Good reading on Ni-Cu deposits from the Geological Survey of Kanuckistan...
Nickel-copper deposits
As noted previously, these ores are characterized by an abundance of sulphide. Evidence indicates that much of the sulphur in the sulphides was derived from wallrocks. It is likely that this abnormal abundance of sulphur in the magma is what leads to considerable oversaturation of sulphur in the magma, thus producing large quantities of sulphide liquid. As mentioned previously, Ni, Cu and PGE enter preferentially into the sulphide liquid rather than the silicate magma. On cooling of the system, the liquid sulphide crystallizes over a large temperature range to eventually form the common mineral assemblage pyrrhotite-pentlandite-chalcopyrite.
* Meteorite-impact subtype
The Sudbury camp is the only representative of this type of Ni-Cu deposit in the world. Because meteorite impacts are random events on the earth's surface, there is no possible regional geological control on their distribution, with the exception that subsequent geological events could obscure or obliterate their traces. In the case of the resulting Sudbury Igneous Complex (SIC; Fig. 5), it is well preserved although strongly deformed by later compressional events.
The meteorite impact took place at 1850 Ma at the boundary between Neoarchean gneisses (about 2711 Ma) to the north and overlying Huronian Supergroup, Paleoproterozoic volcano-sedimentary rocks (about 2450 Ma) to the south. The impact produced a crater some 200 km in diameter, as well as radiating dyke-like fracture/breccia zones that penetrated the surrounding wallrocks. The impact melted the rocks of the target site and generated a high temperature melt layer that occupied the floor of the impact crater. On cooling, the melt differentiated into a lower norite unit, and an overlying granophyre, separated by a thinner gabbro layer. Contacts between these units are gradational, and finer scale layering is absent. A discontinuous more mafic basal layer termed the "sublayer" contains most of the Ni-Cu ores and abundant xenolithic clasts. The melt also invaded some of the radiating breccia zones, forming long quartz diorite dykes ("offsets") extending many kilometres outward from the SIC, which also contain Ni-Cu ores.
Subsequent regional overthrusting from the south compressed the southern half of the SIC and produced the presently exposed elongate basin 65 km long and 27 km across. The inward dip of the complex averages about 30° along the north range and 45° to 60° along the south range. The total thickness of the complex is about 2.5 km.
The target rocks contained significant amounts of sulphur in the form of sulphides, which in the melt produced abundant sulphide liquid that extracted nickel, copper and PGE from the silicate melt. This sulphide liquid along with abundant fragmental material segregated into a basal mafic noritic unit ("sublayer") and collected in depressions ("embayments") along the base of the melt sheet. The Murray mine is in such an embayment (Fig. 5). Sulphide liquid also accompanied melt into the offsets. On cooling, the sulphide liquid crystallized to form Ni-Cu-PGE ores. In some of the embayments, sulphide melt remaining after partial crystallization migrated downward from the SIC into breccia zones in the footwall rocks to produce particularly Cu- and PGE-rich sulphide ore veins and masses up to 400 m below the sublayer.
The resulting ore bodies associated with the sublayer at the base of the intrusion form irregular lenticular sulphide-rich masses, with the longest dimension plunging steeply as at the Murray mine on the South Range (Fig. 6A), and the Strathcona, McCreedy East and Fraser mines on the North Range (Fig. 6B). Clusters of such ore bodies, similarly oriented, lie in the embayments and persist to great depths as at the Creighton mine. The ore bodies in the offsets form discontinuous sulphide-rich sheets or lenses with steep dips sub-parallel to the associated quartz diorite offset. An example is the ore body in the Copper Cliff mine shown in Fig. 6C. A different kind of ore zone occurs at the Falconbridge East mine where the ore is irregularly strung out as discontinuous sheets along the Main fault which separates the felsic norite of the SIC from the Stobie volcanics (Fig. 6D). The deep Cu-PGE-rich ores in the footwall below the SIC form sets of sub-parallel stringers and veins of massive sulphides (Fig. 6B).
The sulphide ores consist of the typical magmatic sulphide minerals. In general order of abundance, they include pyrrhotite, pentlandite, chalcopyrite and pyrite. Bornite is present in copper-rich ores, and South Range ores typically contain arsenic minerals including niccolite, maucherite, gersdorffite and cobaltite. The platinum group elements occur as microscopic grains of numerous minerals, the most abundant of which are michenerite (PdBiTe), moncheite (PtTe2) and sperrylite (PtAs2).
