crystallization sequences in the muskox intrusion and

Ceoohimica et Cosmochimica Acta, 1975, Vol. 39, pp. 991 to 1020. Pergamon Press. Printed in Northern Ireland

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 concentrated 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

It is 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 concentrated 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 associations 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 layers and 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

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

0 .- A-2 ;i: i:, I

4 6 kIlometers ________ ._ -_._

1 I-

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 (IRVINE and 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 granophyrio 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. 2-5). The layers range in thickness from IO to 1100 ft,

:YcLIc

UNITS

. . . . ..~.WESSTERITE

-~.....ORTHOPYROXENITE

PERIDOTITE

~~~--clLI”INE CL~NOPYROXENITE

,........,........ C . . . . . .TROCT&lTIC PER,DOT,TE . . . . _____................. OL,V,NE GABBRO . . . . . . . . . _,..,..._..._................. QL,“,NE

CLINOPYROXENITE

. . . ..PEAIDOTITE 7

MARGINAL

OLlVlNE BRCNZIiE GA9BRO j ZoNE

.. BRONZiiE GABBRO J CRYsTALLiNE BASEMENT COMPLEX

Fig. 2. Drill-hole sections of the Muskox intrusion, showing the main cyclic units and the two chromite-rich layers. mt, magnetite zone; il-mt, ilmenite-magnetite

zone, Diabase dykes have been omitted for clarity.

totaling about 6000 ft. about 5”).

They extend between the marginal zones and dip gently to the north (at Some have been traced in outcrop for as far as 15 miles, and it is evident from drill-

hole intersections that many of them have an area1 extent in excess of 100 square miles. The rocks of the layered series are entirely cumulates, and the series ranges generally from

dunite at the base through various pyroxenites and peridotite to two-pyroxene gabbro at the top, The oyclic units fall into three general classes, distinguished in principle by different sucoessions of rock layers. These sequences developed because the cumulus minerals precipitated in close accordance with the crystallization orders of their parental liquids and so represent phase layeting (cf. HESS, 1960). The repetition of units reflects the repeated influx of relatively undifferentiated ‘new’ liquid into the intrusion (IRVINE and SMITH, 1967; IRYINE, 197Oa).

The characteristic rocksuccessions and corresponding crystallization orders for the main types of cyclic units are summarized in Table 1. The order in which the olasses are listed is essentially their order of occurrence in the layered series; hence the differences between them arise because


Table 1. Rock layer sequences and crystallization orders characteristic of the principal cyclic units in the Muskox intrusion.

Class

1

II

Rock sequence

Dunite; olivine clinopyroxenite; olivine gebbro

Dunite; olivine clinopyroxenite websterite

Crystallization order

01; epx; pl; opx

01; cpx; opx; pl

III Peridotite; orthopyroxenite; websterite; two-pyroxene gabbro

01; opx; cpx; pl

Abbreviations: 01, olivine; cpx, dinopyroxene; opx, orthopyroxene; pl, plagioclase.

LAYER ROCK PPM PPH MODlL *

TYPE Ni Cr CYCLlC

NO. Chromite UNIT

Fig, 3. Data from Muskox drill-hole 18618, showing the stratigraphic position of the ohromite-rich layers in their xespeotive cyclic units, together with data on the

abundances of I%, Cr and chromite in the rocks.

T. N. IRVINE

L*YER ROCK

NO.

s

91 G 8’ CL

?

$5 ).

. .

5.

.

,+ 5

PPM VJ ii n L %

Cr Chromite

Fig. 4. Emission spectrographic and modal data from part of the Muskox South drill hole, showing cyclic variations in the abundances of Ni, Cr and ohromite.

orthopyroxene periodically advanced in the orystallization order of the magma. The advance appears to have occurred because the magma became increasingly contaminated with salic material melted from the intrusion roof rocks (IRVINE, 1970a).

The diagnostic chemical trends in the cyclic units are upward trends of decrease in MgO/FeO ratio (IRVINE and BAIIAOAR, 1972, Fig. 9) and Ni content (Figs. 3 and 4). These are essentially fractional orystallization trends for the cumulus mafic minerals (i.e. they represent cryptic layer&g; cf. WAUER and BROFVN, 1968), but they show also in the whole-rock analyses.

