Blade-Shaped Dikes and Nickel Sulfide Deposits: A Model for the Emplacement of Ore-Bearing Small Intrusions

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.

3D Geological Modeling of the Double Eagle – Black Thor Intrusive Complexes, McFaulds Lake Greenstone Belt, Ontario, Canada

“…The resulting model is expressed in three-dimensions as a flattened tube measuring up to 120 m wide by at least 300 m long and constrained by exploration drilling to a depth of 1300 m (Fig. 4.7b), more-or-less corroborating previous descriptions of the Eagle's Nest conduit (see the discussion section of this Chapter; Mungall et al, 2010;Zuccarelli et al, 2017;Barnes and Mungall., 2018).…”

“…By returning the tilted geometry of the restored complex to its original stratigraphic orientation, the boat-shaped structure of the ultramafic sills is emphasized with downward dike-like keels (Fig. 4.10c; Tuchscherer et al 2010;Carson et al 2015;Spath et al 2015;Spath et al 2017;Barnes and Mungall, 2018), specifically within the BTIC. Between the DEIC and BTIC, the geometry of the granodiorite-ultramafic contact resembles a possible high in the granodiorite.…”

“…Along the eastern contact of the ultramafic-mafic intrusion in the BTIC, truncation of the overlying bimodal metavolcanic unit (Fig. 4.11) indicates that active erosion and assimilation of country rock (Laarman, 2014) were main processes involved during magma emplacement (Barnes and Mungall, 2018).…”

“…Evidence for the assimilation of country rock in the Eagle's Nest conduit is present along its southern margin where thermal and thermomechanical erosion has led to stoping of the conduit roof (Fig. 4.12a and b) and preferential widening of the side walls (Barnes and Mungall, 2018;Barnes et al, 2015). Although the irregular lobate shape of the conduit (Fig.…”

“…(Schulte et al, 2010). These intrusions typically form as funnel-, saucer-or boat-shaped sill-like bodies with prominent magmatic layering (Irvine, 1975;Barnes and Mungall, 2018), extending as much as 180 km along strike and up to 15 km thick. However, the vertical extent of some intrusions is still unknown due to erosion and post-ore deformation (e.g., Rustenberg Layered Suite of the Bushveld Complex, South…”