Sudbury ores have many of the same textural features as other magmatic Ni-Cu sulphide ores. Massive ores (Fig. 7C) consist mainly of an annealed mosaic of subequant pyrrhotite grains with shreddy interstitial pentlandite. Breccia ores (Fig. 7D) contain rock clasts and silicate grains suspended in a matrix of sulphide (mostly pyrrhotite with patchy grains of pentlandite; chalcopyrite often penetrates the rock clasts). A distinctive feature of Sudbury sulphide-rich ores and the hosting sublayer is the presence of clasts of ultramafic rock, not exposed elsewhere but likely attributable to one of the target rocks impacted by the meteorite.
* Rift- and continental flood basalt-associated subtype
Ni-Cu deposits of the rift- and continental flood basalt-associated subtype are the products of the magmatism that accompanies intracrustal rifting events. They include the largest terrestrial deposit (Noril'sk-Talnakh, with 12.6 MmT of contained Ni) and several other near giant class deposits (e.g., Jinchuan, Voisey's Bay, Duluth). The features that these deposits tend to have in common are that they are associated with large magma systems, and that within these systems the sulphide ores tend to be associated with conduits or feeders to the larger igneous masses (in this last respect Duluth is an exception, and has not yet proven to be economic). Much of the sulphur in the sulphides has been derived by contamination of the magma through incorporation of sulphur from adjoining wallrocks. Once formed, and if in sufficient quantity, the sulphides tend to settle gravitationally in the moving magma, and collect in the conduits at points where magma velocity is reduced.
Noril'sk-Talnakh: The Ni-Cu-PGE ores of the Noril'sk-Talnakh camp are spatially associated with the huge Siberian flood basalt magmatic suite. In the Noril'sk-Talnakh area, the sedimentary strata form a gentle north-south trending syncline. Intruded into this sequence are elongate, gently dipping sill-like mafic bodies that lie below the 3.5 km thick lava sequence. These are the units with which the ores are associated (Fig. 8), and which are considered to be feeders to the overlying volcanic rocks. All the ore-bearing sills lie within 7 km of the NNE trending Karayelakh fault, which is thought to be part of the conduit system. The sills have thicknesses of a few tens of metres, lateral extents of a few hundred metres, and lengths of a few kilometers. They consist of a variety of layer-like gabbro-dolerite units (Fig. 9). The lowermost unit consists of a olivine-free gabbro-dolerite contact facies overlain by coarser-grained "taxitic" olivine gabbro-dolerite which passes upwards into picritic gabbro-dolerite. Olivine-free gabbro-dolerite and anorthosite units make up the upper partions of these bodies. The sills are enveloped by metamorphic aureoles of exceptional thickness (up to 200 metres), and hence are considered to have been conduits for the passage of very large volumes of magma.
Three distinct types of Ni-Cu-PGE ore occur in specific associations with the mineralized sills, and contribute to the total resources of the Noril'sk-Talnakh ore field (Table 1):
1. Massive sulphide ores occur as flat-lying sheets and lenses at the base of the sills, in some cases protruding downward into the footwall rocks (Figs. 8, 9). One such massive sulphide orebody attains a thickness of over 50 m and lateral dimensions of hundreds of metres. Some of the larger ore bodies display remarkable sulphide zonation ranging from pyrrhotite dominated chalcopyrite-pentlandite assemblages in the outermost and lower parts to progressively more copper-rich zones and eventually mainly Cu sulphides, chalcopyrite, cubanite and mooihoekite together with pentlandite in the central upper parts. The latter can have up to 25-30% Cu, 3-6% Ni, 50-60 ppm Pt and 60-200 ppm Pd. This zonation of sulphides is believed to result from fractionation in situ. The mechanism of early cumulate separation and basal segregation of a pyrrhotite-like iron sulphide leaves a copper-PGE-rich supernatant liquid to crystallize last. These Ni- and Cu-rich massive sulphide ores have been the mainstay of Noril'sk production for much of the camps history.
2. Copper breccia ores as semiconformable sheet-like zones occupy the upper contacts of the sills with the overlying rocks ("stringer-disseminated ores" in Fig. 9). The breccia comprises fragments of both the intrusion and wallrocks in a matrix of mainly massive sulphide. Sulphide stringers and disseminations accompany the breccias.
3. Disseminated sulphide ores form lenticular to layer-like horizons within picritic gabbro-dolerite units in the interior of the sills. The sulphides generally take the form of centimetre size spheres of chalcopyrite, pentlandite and pyrrhotite dispersed through the hosting gabbro-dolerite. This was the first mined ore type at Noril'sk, then it declined in importance with discovery of the massive sulphide ores. However it is presently a significant component in mining reserves again due to the high price of platinum.
* Komatiitic volcanic flow- and sill-associated subtype
Komatiitic Ni-Cu deposits are widely distributed in the world, mainly in Neoarchean and Paleoproterozoic terranes. Major Ni-Cu producing camps and other prominent deposits are found in Australia, Canada, Brazil, Zimbabwe, and Finland.
