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Crystallization Sequences in the Muskox Intrusion and Other Layered Intrusions-II

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Crystallization Sequences in the Muskox Intrusion and Other Layered Intrusions-II Ceoohimicaet CosmochimicaActa, 1975, Vol. 39, pp. 991 to 1020. PergamonPress. Printed in NorthernIreland Crystallization sequences in the Muskox intrusion and other layered intrusions-II. Origin of chromitite layers and similar deposits of other magmatic ores T. N. IRVINE Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20008, U.S.A. (Received 20 July 1974; accepted in. revised form 30 September 1974) Abstract-A mechanism of origin for chromite-rich layers in stratiform ultramafic-gabbroic intrusions is proposed whereby the layers are precipitated on occasions when the basic parental magma of the intrusion is suddenly extensively contaminated with granitic liquid melted from salic roof rocks. It is inferred that the increase of silica and alkalies in the basic liquid should cause it to become more polymerized with a lower frequency of octahedral sites, so that on continued crystallization, Crs+ is preferentially expelled (into chromite) owing to its large octahedral crystal-field stabilization energy. The feasibility of this process is demonstrated by experimental data on forsterite-picrochromite crystallization relations in the system K,O-&IgO- CrsOs-SiO,, and its apparent applicability to magmas is illustrated through a comparison of the differentiation patterns of Cr and Wi in the Muskox intrusion. The granitic melt is produced because most of the crystals formed in the intrusion accumulate on its floor, leaving the roof rocks to be continuously exposed to the high temperature of the basic magma. Between episodes of contamination, the melt tends to accumulate on top of the basic magma and to remain separate because of its low density and high viscosity. If not assimilated, it eventually resolidifies as granophyre. In the Muskox intrusion there are two chromite-rich layers, each occurring in a stratigraphic unit showing the layer sequence, peridotite-chromitite-orthopyroxenite. This sequence is explained in a model in which the basic magma is contaminated while coprecipitating olivine and minor chromite. A period follows when chromite precipitates alone, and then, because the liquid is enriched in silica, orthopyroxene crystallizes instead of olivine. Variations on the model are described that simulate the main layer sequences involving chromitite in the Stillwater, Great Dyke and Bushveld intrusions. Evidence of contamination is found in the concentrated chromite crystals in the form of small spherical, composite silicate inclusions, rich in alkalies, apparently representing trapped droplets of the contaminant granitic melt in various stages of assimilation. It is suggested that the same type of contamination mechanism may also yield concentrated deposits of magnetit,e and of immiscible sulphide liquid. INTRODIJCTION ONE OB the most fascinating occurrences of the mineral chromite is as thin concen- trated layers in stratiform ultramafic-gabbroic intrusions. These layers are especially common in the Bushveld Complex and Great Dyke in southern Africa and in the Stillwater Complex in Montana, bodies in which dozens of examples ranging from a few inches to about 15 ft in thickness have been traced for distances of tens of miles.* The layers are of interest both as major ore deposits and as a remarkable phenomenon suggestive of important igneous processes. * In the Stillwater Complex, there are several zones of chromitite layers ranging from a few inches to about 15 ft in thickness that have been traced for 15 miles and one that extends for almost 30 miles (JACKSON, 1963). In the Bushveld Complex, the Leader and Steelpoort chromite layers or ‘seams,’ which are respectively 1 and 4 ft thick, have been traced together with several thinner seams for more than 40 miles (CAMERON and DESBOROUGH, 1969), and they are roughly 991 992 T. N. IRVINE Itis generally agreed that the layers are deposits of chromite crystals settled from magma, and some occurrences show strong evidence of having accumulated under the influence of currents (e.g. CAMERON and DESBOROUGH, 1969). There have been various suggestions as to the mechanism of chromite enrichment--that it was concen- trated by current sorting or that it was precipitated preferentially in response to changes of pressure, water content, or oxygen fugacity in the magma-but none of the advocated processes would appear to explain the variety of features and associ- ations shown by chromitite layers, and none has been developed in terms of the overall crystallization history of an intrusion. In the present paper an attempt is made to outline and substantiate a mechanism of origin for chromitite layers based mainly on two occurrences in the Muskox intrusion in the Canadian Northwest Territories. The Muskox chromite-rich layers are unimposing in comparison with many of the occurrences referred to above. One is everywhere less than an inch thick; the other at its best consists of only a 4-in. unit of concentrated chromite in a total zone of chromite enrichment of about 1 ft. These layers, however, have