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Lightfoot P.C.,Vale | Keays R.R.,Monash University | Evans-Lamswood D.,Vale Newfoundland and Labrador Ltd | Wheeler R.,Vale Newfoundland and Labrador Ltd
Mineralium Deposita | Year: 2012

The Voisey's Bay deposit is hosted in a 1.34-Ga intrusion composed of troctolite, olivine gabbro, and ferrogabbro. The sulfide mineralization is associated with magmatic breccias that are enveloped by weakly mineralized olivine gabbros and troctolites, and also occurs as veins along structures in adjacent paragneiss. A dyke is connected to the base of the north wall of the Eastern Deeps Intrusion, and the entry point of this dyke into the chamber is the locus of the Eastern Deeps nickel sulfide deposit. A detailed exploration in the area between the Eastern Deeps and the Ovoid has shown that these intrusions and ore deposits are connected by a splayed dyke. The Eastern Deeps Deposit is surrounded by a halo of moderately to weakly mineralized variable-textured troctolite (VTT) that reaches a maximum thickness above the axis of the Eastern Deeps Deposit along the northern wall of the Eastern Deeps Intrusion. The massive sulfides and breccia sulfides are petrologically and chemically different when compared to the disseminated sulfides in the VTT, and there is a marked break in Ni tenor of sulfide between the two. Sulfides hosted in the dyke tend to have low metal tenors ([Ni] 100 = 2.5-3. 5%), sulfides in Eastern Deeps massive and breccia ores have intermediate Ni tenors ([Ni] 100 = 3.5-4%), and disseminated sulfides in overlying rocks have high Ni tenors ([Ni] 100 = 4-8%). Four principal processes control the compositions of the Voisey's Bay sulfides. Coarse-grained loop-textured ores consisting of pyrrhotite crystals separated by chalcopyrite and pentlandite exhibit a two orders of magnitude variation in the Pd/Ir ratio which is due to mineralogical variations where pentlandite is enriched in Pd and Ir is dispersed throughout the mineral assemblage. A decrease in Ir and Rh from the margin of the Ovoid toward cubanite-rich parts at the central part of the Ovoid is consistent with fractionation of the sulfide from the margins toward the center of the Ovoid. The Ovoid ores have higher Ni and Pd tenor than the Eastern Deeps massive sulfides; this is consistent with both a higher R factor and greater degree of silicate parental magma evolution in the Ovoid than the Eastern Deeps. The disseminated sulfides surrounding the Eastern Deeps deposit have some of the highest Ni and Pd tenors at Voisey's Bay, which are indicative of not only more primitive magmas but also higher R factors than the Ovoid or the Eastern Deeps. VTT and normal-textured troctolite of the Eastern Deeps that contain trace sulfide have 0.1-3 ppb Pt and 0.1-3 ppb Pd, whereas weakly to heavily mineralized variable troctolites in the same unit have one to two orders of magnitude higher abundances of Pt and Pd. Troctolites and olivine gabbros from other parts of the Voisey's Bay Intrusion and other Nain Plutonic Suite Intrusions, including the Kiglapait, Newark Bay, Barth Island, Mushua, and Nain Bay South Intrusion, also have low platinum group element abundances. Although it is possible that this is a signature of a widespread sulfide saturation event that pre-dated ore formation at Voisey's Bay, it is more likely that platinum group element (PGE) depletion is a product of the source melting process where low degrees of melting resulted in the retention of PGE in the mantle source. If so, this indicates that PGE depletion should be used with caution as an exploration tool in the Nain Plutonic Suite. © 2011 Springer-Verlag.