The komatiitic subtype of Ni-Cu sulphide deposits occurs for the most part in two different settings. One setting is as komatiitic volcanic flows and sills in mostly Neoarchean greenstone belts. Greenstone belts are typical terranes found in many Archean cratons, and may represent intracratonic rift zones. They are generally composed of strongly folded, basaltic/andesitic volcanics and related sills, siliciclastic sediments, and granitoid intrusions. They have been metamorphosed to greenschist and amphibolite facies, and typically adjoin tonalitic gneiss terranes. Komatiitic rocks form an integral part of some of these greenstone belts. Examples are the Kambalda camp and the Mt. Keith deposit, respectively, from two greenstone belts in Western Australia. The second setting is as Paleoproterozoic komatiitic sills associated with rifting at cratonic margins. Prime examples are the Raglan horizon in the Cape Smith-Wakeham Bay belt of Ungava, Quebec, and the Thompson camp of the Thompson nickel belt, northern Manitoba. The komatiitic rocks are set in a sequence of volcano-sedimentary strata unconformably resting on Archean basement, and moderately (Raglan) to intensely (Thompson) folded and deformed.
Ultramafic komatiitic rocks are magnesium-rich (18-32% MgO), and therefore the precursor magmas are very hot and fluid. Because of their primitive (high Mg, Ni) composition, the Ni:Cu ratio of the associated sulphide ores is high, in many cases 10:1 or more. The sulphur in the sulphide ores has been derived in significant proportion by contamination from sulphidic wallrocks. The commonly observed close spatial association of these deposits and their hosts with sulphidic sedimentary footwall rocks, and the similarity of sulphur isotopes and other chemical parameters of the magmatic and sedimentary sulphides strongly suggests that the sulphur in these deposits was derived locally from the sediments. This contrasts to some degree with deposits like Noril'sk and Voisey's Bay where, while it is clear that sulphur came from an extraneous source, that source was not likely so near at hand.
Two types of Ni-Cu sulphide ores characterize these deposits. Sulphide-rich ores comprising massive, breccia and matrix-textured ores (Figs. 7C, 7D and 7B, respectively) consisting of pyrrhotite, pentlandite and chalcopyrite occur at the basal contact of the hosting ultramafic flows and sills. These deposits are generally small, in the order of a few million tonnes, and the grades are in the 1.5 to 4 % range. The second type, sulphide-poor disseminated ore (Fig. 7A) forms internal lens-like zones of sparsely dispersed sulphide blebs, which consist mainly of pentlandite. Deposits of this type also occur in both sills and flows but the largest deposits are in sills, with ore tonnages of 10s to 100s of millions, though grades are a modest 0.6 to 0.9 % Ni.
The Timmins Nickel deposits are supposed to be like this group ...
* Komatiitic Ores in Greenstone Belt Setting
Canadian examples of this kind of Ni-Cu deposit are best developed in the Abitibi Greenstone Belt. The Alexo, Langmuir, Redstone and Texmont mines in the Timmins, Ontario area and the Marbridge mine in the Val d'Or area have been minor producers. The deposits in Western Australia are much larger and more economically significant.
Kambalda, Western Australia: Ni sulphide ores of the Kambalda camp are typical of the basal contact deposits associated with ultramafic flows in greenstone belts. They occur in the Kambalda Komatiite, which is a package of ultramafic flows (2710 Ma) that has been folded into an elongate doubly plunging anticlinal dome structure about 8 km by 3 km (Fig. 10). The underlying member of this succession is the Lunnon Basalt, and the overlying units are a sequence of basalts, slates and greywackes (2710 to 2670 Ma). The core of the dome is intruded by a granitoid stock (2662 Ma) whose dykes crosscut the komatiitic hosts and ores.
The Kambalda Komatiite is made up of a pile of thinner, more extensive "sheet flows" and thicker "channel flows" which have created channels by thermal erosion of the underlying substrate. The flows that contain ore are channel flows, which may be up to15 km long and 100 m thick, and occupy channels in the underlying basalt. Flows in the pile are commonly interspersed with interflow sediment, typically sulphidic.
Most of the ore bodies are at the basal contact of the lowermost channel flows (accounting for 80% of reserves), though some do occur in overlying flows in the lower part of the flow sequence (Fig. 11). The ore bodies typically form long tabular or lenticular bodies up to 3 km long and 5 m thick. The ores generally consist of massive and breccia sulphides (Fig. 7C, D) at the base, overlain successively by matrix-textured sulphides (Fig. 7B), and disseminated sulphides (Fig. 7A). The sediment that underlies the flow sequence is generally absent beneath the lowermost ore-bearing channel flow, due to thermal erosion by the flow. Structural deformation renders the shape and continuity of ores more complicated in many instances. Because of their weaker competency compared to their wallrocks, sulphide zones are in many cases strung out along, or cut off by faults and shear zones.
