Formation of bifurcating chromitite layers of the UG1 in the Bushveld Igneous Complex, an analogy with sand volcanoes

Journal of the Geological Society, Nov 2004 by Nex, P A M

Abstract:

Chromitite layers occur frequently within the Critical Zone of the Bushveld Complex. In particular, Dwars River in the eastern Bushveld is well known for the intimate association of chromitite and anorthosite layers. An explanation has been deduced for bifurcations of chromitite layers and other features frequently found in exposures of the UG1 and its footwall. It is proposed that the unconsolidated footwall anorthosite experienced liquefaction possibly caused by major magma influx and associated seismicity. This resulted in structures analogous to those seen in sediments that undergo liquefaction, including sand volcanoes or boils. Bifurcations of chromitite layers occur where there is 'background sedimentation' of chromite at the same time as periodic extrusion of plagioclase plus magma slurry at the magma-cumulate-pile interface. These extrusions form 'waterlilies' of anorthosite that build up on top of each other to form domes. Simultaneously, chromite accumulates and forms multiple layers, which vary in thickness from those formed by a single layer of chromite grains to those over 0.5 m thick. Subsequent post-depositional structures suggest continued liquefaction in the footwall rocks, mobility of residual anorthositic magma and brittle deformation of chromitite layers.

Keywords: Bushveld Igneous Complex, chromitite, anorthosite.

The Bushveld Igneous Complex within South Africa is the world's largest layered intrusion and is justly famed for its magmatic ore deposits of chromite, platinum group elements and vanadium. It comprises volcanic rocks (Rooiberg Group), a mafic layered suite (Rustenburg Layered Suite) and sheeted granites (Lebowa Granite Suite) emplaced onto and within sediments of the Transvaal Supergroup (Fig. 1) at c. 2054-2060 Ma (Walraven & Hattingh 1993; Walraven et al. 1990; Buick et al. 2001). The layered mafic rocks of the Bushveld Igneous Complex have been divided into a number of zones based on the cumulate lithologies (Fig. 2a). The Lower and Marginal Zones are composed of laterally discontinuous norites, harzburgites and pyroxenites; the Critical Zone is made up of varying cycles of chromitite, pyroxenite, norite and anorthosite, which form spectacularly layered rocks. The Main Zone is dominated by gabbronorites and minor anorthosites, whereas the Upper Zone is formed by gabbros and ferrogabbros, and in addition is noted for its magnetitite layers. Chromitite layers are confined to the Critical Zone. This is subdivided into upper and lower portions, with the boundary between the two marked by the first appearance of cumulus plagioclase (Fig. 2b). Chromitite layers within the Critical Zone have been divided into lower, middle and upper groups (Cousins & Feringa 1964), each of which contains a number of chromitite layers. The focus of this study is the UGl chromitite layer, which is the lowest of the Upper Group chromitite layers. It is also the first substantial chromitite layer within the upper Critical Zone and is underlain by a much thicker anorthosite than any other chromitite layer.

Exposures of the UGl chromitite layer are well known from Dwars River in the eastern Bushveld (e.g. Wager & Brown 1968; Ashwal 1993; Cawthorn 1996). Outcrops at Dwars River are dominated by the intimate association of chromitite and anorthosite layers and also by bifurcating chromitite layers together with evidence of disruption and disturbance of the combined chromitite- anorthosite layers (Fig. 3a and b). The disturbances have been documented in part by Lee (1981), who broadly termed them 'post-depositional structures' and considered them the consequence of a reversed density gradient between the >1 m thick UGl chromitite layer and its underlying anorthosite foot-wall sequence. This paper is based on field and underground mapping of the UGl, and proposes a model for the formation of chromitite bifurcations and regards the disruption of chromitite and anorthosite layers as the culmination of multiple processes. Exposures of the UGl have been examined in detail both at Dwars River and underground at Impala, Rustenburg and Lonmin platinum mines in the western limb of the Bushveld Igneous Complex (lower inset, Fig. 1). Details of those features relevant to the genesis of chromitite bifurcations are presented here.

Field mapping

The immediate footwall (underlying lithology) of the UG1 chromitite layer is a mottled anorthosite that grades downwards into layered anorthosite and norite with minor pyroxenite. Mottled anorthosite is found frequently associated with structures associated with fluid or residual magma movement and exhibits both gradational and cross-cutting relationships with host rock cumulates. This has been noted before both from underground exposures of the Critical Zone (de Klerk 1982; Viljoen et al. 1986, fig. 24d) and surface exposures in the Main Zone (Quadling 1996). At Dwars River disharmonie folds within UGl footwall lithologies are interpreted as slump structures associated with fluid escape or residual magma escape and 'soft-sediment deformation' (e.g. Lee 1981). The footwall mottled anorthosites of the UGl contain ellipsoidal oikocrysts of clinopyroxene with a major axis up to 50 mm long. These oikocrysts frequently contain cores of remnant cumulus orthopyroxene with a corona of oikocrystic clinopyroxene (Fig. 4). These textures are consistent with a reaction between cumulus orthopyroxene and interstitial melt. This is in contrast to spotted anorthosites that contain occasional cumulus orthopyroxene and only minor interstitial, intercumulus clinopyroxene.

The UGl chromitite layer itself generally consists of multiple chromitite layers with interleaved anorthosite (Fig. 2c) although in some places it has been noted to consist of a single chromitite layer (de Klerk 1982; Lea 1996; this study). It is also important to note that chromitite bifurcations are not restricted to the UGl layer and are also present in the LG5 and UG2 chromitites (Hatton & von Gruenewaldt 1989; Lonmin Platinum Geology Department 2001). The chromitites of the UGl consist of chromite grains enclosed by oikocrystic plagioclase and orthopyroxene. Where enclosed by silicates the chromite grains are generally 0.1-0.2 mm in size. Where there is lower content of interstitial silicates then the chromite grains are larger (up to 2 mm), reflecting annealing of chromite grains during cooling (Hatton & von Gruenewaldt 1989).