Offset Dike Research Articles

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.

Geology, Geochemistry, and Mineralogy of the Worthington Offset Dike: A Genetic Model for Offset Dike Mineralization in the Sudbury Igneous Complex

Geology, Geochemistry, and Mineralogy of the Worthington Offset Dike: A Genetic Model for Offset Dike Mineralization in the Sudbury Igneous Complex

Mineralogy of Ni-Cu-Platinum-Group Element Sulfide Ore in the 800 and 810 Orebodies, Copper Cliff South Mine, and P-T-X Conditions during the Formation of Platinum-Group Minerals

The 800 and 810 orebodies are two major deposits in the Copper Cliff South mine, located at opposite contacts between the quartz diorite of the Copper Cliff offset dike and metapelite of the McKim Formation. The Copper Cliff South mine is one of the richest Cu-Ni- platinum-group elements (PGE) deposits in the Copper Cliff offset dike. The ore is composed of variable proportions of pyrrhotite, chalcopyrite, and pentlandite and occurs as massive sulfides, semimassive sulfides with inclusions of quartz diorite and wall rocks, net-textured sulfides, disseminated sulfides, and veins. Sphalerite and magnetite are also present in minor amounts. The primary magmatic minerals in the quartz diorite are plagioclase, quartz, amphibole, and biotite. The most common platinum-group mineral (PGM) in the Copper Cliff South mine is sperrylite (PtAs 2 ). It occurs in every textural type of ore, typically enclosed by chalcopyrite, pyrrhotite, pentlandite, cobaltite, and hydrous silicates such as chlorite, amphibole, and epidote. Sperrylite may be homogeneous, zoned (with Sb-rich cores and Sb-poor rims), associated with hollingworthite (RhAsS) and cobaltite, or it may contain inclusions. The Pd-bearing minerals are froodite (PdBi 2 ) and michenerite (PdBiTe), which are most commonly enclosed by silicates (chlorite, biotite, quartz) or chalcopyrite. Hollingworthite invariably occurs in the center of zoned cobaltite in veins or in disseminated sulfides. Argentian gold (Au 0.56 Ag 0.44 ) occurs as small grains in chalcopyrite and in a veinlet with parkerite (Ni 2 Bi 3 S 2 ). The most abundant Ag mineral is hessite (Ag 2 Te), which is present in almost every sample with galena, gersdorffite, and locally PGM. Volynskite (AgBiTe 2 ) occurs as a zoned grain with tsumoite (BiTe) in the center. The most common trace minerals spatially and genetically associated with PGM are tsumoite, galena, gersdorffite, cobaltite, hessite, melonite, and native Te. The Copper Cliff South mine area was affected by more than one hydrothermal event, deformation, and metamorphism, all of which affected the primary igneous textures and mineralogy of the rocks. The first hydrothermal event was responsible for the extensive remobilization of metals, including some PGE. This is revealed by the presence of PGM in large veins in quartz diorite and metapelite and by chlorite, amphibole, epidote, biotite, and quartz that often enclose PGM. The temperature at the time of this event was estimated in samples containing garnet and biotite in equilibrium to be 327° to 540°C. Pressure, estimated using amphibole chemistry, may have been as high as 3 to 4 kbars. The fluid responsible for the transport of metals was very rich in Cl and oxidizing. The fluid composition, temperature, and/or pressure changed with time, suggested by the presence of zoned minerals, including sperrylite, gersdorffite, and cobaltite. Based on the textural relationships, sperrylite precipitated before tellurides and galena, and after or at the same time as michenerite. A second hydrothermal event produced calcite-bearing veinlets that cut larger veins.