I think the Timmins deposits are more like the ones below in fact.
* Komatiitic Ores in Rifted Cratonic Margin Setting
There are two major Canadian nickel belts in rifted cratonic settings, both being segments of the Circum-Superior Belt that encircles a large part of the northern Superior province. One is the Raglan horizon in the Cape Smith-Wakeham Bay Belt in the Ungava peninsula of northern Quebec, and the other is the Thompson Nickel Belt in northern Manitoba.
Raglan Horizon, Cape Smith-Wakeham Bay Belt: The Raglan Horizon is a series of Ni-Cu ore-bearing komatiitic sills emplaced along the northern contact of the Povungnituk Group, at the base of the overlying Chukotat Group (Fig. 12). Together these form the southerly leading edge of the Cape Smith-Wakeham Bay Belt, northern Quebec, a thin-skinned thrust belt which overrides the Archean craton. The Povungnituk Group consists of basaltic and rhyolitic volcanic and clastic sedimentary rocks, the products of continental rifting. The Chukotat Group comprises massive and pillowed basalts and related mafic/ultramafic sills.
In addition to the Raglan Horizon of komatiitic sills along the Chukotat contact, there is another wide zone of komatiitic differentiated mafic/ultramafic sills in the interior of the Povungnituk Group. These Paleoproterozoic suites of komatiitic magmatic rocks (1918 Ma) differ from the greenstone type of komatiites in their lower MgO content (up to only 16 to 18 %) and consequently Ni:Cu ratios of the ores are lower, averaging about 3:1. There are a number of economic Ni-Cu deposits in the Raglan Horizon, and as well there are many Ni-Cu occurrences elsewhere in this horizon and in the ultramafic units lower in the Povungnituk Group. The Raglan sills appear to have richer, more abundant sulphide ore, likely because the clastic sediments they intrude are sulphide-rich, and have provided much of the sulphur that contributed to formation of the ores.
The Ni-Cu sulphide deposits of the Raglan Horizon have much the same development of ore types as the komatiitic greenstone deposits. The Raglan deposits are basal contact deposits consisting of massive and breccia sulphides at the basal contact, overlain in turn by matrix-textured ores and disseminated sulphides. Tectonic deformation has disrupted and mobilized some of the ore bodies. Because of their remoteness and accompanying higher production costs, only the richer ores can profitably be mined.
Thompson Nickel Belt: The Thompson Nickel Belt (TNB) is a portion of the Paleoproterozoic Circum-Superior Belt (Fig. 13), the rifted cratonic margin of the Archean Superior province. The Ni sulphide ores that characterize the Nickel Belt are associated with ultramafic komatiitic sills (1883 Ma) that intrude a sequence of Paleoproterozoic sedimentary cover rocks (Ospwagan Group). The latter consists of conglomerates, greywackes, iron formation, pelitic and calcareous sediments capped by mafic to ultramafic volcanics. Most rocks have suffered several periods of intense deformation and amphibolite to granulite facies metamorphism (about 1820 Ma). Paleoproterozoic strata are tightly infolded with the Archean basement gneisses. Original relationships are strongly deformed and obscured. The Belt on the northwest side abuts against the Paleoproterozoic Churchill province along the relatively late "Churchill-Superior Boundary Fault".
The ultramafic sills with which the ore is associated intrude the Pipe Formation of the Ospwagan Group. The Pipe Formation consists of pelitic schists and iron formations. All the known deposits in the Moak Lake - Thompson area are associated with sulphide iron formations of the Pipe Formation. The Pipe 2 and Birchtree ultramafic sills intersect a sulphide iron formation near the base of the Pipe Formation whereas the Thompson ultramafic sill intersects another sulphide iron formation that is higher in the same pelitic unit.
Intense deformation has produced unusual modifications of some of the nickel deposits. Some of the deformational features are due to the weak competency of massive sulphide relative to its wallrocks. The following descriptions are arranged in order of increasing deformational effects experienced by the various deposits. The Pipe 2 nickel deposit consists of massive and stringer sulphide concentrations forming a "U" shape around the nose of the folded ultramafic sill, and representing original basal contact suphide. The Manibridge mineralized ultramafic is laced with pegmatitic dykes that were mobilized out of the surrounding gneisses, and present problems for mining. The Birchtree mine has one ore zone that is an extensive sheet-like shear zone of massive and breccia sulphide. The Soab North Mine consists of a partly mineralized ellipsoidal boudin of ultramafic with a nearly complete enclosing sheath of massive and breccia sulphide. Ore in the Thompson mine is associated with a highly fragmented ultramafic sill, now dispersed into a zone of ultramafic boudins of all sizes aligned in a horizon within the pelitic schist unit. The ore consists of nickeliferous sulphides (pyrrhotite-pentlandite) as impregnations in the pelitic schist in a conformable zone that is coextensive with the ultramafic boudins. Massive sulphides are commonly coarsely recrystallized; pentlandite "eyes" up to several cm are not unusual.