The periodio infusions of ‘new’ liquid indicated by the repetition of cyclio units were apparently introduced laterally by flow between the accumulating layers and the roof contact. (The feeder dyke does not out the layered series, so the magma could not have come from directly below.) The chemical trends in the cyclic units provide evidence that some of the infusions were very large, amounting to practically the whole volume of the exposed part of the intrusion, so it is presumed that the magma originated from a major feeder system or reservoir in the area of the large gravity anomaly to the north and moved southward, pushing the residual, ‘old’ liquid ahead of it and eventually to the surface as volcanic eruptions (IRVINE and SETH, 1967; IRVINE and BARAOAR, 1972, pp. 21-22).

The granophyric roof zone is an irregular unit up to about 1000 ft in thickness, consisting of granophyre and granophyrio gabbro transitional with the top of the layered series. The granophyre aonsists mainly of a micrographic intergrowth of quartz and K-feldspar with variable minor amounts of biotite and ihnenite. Evidence described in a later se&ion indicates that it was largely crystallized from roof-rock melt.


LAYER ROCK

NO. TYPE

OLIVINE FABER0

OLIVINE CLINOPX

DUNITE

3L CLINOPX

,:

Cr,O, in chromite, wt % CYCLIC

UNIT

l_-- .y.

,_ .

k-e . . . _ -.\ ..:, . ,. .

d’- *..- __&

:’ 1, _. .”

. .: ‘r -. “. I

. ,

. ../

. . .

‘.‘“.. J..

:j. .

y:

-i;

._A ”

\ _ .

-. .:- A. .

I - I$.

F-“‘-------- _ $1 -*.

_____- L

.-.

‘-

z r

___a_ __-

1

-- . .

.- -_ ..z -I.

-.. _. .:

_ A

-;j. .

._.. . . l- . ..: . ...--*

P /.

1 . .* ._.

---

1-L-_-

.I- _. *. . .

-. .-.Z..

.I.

. *-_

_;3pLl_L Fig. 5. Electron microprobe data for the same part of the Muskox South drill hole as in Fig. 4, showing the concentrations of Ni in olivine and Ni and Cr,Os in

chromite.

CHROMI~E IN THE MUSEOX INTRUSION

The distribution and crystallization relations of chromite in the intrusion have been described by IRVINE (1967) and IRVINE and SMITH (1969), so only critical features are reviewed here.

Except for the two concentrated layers, almost all the chromite in the intrusion is disseminated in amounts of l-3 per cent in olivine cumulates that are variously classed as dunite, peridotite, feldspathic peridotite or picrite, depending on their content of postcumulus materials. These rocks make up about two thirds of the cross-section of the intrusion and in the layered series form 22 layers with an aggregate thickness of about 4000 ft (Fig. 2). They contain chromite almost throughout.

Most of the chromite occurs as subhedral crystals, 0.5-0.15 mm in diameter, situated either individually or in small clusters between the larger cumulus grains of olivine. Occasional small euhedral crystals are trapped in the olivine as early formed inclusions. These features, together with the observation that the modal ratio of chromite to olivine is approximately that in which picrochromite and forsterite coprecipitate in the system MgO-Cr,Oa-SiO, (KEITH, 1964), suggest that the two

998 T. N. IRVINE

minerals precipita~d simultaneously by fractional crystallization, an inte~retation that is confirmed by other data described below. The chromite in the concentrated layers is similar to the disseminated material but commonly is recrystallized to coarser grain sizes where the crystals ac~urnul~~d in close contact with one another. The layers also contain small but conspicuous concentrations of sulphids minerals, principally pyrrhotite, chalcopyrite and pentlandite, occurring in part as distinctive ellipsoidal globules, l-10 mm in length, that would appear to represent accumulations of immiscible sulphide liquid.

Other main features of the distribution of chromite relate to the cyclic units and are illustrated by the data in Figs. 3 and 4. Three points are noted:

(1) In Fig. 4, the modal abundance of the chromite disseminated in dunite shows a cyclic variation similar to that of Ni, particularly in units 5 and 6. Considering that the Ni variation is essentially due to fractional crystallization of the olivine in the dunite, it is apparent that the chromite must have coprecipitated with the olivine.

(2) The two chromite-rich layers (Fig. 3) are both situated between layers of peridotite and orthopyroxeni~ and have a definite place in the cyclic repetition. In the units in which they occur the rock sequence is peridotite-chromitite-orthopyroxenite-websterite.

