Abstract
Most of the world's major deposits of nickel and copper were formed as accumulations of magmatic sulfide liquid within small mafic-ultramafic intrusions. Ore-hosting intrusions exhibit a variety of forms but typically show greater horizontal than vertical extents, occurring as tube-shaped chonoliths, narrow elongate sills, boatshaped bodies, or sword blade-shaped dikes with originally horizontal principal axes. Sulfide accumulation at the downward termination of blade-shaped dikes is noted in a number of deposits worldwide. Based on evidence from continental dike swarms, Icelandic fissure eruptions, and active shield volcanoes, we suggest that these blade-shaped bodies formed by magma migration within vertical fractures that propagated laterally rather than vertically. Some examples, notably the bladed dikes of the South Raglan trend, have contact relationships indicating processes of active erosion, assimilation, and replacement of country rock. Where such dikes are injected into mixed volcanic-sedimentary sequences, thermal modeling of interaction with nonrefractory country rock predicts repeated formation and collapse of transient chilled margins against narrow zones of partially molten wall rock. Where the wall rock is sulfide-rich sediment, this generates a slurry of sulfide liquid, country-rock xenoliths, and chilled margin fragments that flows down the wall to accumulate at the bottom edge of the dike. This type of mixture is commonly seen as sulfide-matrix ore breccias in intrusion-hosted deposits. Widening of the conduit along zones of easily eroded country rock may result in a transition from a dike-like to a tubular chonolith geometry. Multiple magma pulses within the same magma conduit may give rise to complex superimpositions of ore zones, further complicated by formation of sulfide-silicate melting-infiltration fronts. Hence we argue that the bladed-dike geometry is an end-member type that commonly serves as a precursor form to a range of eventual geometric forms; recognition of this basic geometry provides general insights into the origins of other important deposits.
Highlights
Many of the world’s major Ni-Cu-platinum group element (PGE) sulfide deposits, including those at Noril’sk (Russia), Voisey’s Bay (Canada), Jinchuan and the Kalatongke-Huangshan nickel province in China, and the Midcontinent rift of the USA, are hosted by small mafic or mafic-ultramafic intrusions thought to have developed as conduits within magmatic plumbing systems (Naldrett and Lightfoot, 1999; Lightfoot and Evans-Lamswood, 2015; Barnes et al, 2016)
Where the wall rock is sulfide-rich sediment, this generates a slurry of sulfide liquid, country-rock xenoliths, and chilled margin fragments that flows down the wall to accumulate at the bottom edge of the dike, giving rise in ideal circumstances to a basal zone of sulfide-matrix ore breccia as seen at Savannah
All of the processes alluded to above—lateral propagation of dikes, widening of conduits due to preferential thermal erosion of country rocks, gravity flow of sulfide-silicate-xenolith slurries, and self-enhancing propagation of sulfide vein-dike networks into process zones in country rocks—coupled with postemplacement tilting and random intersection with present-day erosion surfaces, could work together to create the spectrum of mineralized intrusion geometries highlighted in Figure 1: sword blade-shaped dikes, boat-shaped intrusions with downward terminating keels, funnel-dike transitions such as Kalatongke (Fig. 1D), and potentially tube shaped chonoliths
Summary
Many of the world’s major Ni-Cu-platinum group element (PGE) sulfide deposits, including those at Noril’sk (Russia), Voisey’s Bay (Canada), Jinchuan and the Kalatongke-Huangshan nickel province in China, and the Midcontinent rift of the USA, are hosted by small mafic or mafic-ultramafic intrusions thought to have developed as conduits within magmatic plumbing systems (Naldrett and Lightfoot, 1999; Lightfoot and Evans-Lamswood, 2015; Barnes et al, 2016). The Expo Intrusive Suite comprises a group of originally vertical ultramafic dikes that were emplaced into a layered sequence of basalt and sediment on the northern margin of the Superior province, with longaxis orientations approximately parallel to the regional strike.
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