Union.