(3) Chromite tends to be absent in the rocks with cumulus pyroxene. Thus in several cyclic units in Figs. 3 and 4, its modal abundance abruptly drops practically to zero in passing upward from peridotite to orthopyroxenite or from dunite to olivine clinopyroxenite, even though the amount of chromium in the rocks shows a relatively smooth transition across the contact. The ~scontin~ties are att~butgd to magmatic reaction relations between chromite and the pyroxenes such that, when pyroxene began to crystallize, it could accommodate all the chromium the liquid could supply, so chromite stopped forming.

These obse~ations can be further explained by the schematic phase diagram in Fig. 6, illustrating a composite fractional crystallization path for the Muskox liquid. The key features of the diagram are (1) the curved cotectic boundary between the olivine and chromite liquidus fields; (2) the reaction or dist,ribution point b at the ~tersection of the olivine, orthop~oxene, and chromite fields; and (3) the very low concentrations of Cr,O, in the liquid as compared with the pyroxene and chromite. The curvature of the cotectic causes the modal ratio in which olivine and chromite coprecipitate to gradually decrease, ostensibly from 4: 96 to 1: 99 as in Muskox cyclic units 5 and 6 (Fig. 4). When the liquid reaches the dist~bution point, the combination of reaction boundaries requires that both minerals stop forming as pyroxenc begins to precipitate, as apparently happened, for example, in Muskox cyclic unit 19 (Fig. 3). And the phase relations show how the condition that the liquid contains less Cr,O, than the pyroxene can lead to a chromite + pyroxene reaction relationship.

The value of 0.08 wt. y0 Cr,O, indicated for the initial Muskox liquid comes from analyses of the chilled margin. The value of 0.03 per cent for the point st which pyroxene begins to crystallize was obtained by subtraet~g the amount of Cr,O, t-hat would have been removed in the ohvine-chromite cumulates fractionated from the liquid by that stage (see IRMNE and SMITH, 1969). These data are tenuous in themselves but are compatible with analyses reported for chromite-saturated olivine tholeii~s of overall composition similar to the chilled margin (e.g. YODEE and TILLEY, 1962, TabIe 2; EVANS and WRIGHT, 1972, Table 2).

Crystallization sequences in the Mu&ox intrusion and other layered intrusions--l1 N9

Chramites,(-4O%Cr203)

CUMULATE SEQUENCE Minerals Rocks

b-c Orthopyroxen~ Oriho~yrox~nite

o-b Olivine -(Chromite) Dunite, peridotiie

Fig. 6. Schematic projeation illustrating the apparent phase relations of olivine, orthopyroxene and chromite in the Muskox intrusion as indicated by petrographic data. The inset triangle shows the plane of projection, which is part of the more extensive join (Mg, lYe)O-Cr,Os-SiO,. Liquidus relations are modeled after those in the system MgO-Cr,O,--SiO, (KEITH, 1954) but have been drawn to simulate the Mu&ox data, and the main diagram has been distorted for clarity. Boundary

curves with double arrows are reaction boundaries.

Two further points directly pertinent to the origin of &omit&e layers can also be illustrated by means of Fig. 6. The first, a matter of principle, is that fraotional crystallization by it.seZf cannot yield a concentrated deposit of chromite once olivine or pyroxene {or any other silicate mineraI) has begun to form (cf. IRVIZNE and SXITH, 1969, p. 93). Thus an explanation of the situation of the Muskox chromite-rich layers in the middle of cyclic units above layers of peridotite requires some additional process. The other point, a feature that was first suspected on the basis of the model, is that addition of siliceous material to liquids on the olivine-chromite cotectic should tend to shift their compositions into the chromite primary phase field. Figure 6 is

not definitive in this regard inasmuch as it is only a schematic projection, and in fact even the system BlgO-Cr,O,SiO, (KIWII, 1954) does not show any clear ~dication of the suggested effect of silica. There is, however, a theoretical line of reasoning concerning the partitioning of Cr and Ni in silicate melts that leads to the same prediction, and the effect has been substantiated by an experimental study described below.

FRACTIONATION OB NICKEL AND CXEOMIUM IN SILICATE MELTS