Deposits of Ni-Cu-Co-(PGE) sulfide often occur in association with small differentiated intrusions that reside within local transtensional spaces in strike-slip fault zones. These faults often develop in response to incipient rifting of the crust and the development of large igneous provinces due to far-field stresses generated by plume-induced continental drift. We review the geology of a number of large and small nickel sulfide deposits and the associated intrusions, and show that the geometry of the host intrusion and localization of the mineral zones can be classified into three main groups. Further, we show that the morphology of each is controlled by space created in response to deformation on structures. One group of intrusions has the plan shape of an asymmetric rhomboid with the long axis sub-parallel to a fault zone, and contacts which have often been structurally modified during and/or after emplacement of themagma. The typical cross section is a downward-closing cone shape with curved walls and often a dyke-like keel at the base. This morphology is found in the Ovoid and Discovery Hill Zones of the Voisey's Bay Deposit (Canada), the Jinchuan, Huangshan, Huangshandong, Hongqiling, Limahe, Qingquanshan, and Jingbulake (Qingbulake) Intrusions in China, and the Eagle and Eagle's Nest deposits in the USA and Canada, respectively. A second group of deposits is associated with conduits within dyke and sheet-like intrusions; these deposits are often associated with discontinuities in the dyke which were created in response to structural controls during emplacement. Examples include the Discovery Hill Deposit and the Reid Brook Zone of the Voisey's Bay Intrusion, where there are plunging domains of thicker dyke which control the mineralization inside the dyke, and thin discontinuous segments of the dyke which are associated with structurally controlled mineralization in the surrounding country rock gneisses. The Oktyabrysk, Taimyrsk, Komsomolsk, and Gluboky Deposits in the Noril'sk Region of Russia are localized at the base of thicker parts of the Kharaelakh Intrusionwhich appear to be a conduit that follows synformal features in the country rocks located west of the Noril'sk-Kharaelakh Fault. Other examples of dyke-like bodies with both variation inwidth and the development of discontinuities are the Copper Cliff and Worthington Offset Dykes which radiate away from the Sudbury Igneous Complex (Canada). The distribution of ore bodies in these Sudbury Offset Dykes is principally controlled by variations in the thickness of the dyke, interpreted to reflect the presence of conduits within the dyke. A third group ofmineralized intrusions locatedwithin structural corridors have the geometry of oblate tubes; examples include Kalatongke in China, Northeastern Talnakh and Noril'sk 1 in Russia, Babel-Nebo in Australia, and Nkomati in South Africa. Sometimes these oblate tube-like intrusions formin bridging structures between larger intrusions hosted in themore significant structures. Examples include the Tamarack Intrusion in Minnesota, USA, and the Current Lake Complex in Ontario, Canada, both ofwhich containmagmatic Ni-Cu sulfide mineralization. In all of these deposits, the intrusions appear to be open system magma pathways, and so the term "chonolith" can be applied to describe them as a group. All of these intrusions are characterized by a high ratio of sulfide/silicate; there are 1-3 orders ofmagnitude more sulfide in the intrusion than themagma contained in the intrusion is capable of dissolving. The formation of these deposits is considered to have taken place in open systemmagma conduits. It is possible that the metal tenor of the sulfideswere upgraded by equilibration of successive batches of silicate magma passing through the conduit, and equilibrating with a stationary pool of magmatic sulfide. At Voisey's Bay there appears little doubt that the sulfides were injected through a conduit dyke into higher level magma chambers. A similar model has been proposed for the formation of the deposits at Jinchuan and Noril'sk-Kharaelakh. Economically significant nickel sulfide deposits that tend to be high in Ni tenor, are often related to the late injection ofmagma that formdistinct parts of the intrusion, and the localization of mineralization tends to be related to changes in the geometry of the magma chamber. Strongly deformed and metamorphosed komatiite-associated deposits (e.g. Pechenga, Thompson, and the Yilgarn komatiite associations)appear to be the remains of open system magma conduits which are now represented by segmented and boudinaged ultramafic bodies as a result of more than 4 phases of post-emplacement deformation. LIP activity at craton margins has long been recognized as a key control on the genesis of magmatic sulfide deposits; we showthat the principal regional controls of strike-slip tectonics underpin the local geometry of the intrusions, and we provide an explanation for why so many of the global nickel sulfide ore deposits are associated with intrusions that share commonmorphologies and characteristics. Thismodel provides a framework for more detailed structural investigations of nickel sulfide deposits, and it is a predictive framework for mineral exploration. © 2014 Elsevier B.V.

Lightfoot P.C.,Vale | Evans-Lamswood D.,Vale Newfoundland and Labrador Ltd | Wheeler R.,Vale Newfoundland and Labrador Ltd
Northwestern Geology | Year: 2012