Emplacement of Sulfide Deposits in the Copper Cliff Offset Dike during Collapse of the Sudbury Crater Rim: Evidence from Magnetic Fabric Studies

Emplacement of Sulfide Deposits in the Copper Cliff Offset Dike during Collapse of the Sudbury Crater Rim: Evidence from Magnetic Fabric Studies

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

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

Geology, Mineralization, and Emplacement of the Whistle-Parkin Offset Dike, Sudbury

Geology, Mineralization, and Emplacement of the Whistle-Parkin Offset Dike, Sudbury

Sudbury Breccia (Canada): a product of the 1850 Ma Sudbury Event and host to footwall Cu–Ni–PGE deposits

The Sudbury Structure, formed by meteorite impact at 1850 Ma, consists of three major components: (1) the Sudbury Basin; (2) the Sudbury Igneous Complex, which surrounds the basin as an elliptical collar; and (3) breccia bodies in the footwall known as Sudbury Breccia. In general, the breccia consists of subrounded fragments set in a dark, fine-grained to aphanitic matrix. A comparison of the chemical composition of host rocks, clasts and matrices indicates that brecciation was essentially an in-situ process. Sudbury Breccia forms irregular-shaped bodies or dikes that range in size from mm to km scale. Contacts with the host rocks are commonly sharp. The aspect ratio of most clasts is approximately 2 with the long axes parallel to dike walls. The fractal dimension (Dr)=1.55. Although there appears to be some concentration of brecciation within concentric zones, small Sudbury Breccia bodies within and outside these zones have more or less random strikes and steep dips. Sudbury Breccia bodies near an embayment structure tend to be subparallel to the base of the Sudbury Igneous Complex. Sudbury Breccia occurs as much as 80 km from the outer margin of the Sudbury Igneous Complex. In an inner zone, 5 to 15 km wide, breccia comprises 5% of exposed bedrock with an increase in brecciation intensity in embayment structures. Sudbury Breccia may be classified into three types based on the nature of the matrix: clastic, pseudotachylite and microcrystalline. Clastic Sudbury Breccia, the dominant type in the Southern Province, is characterized by flow-surface structures. Possibly, a sudden rise in pore pressure caused explosive dilation and fragmentation, followed by fluidization and flowage into extension fractures. Pseudotachylite Sudbury Breccia, mainly confined to Archean rocks, apparently formed by comminution and frictional melting. Microcrystalline Sudbury Breccia formed as a result of the thermal metamorphism, of the North Range footwall, by the Sudbury Igneous Complex. This produced a zone, approximately 1.2 km wide, wherein the matrix of the breccia either recrystallized or, locally, melted. An overprint of regional metamorphism obliterated contact effects in the South Range footwall. The Ni–Cu–PGE magmatic sulphide deposits may be classified into four types based on structural setting: Sudbury Igneous Complex–footwall contact, footwall, offset, and sheared deposits. Sudbury Breccia is the main host for footwall deposits (e.g., McCreedy East, Victor, Lindsley). Sudbury Breccia locally hosts mineralization in radial (e.g., Parkin and Copper Cliff) and concentric (e.g., Frood–Stobie) offset dikes.

Chemical Evolution and Origin of Nickel Sulfide Mineralizationin the Sudbury Igneous Complex, Ontario, Canada

Chemical Evolution and Origin of Nickel Sulfide Mineralizationin the Sudbury Igneous Complex, Ontario, Canada

Cu-Ni-PGE Mineralization within the Copper Cliff Offset Dike, Copper Cliff North Mine, Sudbury, Ontario: Evidence for Multiple Stages of Emplacement

Detailed petrographic and quantitative (electron microprobe, SEM-EDS) mineral-chemical analyses of sulfide ore from the 100 orebody at the Copper Cliff North mine, Copper Cliff, Ontario, reveal numerous stages of ore development. Sharp contacts between weakly mineralized quartz diorite and blebby to massive sulfides indicate multiple intrusive events within the Copper Cliff offset dike. A spatial association of sulfides and hydrous minerals, such as amphibole, biotite, chlorite and epidote, suggests that most primary ore has been affected by later hydrothermal fluids. Sulfides are often enclosed or bordered by these secondary silicate phases. The ore consists primarily of pyrrhotite, pentlandite, chalcopyrite, with trace michenerite, cobaltite/gersdorffite, irarsite, hessite, tsumoite, and bismuth tellurides. Geological and petrological comparison of the 100 orebody and the adjacent 900 and 890 ore-bodies has revealed distinct similarities in ore distribution. Data show that the 100 orebody is depleted in platinum-group elements (PGE) relative to the 900 orebody. Platinum-group elements from the 100 orebody may have been hydrothermally remobilized and partly emplaced in the 900 orebody, in adjacent veins, or in magmatic breccia. Analysis of Cu, Ni, and PGE assay data of ore samples has characterized their distribution among the eight different ore textures in the orebody. There is a strong association between Pt and interstitial sulfide, and to a lesser extent massive sulfide, but little correlation between chalcopyrite-rich ore and PGE in the 100 orebody, unlike many other deposits in the Sudbury region.