The data in Fig. 4 show very similar whole-rock differentiation patterns for nickel and chromium. (Nickel shows the better developed trends of depletion within cyelic

15

1000 T. N. IRVINE

units, but the trends of chromium are obviously comparable.) There is an important difference, however. In the dunite layers, the nickel was almost entirely precipitated in olivine, which originally made up about 90 per cent of the rock. The olivine is now completely serpentinized in the upper two units, but in the lower units its Ni variation closely parallels that of the whole rock; and the coprecipitated chromite, which survived the serpentinization without apparent change in composition, exhibits the same type of Ni variation in all four units (Fig. 5). By contrast, the chromium in the dunite is almost entirely contained in chromite of constant Cr,O, content (Fig. 5) ;

therefore, its whole-rock variations reflect the modal variations of this mineral (Fig. 4). Thus the difference is that nickel was precipitated as a trace element in a major mineral (olivine), whereas the chromium was precipitated as a major element in an accessory or trace mineral (chromite).

Why, then, the similarity of differentiation pattern? The one thing the two elements do have in common petrologically is that they have originated from the same batches of liquid. It is suggested, therefore, that the similarity is primarily a reflection of the properties of the liquid. In particular, it seems likely that because both elements have exceptionally large octahedral site preference energies due to crystal-field stabilization (cf. BURNS, 1970, Table 6.2), they probably came from much the same type of site in the liquid. Following an argument used by BURNS and FYFE (1964, 1967) in discussion of Ni partitioning in magmas, it is suggested that with the increased polymerization of the liquid relating to the joint enrichment of silica and alkalies and the decrease of liquidus temperature caused by fractional crystallization of the olivine and chromite, there was a marked reduction in the number of octahedral sites in the liquid that were energetically favorable for occupancy. Thus the remaining Cr3+ and Ni2+ in the liquid, along with other octahedrally coordinated ions such as Mg2+ and Fea+, were required more and more frequently, through the following combination of equilibria, either to enter tetrahedral sites or to transfer to octahedral sites in the crystals.

(M~+odahedrsi)~iqni

!l

\ \ (Mnfootnhedral)~ystsla,

// (Mn+~t.tetrahadral)Liqnid

where M”+ = Cr3+, Nisf, Mg2+, Fez+.

Because of their very large octahedral site preference energies, the partitioning of Cr3+ and Ni2+ would be strongly biased in favor of the crystals; hence their equilibria were increasingly shifted to the right, with the effect that they were preferentially expelled from the liquid at similar rates. The Ni2+ ions substituted for Mg2+ and Fe2+ in the minerals, whereas the Cr3+ ions effectively controlled the precipitation of the chromite.

The postulated shift of the nickel equilibria is representative of an effect that apparently is large enough to reverse the preferred direction of nickel partitioning between olivine and liquid for basaltic liquids as compared with olivine melts (cf. BURNS and FYFE, 1964, 1967). The shift should be most closely reflected in a coefficient in which the partitioning is defined relative to magnesium,


K = (Ni/Mg) oliviDe/(Ni/Mg)Liquid, inasmuch as Mg 2+ is similar in size to NP+ but, not being a transition metal ion, shows no crystal-field stabilization. The shift, however, is also represented in the distribution ratio, D = (Ni in olivine)/(Ni in liquid), and it is expedient for present purposes to consider the partitioning in this form. Figure 7 illustrates data on the variation of D for tholeiitic liquids similar to

60 8 6 I 1 , 1 , 1 , 1 ,

(u

.5

.z 0

E 20

e G CL

i IO .$

3

(Ni in olivine)/(Ni in initial liquid)