The Voisey's Bay Ni-Cu-Co sulfide deposit is hosted in a 1. 34 Ga mafic intrusion that is part of the Nain Plutonic Suite in Labrador. Canada. The Ni-Cu-Co sulfide mineralization is associated with mag-matic breccias that are typically contained in weakly mineralized olivine gabbros, troctolites and ferrogab-bros, but also occur as veins in adjacent paragneiss. The mineralization is associated with a dyke-like body which is termed the feeder dyke. This dyke connects the shallow differentiated Eastern Deeps chamber in the east to a deeper intrusion in the west termed the Western Deeps Intrusion. Where the conduit is connected to the Eastern Deeps Intrusion, the Eastern Deeps Deposit is developed at the entry line of the dyke along the steep north wall of the Eastern Deeps Intrusion. The Eastern Deeps Deposit is surrounded by a halo of moderately to weakly mineralized Variable-Textured Troctolite (VTT) that reaches a maximum thickness above the ENE-WSW axis of the Eastern Deeps Deposit At depth to the west, the conduit is adjacent to the south side of the Western Deeps Intrusion, where the dyke and intrusion contain disseminated magmatic sulfide mineralization. The Reid Brook Zone plunges to the east within the dyke, and both the dyke and adjacent paragneiss are mineralized. The Ovoid Deposit comprises a bowl-shaped body of massive sulfide where the dyke widens near to the present-day surface. It is not clear whether this deposit was developed as a widened-zone within the conduit or at the entry point into a chamber that is now lost to erosion. The massive sulfides and breccia sulfides of the Eastern Deeps are petrologically and chemically different when compared to the disseminated sulfides in the VTT; there is a marked break in Ni tenor (Ni content in 100% sulfide, abbreviated to [Ni] 100) and Ni/Co of sulfide between the two. The boundary of the sulfide types is often marked by strong sub-horizontal alignment of heavily digested and metamorphosed paragneiss fragments, development of barren olivine gabbro, and by a change from typically massive sulfides and breccias sulfides into more typical variable-textured troctolites with heavy to weak disseminated sulfide. Sulfides hosted in the feeder dyke tend to have low metal tenors ([Ni] 100 = 2. 5%-3. 5%); sulfides in Eastern Deeps massive and breccia ores have intermediate Ni tenors ([Ni] 100= 3. 5%-4%) and disseminated sulfides in overlying rocks have high Ni tenors ([Ni] 100 = 4%-8%). Conduit-hosted mineralization and mineral zones in the paragneiss adjacent to the Reid Brook Deposit tend to have lower Ni tenor than the Ovoid and Eastern Deeps Deposits. The tenor of mineral hosted in the country rock gneisses tends to be the same as that developed in the conduit; the injection of the sulfide into the country rocks likely occurred before formation of monosulfide solid solution. The Ovoid Deposit is characterized by coarse-grained loop-textured ores consisting of 10cm-2m sized pyrrhotite crystals separated by chalcopyrite and pentlandite. A small lens of massive cubanite surrounded by more magnetite-rich sulfide assemblages represents what appears to be the product of in-situ sulfide fractionation. Detailed exploration in the area between the Reid Brook Zone and the Eastern Deeps has shown that these intrusions and ore deposits are connected by a branched dyke and chamber system in a major west-east fault zone. The Eastern Deeps chamber may be controlled by graben-like fault structures, and the marginal structures appear to have controlled dykes which connect the chambers at different levels in the crust. The geological relationships in the intrusion are consistent with emplacement of the silicate and sulfide laden magma from a deeper sub-chamber (possibly a deep eastward extension of the Western Deeps Intrusion where S-saturation was initially achieved). The silicate and sulfide magmas were likely emplaced through this conduit into the Eastern Deeps intrusion as a number of different fragment laden pulses of sulfide-silicate melt that evolved with different R factors and in response to some variation in the degree of evolution of the parental magma. S isotope and S/Se data coupled with geological evidence point to a crustal source for the sulfur, and the site of equilibration of mafic magma and crustal S is placed at depth in a sulfidic Tasiuyak Gneiss. The structural control on emplacement of small intrusions with transported sulfide is a feature found in different nickel sulfide deposits around the world. Champagne glass-shaped openings in sub-vertical chonoliths are a common morphology for this deposit type (e. g. the Jinchuan, Huangshan, Huangshan-dong, Jingbulake, Limahe, Hong Qi Ling deposits in China, the Eagle deposits in the United States, and the Double Eagle deposit in Canada). Some of the structures of the Midcontinent Rift of North America also host Ni-Cu-(PGE) deposits of this type (e. g. the Current Lake Complex in the Quetico Fault Zone in Ontario, Canada and the Tamarac mineralisation in the Great Lakes Structural Zone of the United States). Other major nickel deposits associated with flat structures adjacent to major mantle-penetrating structures include the Noril'sk, Noril'sk II, Kharaelakh, NW Talnakh, and NE Talnakh Intrusions of the Noril'sk Region of Russia, the Kalatongke deposit in NW China, and Babel-Nebo in Western Australia. These deposits are all formed in mantle-penetrating structural conduits that link into the roots of large igneous provinces near the edges of old cratons.

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