* Other mafic/ultramafic intrusion-associated subtypes
The host mafic/ultramafic intrusions associated with these Ni-Cu sulphide deposits include a variety of types; multi-phase stocks (Lynn Lake, Proterozoic; Råna, Silurian), multi-phase chonoliths (Kotalahti, 1885 Ma), multi-phase sills (Kanichee and Carr Boyd Rocks, Archean), highly deformed sills (Selebi-Phikwe, Archean). The styles of mineralization are also varied, including massive sulphides, breccia sulphides, stringers and veins and disseminated sulphides. Voisey's Bay is the most important example
Voisey's Bay: The Ni-Cu sulphide ores at Voisey's Bay are associated with the troctolitic Voisey's Bay Intrusion, a part of the anorogenic Nain Plutonic Suite in Labrador. These deposits have similarities to those at Noril'sk in that the role of a feeder system appears crucial to the accumulation of sulphides.
The troctolitic intrusions (1290-1340 Ma), a member of which hosts the Voisey's Bay ores, straddle the collisional suture (~1850 Ma) between the Archean Nain province gneisses (2843 Ma) to the east and the Paleoproterozoic Churchill (Rae) province gneisses to the west (Fig. 14). These intrusions constitute a large magmatic system that includes granites, anorthosite, ferro-diorite and troctolite. Of significance for the Voisey's Bay Ni-Cu ores is that the Voisey's Bay Intrusion, which hosts the ores, intrudes sulphide-bearing Tasiuyak gneiss of the Churchill province. These gneisses appear to have been the source of much of the sulphur essential for forming the magmatic sulphides.
The Voisey's Bay Intrusion (Fig. 15A) consists of (1) a deep "Western Subchamber" of troctolite-olivine gabbro that is connected by (2) a subvertical mineralized feeder dyke of ferrodiorite, olivine gabbro and troctolite, which extends and flattens generally eastward for about 3 km to (3) the "Eastern Deeps" troctolitic chamber, the largest exposed part of the intrusion. Along this strike length, three main Ni-Cu sulphide zones constitute integral widened parts of the feeder dyke. The Reid Brook mineralized zone (Fig. 15B) in the west is a near vertical, thickened part of the feeder dyke with a central mineralized "Leopard Troctolite" (augite oikocrysts), sheathed in a mineralized breccia and transected by steep massive sulphide veins. The Ovoid deposit (Fig. 15C) is the richest ore zone. It is a flat-lying spoon-shaped lens of massive sulphide enveloped in mineralized "Leopard" and variable-textured troctolite and breccia representing a widened part of the feeder dyke. The Eastern Deeps zone (Fig. 15D) is located where the feeder dyke widens out into the base of the Eastern Deeps troctolite chamber. At the core of this junction is a massive sulphide lens that expands and extends into the Eastern Deeps chamber. The massive sulphide is enclosed in a complex mineralized sheath of variable textured troctolite, "Leopard" troctolite and breccia, similar to the assemblages accompanying the Reid Brook and Ovoid mineralized ores.
The feeder system and the Eastern Deeps zone are extensively mineralized in addition to the three zones mentioned. However these ores represent sulphide-enriched locations in the feeder system where it widened and slowed the through-going flow of magma. As a result, the suspended droplets of liquid sulphide settled gravitationally out of the flowing magma and produced accumulations of ponded liquid sulphides. These crystallized to form massive Ni-Cu sulphide. Each of the main ore zones include veins of crosscutting massive sulphide that transect the other rock units, attesting to the later mobility of liquid sulphide.
Sulphide assemblages consist of the usual pyrrhotite-pentlandite-chalcopyrite, with additional troilite and magnetite. Pyrrhotite grain size is exceptionally coarse, up to 20 cm, in the massive sulphide ore, while pentlandite forms finer exsolution grains and lamellae. The Ni, Cu, and Co resources for the Voisey's Bay deposit / camp are given in Table 2.
Platinum Group Element Deposits
Economic Platinum Group Element deposits are extremely rare. Two camps, Bushveld and Noril'sk-Talnakh, supply the majority of the world's PGE, though Noril'sk-Talnakh is not primarily a PGE deposit. Stillwater is the only other significant PGE producer of this subtype. Lac des Iles, small by comparison, is Canada's only producer of this subtype of deposit.