Fig. 7. Variation curves for Ni in olivine formed by fractional crystallization, computed for the situation that the distribution coefficient, D, varies with the amount of normative olivine in the liquid as shown in the inset diagram. The value of D for the systemMgaSiO,-NisSiO, is estimated fromthe data of RINGWOOD (1956); the value for Kilauea tholeiite is from H~~LI and WRIOHT (1967). The other data points are from olivine orystallized experimentally from melted Kilauea lava.9 spiked with small amounts of NiO. The experiments were run in a gas- mixing furnace at oxygen fugacities approximately equivalent to the quartz- fayalite-magnetite buffer. Products were quenched and analyzed by electron microprobe. Negative amounts of normative olivine are computed equivalents of normative quartz. The curve drawn through the data points has the equation, D = -0.91 + 2.969 x 10-3(100 -X0,) + 3.248 x 104(100 - XoJ2, where X0,

is the percentage of normative olivine. Compare with the Ni data in Fig. 5.

the Muskox magma and shows the kinds of fractional crystallization trends that should develop in olivine as a result of this variation. The tendency of the fractionation curves to be convex upward with respect to Ni content is diagnostic. Compari- son with Fig. 4 reveals that the olivine in Muskox cyclic units 4 and 5 does in fact show this type of Ni variation* (as does the chromite in cyclic units Pi’); hence it

* It will be noted that the olivine in cyclic units 4 and 5 shows the type of Ni variation that would be expected if it had been fractionated from liquid initially containing about 15 per cent normative olivine. The data on which the fractionation curves are based are still provisional, so no firm conclusion can be drawn in this regard, but it may be noted that, although the Muskox chilled margin is just saturated in silica, the intrusion may well at times have contained more primitive liquid with IO-15 per cent normative olivine.

1002 T. N. &VINE

appears that the inferred shift of equilibria did indeed occur in the Muskox magma.

In the case of chromium, the shift of equilibria should bear on the proportion of chromite precipitated and, therefore, should be reflected in the trend of the olivine- chromite cotectic. The trend should be such that the addition of constituents that act to polymerize silicate melts, such as silica and alkalies, should tend to shift the liquid compositions from the cotectic into the chromite liquidus field. As noted above, this is the same prediction that arose from the phase diagram model in Fig. 6. It prompted the experimental study that will now be described.

FORSTERITE-PICROCHROMITE CRYSTALLIZATION ON THE JOIN MgO.SiO,-Cr,O,-K,O*6SiO,

The specific objective of this investigation was to test whether compositions in the forsterite liquidus field on the join MgO*SiO,(enstatite)-Cr,O, could be transposed to the picrochromite field by the join addition of SiOz and K,O. The experiments were performed by 1-atm quenching methods. Starting materials were prepared from weighed quantities of dried MgO, cristobalite, Cr,O,, and a powdered crystalline aggregate of K,O*bSiO, composition. These were thoroughly mixed, fused at temperatures 30-100°C above their liquidus, and cooled quickly. The experiments were run at temperatures controlled to f2”C, usually for 1 hr. Run products were identified optically, and in selected cases were analyzed by electron microprobe.

A prime consideration in choosing the system was to avoid solid solution in the crystalline phases in order to concentrate on the effects relating to changes in liquid composition. This goal was evidently satisfied, because the forsterite showed a maximum of only 0.5 wt.% Cr203, and the picrochromite, only traces of silica. The join enstatite-Cr,O, was selected as a starting line because it has forsterite and picrochromite on the liquidus at temperatures low enough to be reached conveniently. The composition K,O*6SiO, was used because it is rich in silica and yet can be mixed with enstatite in amounts up to almost 70 wt,% and still have compositions in the forsterite liquidus field (Fig. 8). From the latter feature, a large working range in which to examine the forsterite-picrochromite cotectic was anticipated. It should be noted t,hat the investigated join is not ternary and can show only a line of intersection or trace of the cotectic surface as developed in the quaternary system K,O-MgO-Cr,O,-SiO,.

The experimental results (Fig. 9) show that the addition of K,O.6SiO, will indeed transpose particular compositions from the forsterite liquidus to the picrochromite field, and the phase diagram prepared from the data indicates that it should shift any composition on the cotectic trace into the spine1 field.* The system is