been traced in outcrop for about 12 miles, and from drill-hole intersections it is apparent that their area1 extent is at least 40 square miles. They are similar to the Bushveld, Great Dyke, and Stillwater chromitite layers in various details, and perhaps most important, they are contained in an intrusion of convenient size that is well preserved structurally, fully exposed from floor to roof in cross-section, and remarkably systematic in its differentiation. The postulated origin for the chromite-rich layers is that they precipitated on occasions when their parental magma deviated from its normal course of crystallization owing to extensive contamination by granitic melt derived from the roof of the intrusion. The paper is, in some respects, a sequel to a previous contribution dealing with the crystallization relations of olivine, pyroxene and plagioclase in the Muskox and other layered intrusions (IRVINE,1970a). GEOLOGY OF THE MUSKOX INTRUSION The Muskox intrusion was mapped and first described by SMITE (1962) and has been the subject of numerous subsequent publications (see IRVINE and BARAUAR, 1972). As exposed it is a north-northwesterly trending body, about 74 miles long, crossed in the middle by the Copper- mino River (Fig. 1). South of the river it appears as a vertical dyke, 500-1700 ft wide, that (footnote cont’d) correlative with the ‘Main chromite seam,’ which extends discontinuously many times as far. Lesser concentrations of chromite in layers only an inch or so in thickness are ubiquitously associated with the platiniferous Merensky reef, which has been traced for about 80 miles in the eastern part of the complex and 120 miles in the western part (cf. WAGER and BROWN, 1968). In the Great Dyke, WORST (1960)has distinguished 31 chromitite layers, ranging from 1 to 18 in. in thickness, extending along large segments of the 330-mile length of the body. One example, about 10 in. thick, was traced for 73 miles; another with an almost constant 4-in. thickness wa,y followed for 55 miles up one side of the dyke and for almost as far back down the other. The Stillwater and Great Dyke chromitite layers occur mainly with layers of peridotite or dunite in more or less systematic succession with layers of orthopyroxenite. The Bushveld chromitite is principally associated with orthopyroxenite and anorthosite in sections of very complicated stratigraphy. The BushveldComplex also contains layers of magnetite, comparable in appearance and extent to the chromitite layersand similarly inter&ratified with anorthositic rooks, that would seem to require a similar explanation. Crystallization sequences in the Muskox intrusion and other layered intrusions-II 993 LEGEND Diabose dykes and basic ~111s not shown Coppermine River bcsalr Dolomite Granophyre Gronophyre-rich gabbros 1 I- Two-pyroxene qobbros Oilvine gabbros ; / m Picrtt!c websten!e Webaterite, orthopyroxenite OIlvine clinopyroxenlte [x Gwnit~c rocks [la Melavolconic rocks m Metosedimentory rocks Geologic boundary (defined. showtng dip. opproxvnote; ,’ assumed) ‘.’ ..-..r,ti Fault (defined: assumed) ” / 0 t 2 3 4 Smiler t---a--+ :.n .- A-2 ;i: i:, I 0 4 6 kIlometers ________ ._ -_._ Fig. 1. Generalized geological map of the Muskox intrusion, showing the locations of the main diamond drill holes. apparently projects beneath the main body of the intrusion to the north and so is called the feeder dyke. The main body, which is estimated to have been emplaced at a depth of less than 5000 ft (IRVINEand BARAGAR, 19’72, p. IO), is funnel-shaped in cross-section with lower ~valls dipping inward, generally at 25-35°, and roof inclined gently to the north. It has been tilted about 5* to the north and consequently has eroded so that its deepest levels are exposed in the south and its outcrop width gradually increases northward (reaching a maximum of about 7.5 miles where it plunges beneath its roof). Further north, the intrusion can be traoed beneath its roof rocks and younger cover for at least another 20 miles on the basis of an neromagnetic anomaly, which then merges with a major gravity anomaly that continues nort~l~~~esterly for about 150 miles. The exposed rooks, therefore, appear to represent only the southern extremity of a much larger plutonic complex. The feeder dyke is composed mainly of bronzite gabbro but along most sections of its length contains either one or two parallel internal zones of picrite. The gabbro is locally chilled along 994 T. N. IRVINE the dyke walls, and its composition, which is equivalent to silica-saturated tholeiitic basalt, appears to be approximately representative of the parental magma of the intrusion (IRVINE, 1970a). The main body of the intrusion comprises two marginal zones, a layered series, and a grano- phyrio roof zone. The marginal zones line the inward-dipping footwall contacts and are generally 400-700 ft thick. They grade inward (or upward) from bronzite gabbro at the contact through pierite and feldspathic peridot&s to peridotite. The layered series consists of 42 layers of 18 different rook types and has been divided into 26 cyuhc units, these being repeated stratigraphic divisions characterized by specific litholo~o~ sequences or chemical trends (Figs.
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