An obvious feature of the few economic PGE deposits in the world is the great size of their hosting intrusions. An apparent exception is the smaller Lac des Iles intrusion, but it is just one of a number of comagmatic intrusions in the area which together constitute a significant magma system. Because of the very low contents of PGE, large quantities of magma are necessary to provide an adequate source of PGE in order for it to be concentrated into significant economic deposits.
Another feature shared by most known examples is the small amount of sulphide (less than 3 %) with which the PGE are associated. The sparsely disseminated sulphide is mainly chalcopyrite but also includes pentlandite and pyrrhotite. The small amount of sulphide is due to the fact that the only sulphur involved is the original mantle sulphur, with little or no addition from the intruded wallrocks. Because the solubility of sulphur in mafic magmas is quite low, the amount of sulphide produced when the magma reaches saturation is very small, resulting in small, sparsely dispersed sulphides. This is in distinct contrast with Ni-Cu sulphide deposits in which the ore consists of rich concentrations of sulphide that result from so much sulphur having been added from wallrock.
The PGE minerals occur in very minute quantities that have apparently exsolved from the iron and base metal sulphides at various temperatures during cooling. They include a host of known as well as unnamed minerals. Pentlandite is the only common sulphide mineral that contains a significant amount of any PGE, and that is Pd.
Two distinct modes of PGE deposits are (1) the reef type, and (2) the magmatic breccia type. Of the two, only the reef type has proved to be a major producer.
* Reef Subtype
The reef or stratiform subtype of PGE deposits invariably occurs in large, well-layered mafic/ultramafic intrusions. The most important examples include the Merensky Reef and UG-2 chromitite reef of the Western and Eastern Bushveld, the J-M Reef of the Stillwater Complex, and the Main Sulphide zone of the Great Dyke. Other examples include the PGE zones in the Munni Munni (Australia) and the Rincon del Tigre (Bolivia) layered intrusions. All PGE reefs are typically more or less conformable, relatively thin layers (from less than one to a few metres)within the well-layered sequence of the intrusions. There are no known significant examples in Canada.
The genetic model of formation of the Merensky and J-M reefs remains controversial. Because of their great lateral extent (virtually a single layer within the whole of each large intrusion) and the thinness of the reefs, it is appealing to call on a magmatic process operating during the course of formation of the layered intrusions. The most generally accepted model involves the mixing of (a) the residual magma remaining after partial crystallization with (b) a new pulse of magma emplaced above it. It has been demonstrated experimentally that this mixing mechanism can induce sulphide saturation. The newly formed sulphide droplets thus produced then scavenge PGE from the silicate magma and settle to form a sparse sulphide concentration with a rich PGE content as a thin layer on the floor of the overlying magma. An alternative model proposes PGE carried upward by rising fluids.
Bushveld Complex: The Bushveld Complex is a mafic/ultramafic layered intrusion (2060 Ma) that extends over an area of 240 by 350 km in the Kapvaal craton (Fig. 16A). It is noted not only for its large size but also for the remarkable lateral extent of the Merensky Reef and the UG-2 chromitite, the two producing PGE horizons. The Complex's total thickness of over 7 km is made up of four stratigraphic zones: (1) the Lower Zone of bronzitites, harzburgites and dunites, (2) the Critical Zone of chromitite, pyroxenite, norite and anorthosite, and includes the Merensky Reef and UG-2 chromitite as well as numerous additional chromitites, (3) the Main Zone of norite and gabbronorite with minor anorthosite and pyroxenite, and (4) the Upper Zone of anorthosite, leucogabbro and diorite, notable for numerous magnetitite layers up to 6 m thick.
The whole of the sequence represents a simple progression of cumulus minerals (Fig. 16B), but the actual succession of layered units is complex. Much of the Critical Zone is made up of cyclic units, each consisting of all or part of an upward sequence of chromitite, pyroxenite, norite and anorthosite.
The Merensky Reef occurs near the top of the upper part of the Critical Zone, and the UG-2 chromitite at varying depths below the Merensky: about 30 m below at Union, 150 m below at Rustenburg and 350 m below near Lebowa. The Merensky Reef lies at the base of the Merensky cyclic unit, below the basal pyroxenite (Fig. 16C). It generally comprises a thin pegmatoidal feldspathic pyroxenite layer about 1 m in thickness, bounded above and below by very thin chromitite layers, and containing sparsely disseminated Cu-Ni sulphides (up to 3 %). The UG-2 chromitite occurs at the base of the UG-2 cyclic unit. It ranges from 70 to 130 cm in thickness, and has the same lateral extent as the Merensky Reef (see Fig. 16A). Estimated resources contained in the two reefs and the Platreef (discussed later) are shown in Table 3.
The PGE grade of the Merensky Reef is surprisingly uniform throughout the lateral extent of the unit, ranging between 4.9 and 7.3 g/t. This is despite considerable variation along strike in the platinum group mineral assemblages, which include alloys, sulphides, tellurides and arsenides.