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.

U-Pb geochronology of the sublayer environment, Sudbury igneous complex, Ontario

Cu-Ni sulfide deposits of the Sudbury Igneous Complex are hosted by the inclusion-rich sublayer, which occurs discontinuously between the footwall and main mass of the Sudbury Igneous Complex, by brecciated footwall in the vicinity of the Sudbury Igneous Complex, and by offset dikes protruding for tens of kilometers into the country rocks. The genesis of mafic-ultramafic inclusions in the sublayer and the relationships between the sublayer, main mass, and offset dikes have long been at the center of debate on Sudbury geology. This paper reports U-Pb isotope data on zircon and baddeleyite from samples representing norite matrix, melanorite and olivine melanorite pods, a metapyroxenite inclusion, and diabase from the sublayer at the Whistle embayment. These samples yield ages ranging from 1848.1 + or - 1.8 to 1849.1 + or - 1.1 Ma, similar to a U-Pb age of 1849.8 + or - 2.0 Ma obtained for zircons in quartz diorite from the Copper Cliff offset, and indistinguishable from the previously published average age of 1850 + or - 1 Ma for the main mass of the Sudbury Igneous Complex. None of the samples yield evidence for inherited zircon components. The isotopic data prove that the exotic mafic-ultramafic inclusions and pods in the sublayer are not xenoliths scavenged from older layered complexes but require the existence of primitive, moderately Mg-rich magmas that crystallized in the early stages of development of the Sudbury Igneous Complex. Such Mg-rich cumulates are unlikely to have crystallized from melted average upper crust.

Impact melt dikes in the Sudbury multi‐ring basin (Canada): Implications from uranium‐lead geochronology on the Foy Offset Dike

Abstract To investigate the origin of Offset Dikes and their age relationships to major impact generated lithologies in the Sudbury multi‐ring impact structure, such as the Main Mass of the Sudbury “Igneous” Complex, zircon and baddeleyite were dated by the U‐Pb chronometer. The rocks analysed are one diorite and two quartz diorites from inside the Foy Offset, one quartz diorite from the contact zone, and two country rock samples collected at 10 and 30 m distances from the contact within the Levack Gneiss Complex. The 21 analysed zircon and baddeleyite fractions yield a crystallization age of 1852 +4/‐3 (2σ) Ma for the accessory minerals in the Foy Offset Dike and an age of 2635 ± 5 Ma for the shocked Levack country rock, in which zircons show significant shock effects (multiple sets of planar fractures), in contrast to the totally unshocked zircons of the Offset Dike. Within given errors, the new age of 1852 Ma is identical to the pooled 1850 ± 1 Ma U‐Pb age determined by Krogh et al. (1984) as the crystallization age of accessory phases in different lithologies of the Sudbury “Igneous” Complex, which has been interpreted to represent the coherent impact melt sheet of the Sudbury Structure. This excellent agreement of the ages substantiates that emplacement of the Offset Dikes occurred coevally with the formation of the impact melt sheet. Total absence of inherited zircons in the central part of the Foy Offset indicates melting of the precursor material at temperatures well above 1700 °C, which emphasizes the origin of the dike lithologies by impact melting.

