MINING GEOLOGY/Magmatic Ores 643 from mantle-derived mafic, or ultramafic silicate magmas in lava channels, magma conduits, or places where conduits feed into larger bodies The Sudbury mining camp, Canada, the world’s largest collection of Ni-Cu-PGE deposits, is a unique exception The Sudbury Igneous Complex and its base-metal sulphide ore deposits formed by the batch equilibration and gravitational segregation of sulphide liquid with several thousand km3 of shock-melted continental crust following a large meteorite impact into a shallow marine basin filled with sulphidic sediments Base Metal Sulphide Deposits A more common path to sulphide saturation in fertile, previously sulphide-undersaturated magmas is the assimilation of crustal rocks rich in sulphide This has the combined effect of lowering the temperature of the magma while sharply increasing the amount of sulphur it contains Addition of large quantities of sulphide can drive a magma into a state of extreme sulphide-oversaturation, so that the sulphide equilibrates at a relatively low silicate/sulphide ratio (100 < R < 5000; eqn [3]) The result can be the generation of sulphide in sufficient quantity to separate and form economically attractive tonnages of the base metal sulphides, however, at these relatively lower silicate/ sulphide ratios, the concentrations of the precious metals in the resulting sulphide tend to contribute a relatively small part of the total value of the ores Emplacement of magmatic sulphides The dominant mode of transport of magmas in the crust is by the emplacement of dykes and sills Figure provides schematic illustrations of the most important mechanisms for sulphide segregation Two phenomena related to dyke and sill emplacement are of particular importance in the generation of magmatic sulphide deposits These are the formation of long-lived conduits, and the formation of channelized lava flows Because thin, tabular sheets of magma cool rapidly, flow along dykes and sills tends to become focused along their widest portions, which then enlarge themselves by heating and assimilating their wall rocks The result is the generation of long-lived bladeshaped or tubular magma conduits in which flowing magma cools relatively slowly Similarly, large fissure-fed lava flow fields are thought to develop distributary systems resembling deltas, in which deeply incised lava channels proximal to the vent feed smaller branching channels and distal sheet or pillowed flow lobes Within either a conduit or a lava channel, it is possible for a steady state to be reached, in which the temperature of magma passing a given point remains constant for extended periods of time If the rate of supply of sulphide by degassing or Figure Cartoon of principal mechanisms for the generation and segregation of magmatic base metal sulphides The image shows a cross section of the upper crust, with brown areas representing sulphur rich crustal rocks which act as sources of sulphur for dykes and plutons of mantle derived magma (darker green) or lava flows (lighter green) Sulphide liquid collects where it formed (Duluth), in traps within conduits or where duits enter magma chambers (Voisey’s Bay), in subvolcanic sills (Noril’sk), or in lava channels cutting through sulphidic sediments (Kambalda) erosion of the channel floor or conduit walls, and the rate of cooling and crystallization also approach a steady state, then sulphide liquid first reaches saturation in the magma always as it passes a single point within the conduit or channel Sedimentation of sulphide droplets from the flowing lava, perhaps also with a co-saturated phenocryst phase (olivine or pyroxene), can thus lead to highly efficient sorting of the phase and the elements it concentrates from a very large volume of magma into a small, localized volume of rock A related possibility with a similar result is that a magma which has become saturated with sulphide liquid may carry the exsolved sulphide suspended as small droplets in the rapid and turbulent flow within the conduit A transition within the dyke to laminar flow or a sudden drop in flow velocity due to its passage through a magmatic breccia or a widening of the conduit will then cause the entrained sulphides to settle together in one place within the conduit In practice it may be impossible to distinguish between the two sulphide collection mechanisms described above Classic examples of valuable sulphide deposits accumulated from flowing magma in conduits or lava channels are given in Table 2, with the Sudbury Camp for comparison If sulphides are deposited in the place at which sulphide saturation occurred, without being transported and collected from a large volume into a small concentration, then a low-grade disseminated