A feature common to sulphide reef type deposits in layered intrusions is that they tend to occur at or some distance above the contact between the lower ultramafic zone and the upper mafic zone. The Bushveld and Stillwater reefs occur some distance above the contact, and the Hartley and Munni Munni reefs lie immediately below this contact.
* Magmatic Breccia Subtype
The magmatic breccia subtype of PGE mineralization is characterized by a large zone of sparsely disseminated sulphide in a mafic magmatic host that has a high proportion of breccia clasts, both cognate and exotic. The most important example of this subtype is the Platreef camp in the Northern Bushveld Complex, South Africa. Two similar Canadian deposits are in the River Valley intrusion and the Marathon deposit in the Coldwell Complex. These deposits all comprise semiconformable zones of PGE mineralization in a basal breccia unit of a layered mafic/ultramafic intrusion. The Lac des Iles PGE deposit in Canada is different from the preceding examples in that the intrusion is a multiphase stock-like body rather than a layered intrusion. Nevertheless the deposit comprises disseminated sulphide in a mafic magmatic breccia (Fig. 17) and on this basis is grouped in this subclass.
Lac des Iles: The Lac des Iles intrusion (2738 Ma) intrudes a Neoarchean gneissic tonalitic terrane. It is one of a 30 km diameter ring of similar intrusions, and on a larger scale, part of an ENE trending zone of mafic plutons. The intrusion consists essentially of a gabbronorite elliptical core, enveloped by a border unit of varitextured gabbro (Fig. 17). The Roby Zone (ore zone) lies between these two units at the west end of the intrusion and is made up of a combination of varitextured gabbro which is matrix to a heterolithic gabbro breccia. The varitextured gabbro contains abundant coarse-grained and pegmatitic patches, and the clasts in the heterolithic breccia are mostly cognate mafic rock types. A 20 m wide north-trending dyke-like pyroxenite lies between the Roby Zone and the barren gabbronorite to the east, and effectively marks the eastern boundary of mineralization.
The PGE mineralized Roby ore zone is 950 m long by 815 m wide and is distinguished by the presence of up to 3 % irregularly disseminated sulphides. These include chalcopyrite, pyrrhotite, pentlandite and pyrite as sub-millimetre to a few cm size grains and patches. Sulphide mineralization is coextensive with the varitextured gabbro breccia. PGE mineralization is mainly Pd (Pd:Pt = 9:1) and is locally erratically distributed, but on a mine scale is more or less uniform (Fig. 18). A higher-grade zone (about 5 g/t) is focused on a 400 m long portion of the western part of the pyroxenite dyke and a parallel portion of the adjoining varitextured gabbro/heterolithic breccia. Within this higher-grade zone the silicates are hydrothermally altered to amphibole, chlorite and saussuritized feldspar. Figure 18 is an west-east section showing the PGE grade distribution. The PGE minerals are mainly braggite, merenskyite and kotulskite.
The stock-like Lac des Iles PGE deposit may represent a conduit for mineralized magmatic breccia. If intruded to a higher level in the crust, such a magmatic breccia could have been emplaced as the stratiform basal PGE-mineralized breccia unit of a layered intrusion such as the Platreef, the River Valley intrusion or the Marathon deposit.
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Genetic and exploration models
Because magmatic Ni-Cu-PGE sulphide deposits are invariably associated with mafic and/or ultramafic magmatic bodies, such bodies constitute the first order target for exploration. From the preceding accounts it is clear that the different types of deposits are associated with different suites of mafic/ultramafic rocks, each of which have somewhat different but typical attributes.
District Scale
The Voisey's Bay discovery has emphasized, as is also the case at Noril'sk-Talnakh, the importance of relatively small intrusions as parts of large magmatic systems. Their role as conduits for large volumes of magma provides sites for accumulations of settled sulphide out of the passing magma. At Voisey's Bay, a dyke-like conduit led from one magma chamber to a higher one and contains the ores. At Noril'sk-Talnakh, sills are the conduits that appear to have fed the flood basalts, and in which the sulphide ores accumulated. Though of different geometries, the conduits record the passage of differing magmas by exhibiting significant differentiation: well layered at Noril'sk-Talnakh (Fig. 9), distinct dyke facies at Voisey's Bay (Fig. 15B, C). In the case of the Jinchuan deposits, the exposed ore-laden intrusion itself appears to be a feeder to a much larger layered magmatic complex now largely removed by erosion. Consequently, within a large mafic magmatic province, the target reduces to identifying smaller differentiated cognate intrusions that could represent magma conduits.