Floor‐fractured crater models of the Sudbury Structure, Canada: Implications for initial crater size and crater modification

Abstract The pattern of radial and concentric offset dikes at Sudbury strongly resembles fracture patterns in certain volcanically modified craters on the Moon. Since the Sudbury dikes apparently formed shortly after the impact event, this resemblance suggests that early endogenic modification at Sudbury was comparable to deformation in lunar floor‐fractured craters. Although regional deformation has obscured many details of the Sudbury Structure, such a comparison of Sudbury with lunar floor‐fractured craters provides two alternative models for the original size and surface structures of the Sudbury basin. First, the Sudbury date pattern can be correlated with fractures in the central peak crater Haldane (36 km in diameter). This comparison indicates an initial Sudbury diameter of between 100 and 140 km but requires loss of a central peak complex for which there is little evidence. Alternatively, comparison of the Sudbury dikes with fractures in the two‐ring basin Schrödinger indicates an initial Sudbury diameter of at least ∼ 180 km, which is in agreement with other recent estimates for the size of the Sudbury Structure. In addition to constraining the size and structure of the original Sudbury crater, these comparisons also suggest that crater modification may reflect different deformation mechanisms at different sizes. Most lunar floor‐fractured craters are attributed to deformation over a shallow, crater‐centered intrusion; however, there is no evidence for such an intrusion at Sudbury. Instead, melts from the evolving impact melt sheet probably entered fractures formed by isostatically‐induced flexure of the crater floor. Since most of the lunar floor‐fractured craters are too small (<100‐km diameter) to induce significant isostatic adjustment, crater modification by isostatic uplift apparently is limited to only the largest of craters, whereas deformation over igneous intrusions dominates the modification of smaller craters.

Genesis of the Foy (?) offset and its sulfide ores; the paleomagnetic evidence from a study in Hess Township, Sudbury, Ontario

A possible extension of the Foy offset, which outcrops to the north of the Sudbury basin, was examined at three localities in Hess Township. A specific geologic feature was examined at each locality: (1) a dike cutting the offset dike, (2) the contact between the country rock and a dike, and (3) a contact with a second dike and a sulfide zone. Analysis of these features yield the following results:Locality 1. Two older remanence directions of D = 346 degrees , I = 56 degrees (O1) and D = 287 degrees , I = 64 degrees (O8) occur in the offset, and a younger direction D = 056 degrees , I = 75 degrees (O6) occurs in the dike.Locality 2. A fully executed contact test was performed giving the dike direction of D = 051 degrees , I = 75 degrees (O6) in the offset.Locality 3. Magnetic susceptibility, remanence intensity, thermal demagnetization characteristics, and remanence directions provide evidence for five distinct zones in the offset at this locality. The fine-grained central portion of the offset marked by a crosscutting dike has a unique direction D = 040 degrees , I = 63 degrees and is geochemically distinct from the remainder of the offset at this locality. Two zones of sulfide mineralization on either side of this dike have distinct remanence directions. One of these is identical to the direction found in the barren diabase of both contact zones.From this data it may be concluded that in the area studied the Foy offset is a multiple intrusion. The remanence acquisition sequence, and therefore the sequence of intrusive events, is identical to that in the main irruptive. Hence the irruptive and the offsets have the same intrusive history. At least one phase of sulfide acquired its remanence direction at a time distinct from that related to the multiple intrusive events. Hydrothermal remobilization is the most probable mechanism.

Large Lateral Displacement on Garlock Fault, Califonia, as Measured from Offset Dike Swarm

A dike swarm in the Spangler Hills, north of the Garlock fault, is believed to be the offset extension of a similar swarm in the Granite Mountains, south of the fault. The projected intersections of the swarms with the fault are about 40 miles apart. The sense is left lateral. The swarm in the Spangler Hills can be traced north from the fault for about 145 miles; the swarm in the Granite Mountains can be traced south of it for at least 7 miles. Both swarms are about 12 miles wide. The dikes in the two swarms have similar widths; their trends differ by about 25, however, and this is attributed to rotation of the southern fault block. Fine-grained diorite porphyry forms most of the dikes in both swarms, although microcrystalline granite porphyry forms dikes near their eastern edges. The host rocks are mostly quartz monzonite or granodiorite. No other comparable swarms crop out along the fault. The dikes were probably emplaced during very late Mesozoic time. As the Garlock fault was probably formed in late Mesozoic or early Tertiary time, the offset of the vertical swarm approximates the total horizontal offset along the fault.

The "offset dikes" of the nickel intrusive. Sudbury, Ontario

The "offset dikes" of the nickel intrusive. Sudbury, Ontario