Komatiitic deposits occur in small to medium sized sills and flows that invariably include ultramafic rocks, either alone or with mafic differentiates, usually gabbros. Those in greenstone belts tend to occupy a limited range of stratigraphy at the district or regional scale. Thus they form clusters of ultramafic lenses along strike as at the Langmuir and Redstone mines near Timmins, Ontario, or whole formations as at Kambalda (Fig. 10). Similarly the komatiitic deposits in cratonic margin rift settings occur in lenticular ultramafic sills strung out along strike in long linear belts as at Thompson (Fig. 13) and Raglan Horizon (Fig. 12). These groupings of target rocks focus exploration at a district scale.
Ultramafic rocks associated with any of the deposit types have in most terranes (especially greenschist facies metamorphism) undergone serpentinization with the accompanying generation of magnetite. Consequently these bodies have well-defined magnetic response. Low-level aeromagnetic surveys thus are indispensable at early exploration stages, especially in poorly exposed areas.
Large layered intrusions have generally been recognized in many regions, but there may still be unidentified bodies in some little mapped areas. Magnetic and gravity surveys could be of use in this circumstance.
Local Scale
Sulphide-rich Ni-Cu deposits achieve their concentrations mostly through the settling effects of gravity. Consequently in virtually all magmatic bodies (sills, flows and dykes), the sulphide-rich ores are most likely to be found at the base of those bodies. Determination of the base of a given body is thus an important part of exploration targeting. However if it is clear that faulting is a possibility, the distribution pattern of sulphide-rich zones may be more complex. For instance in the Thompson Nickel Belt, some of the sulphide ores are extended far beyond the parent ultramafic bodies.
Electromagnetic surveys designed to detect conductors should be effective on the sulphide-rich (i.e., massive, breccia and matrix-textured sulphides) deposits. IP methods may identify disseminated sulphides, but presence of serpentinization in the same body may render the technique ineffective.
Within the komatiitic greenstone belt type, the ores are generally located in the lowest flow, which is also the first and generally most primitive in the pile of flows. Some ores may lie at a somewhat higher level.
Because chromite is commonly an associated mineral, geochemical surveys should include Cr as well as the obvious suite consisting of Ni, Cu, Co, Pt and Pd.
Recent Advances
A much better appreciation of the role of magma dynamics in the concentration and enrichment of magmatic Ni-Cu-PGE sulphide deposits has developed in the last decade or two. The importance of changes, particularly decreases in the rate of flow of magmas has become clearer. The location of sulphide concentrations in conduits at Talnakh-Noril'sk and Voisey's Bay, and near conduits in certain of the komatiitic deposits suggests that sulphides accumulate where the flow rate of magma was reduced and the entrained sulphides were able to settle gravitationally to form rich basal concentrations.
Nickel depletion of mafic magmatic rocks in connection with the existence of Ni sulphide deposits has become better documented. It was anticipated that the formation of nickeliferous liquid sulphide in a magma resulted by extraction of nickel from the magma, thereby leaving the magma depleted in nickel. Documentation has supported this theory, and it now plays a part in exploration strategy.
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Knowledge gaps
The nature of science is that there is always more to learn. Trying to define what we need to learn is one of the great challenges of science, and it is as true here as elsewhere.
One of the gaps in our knowledge of Ni-Cu sulphide deposits is knowing what is the most important factor in triggering sulphide saturation in a given magma. Certain things are clear. The magma must have a sufficient dissolved content of Ni, Cu and PGE. Once a liquid sulphide is formed, it will tend to equilibrate with the magma, and this means acquiring the Ni, Cu and PGE from the magma according to the partition coefficients for those elements. It also is clear that much of the sulphur in magmatic Ni-Cu sulphide deposits has been derived from sulphidic wallrocks, usually clastic pyritic sediments. Thus addition of sulphur to the magma by incorporation of such material leads to sulphide saturation. However it is also known that by increasing the silica content of the magma through incorporation of siliceous wallrock, the solubility of sulphide in the magma is decreased, thereby producing sulphide saturation. It remains unclear, which of the two mechanisms is the more critical in producing sulphide saturation. The significance for exploration is whether it is essential to have wallrock rich in sulphide as a source of sulphur in order to better evaluate a priori the nickel potential of a given mafic/ultramafic body. Existing evidence tends to favor the sulphidic wallrock theory, but more investigation of the settings of known nickel sulphide deposits seems worthwhile to evaluate the importance of the alternative theory.
In the case of PGE reef type deposits, there is still ongoing controversy over the main mechanism of concentration of PGE in the thin extensive "reefs" that are hosted in very large layered mafic/ultramafic intrusions. As noted previously, the magmatic theory emphasizing magma mixing is the more favored, but a "fluids from below" theory has some persuasive arguments. This controversy will undoubtedly continue. It is unclear whether there are important exploration ramifications contingent on this question.
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