* It should be noted that this result is not incompatible with the earlier observation that the addition of silica alone to liquids on the forsterito-picrochromite cotectic in the system

MgO-Cr,O,-SiO, (KEITH, 1954) does not apparently shift their compositions into the picrochromite field. In fact, there is a possibility within the framework of existing data that tho trend of Cr variation in liquids on the cotectic even reverses as their normative olivine content and temperature decrease in much the same way as the direction of Ni partitioning reverses in Fig. 7, the trend initially being toward increased concentrations of Cr,03 before turning toward lower concentrations as in Fig. 9.

Crystallization sequences in the Muskox intrusion and other layered intrusions--II 1003

W (Mgo.C_r20.)

Fig. 8. Liquidus relations onthe joinKzO-2MgO-SiO,SiO, (after ROEDDER, 1951) superimposed on relations for the join 2MgO*SiO,-MgOX$Os-SiO, (after KEITH, 1954, shown by light da.shed lines with phase names in parentheses). Liquidus data for composition on the line from point a (located at 99 % MgO*SiO,, 1% Cr,O,f

to IC,O*GSiO, are shown in Fig. 9. (Note: K,O.GSiO, is not a compound.)

extremely simple compositionally in comparison with magmas, but the observed effect may be expected to have broader significance inasmuch as it is the analogue of an effect of crystal field stabilization that apparently is extremely important in determining the behavior of nickel in magmas, and because Cra+ shows an even stronger preference for octahedral sites owing to crystal-field stabilization than NP+ (cf. BURNS, 1970, Table 6.2). The effect is fundamental to the mechanism of origin for chromitite layers that will now be described.

FORMATION OF CIE~OMITITE LAYERS

A model of the chemical aspects of the me~ha~sm as applied to the Muskox layers is illustrated in Fig. 10. It is seen that the fractionation path of a liquid initially crystallizing olivine and minor cbromite can be altered through contamination by siliceous material so that the liquid subsequently precipitates first chromite and then orthopyroxene, giving the rock sequence peridotite-chromititortho- pyroxenite observed in the lower parts of the cyclic units ~onta~g the chromite- rich layers (Fig. 3). (The succeeding websterite layers could be simulated by including CaO as a component so that two pyroxenes could precipitate; IRVINE and SMITH, 1967.) It is noted that the model also accords with the interpretation that contamination was responsible for the periodic advances of orthopyroxene in the crystallization orders in the three classes of cyclic units listed in Table 1.

Figure 1 I shows variations on the model that simulate the main layer sequences involving ohromitite in the Stillwater, Great Dyke and Bushveld eomplexes. The succession of cumulates produced in A is essentially that in the type Stillwater cyclic unit (JACKSON, 1961, I’ig. 11). The sequence in B. either as a whole or in parts,

1004 T. N. IRVINE

Kfl6SiOp KzO

“2’3 , ./3’. ,’ ._”

Weight percent

1300

MgO.Si02

K20.6Si0i

Fig. 9. Phase relations for part of the join MgOSiOs-CrsOs-K,O*BSiO, at 1 atm. Boundary lines on the right face (0% CrsO,) are from ROEDDER (1951); the liquidus on the join MgO*SiO,-CrsO, is consistent with data of KEITH (1954). It is seen that compositions in the forsterite liquidus field can be transposed to the picrochromite field by ‘contamination’ with K,O+SiO,. Compare with Figs. 6, 10 and 11. Abbreviations: Fo, forsterite; Pr, protoenstatite; PC, picrochromite; L,

liquid; ss, solid solution.

matches various sections of Great Dyke stratigraphy (cf. WORST, 1960, Plate ll),

especially the unit described in detail by BICHAN (1969). And the scheme in C, coupled with some means for periodically replenishing the liquid, would give an alternation of chromitite and pyroxenite as observed in the lower part of the Bush- veld Complex (e.g. CAMERON and DESBOROUGH, 1969).* It is not possible at this

* In the model in Fig. 1 Ic, chromite is shown to be succeeded by orthopyroxene in the crystallization history of the Bushveld magma. It should be noted in this regard that, although it has been amply demonstrated that chromite and orthopyroxene frequently accumulated together in the Bushveld complex (e.g. CAMERON and DESBOROUQH, 1969), it has not been shown that they crystallized together.

Crystallization sequences in the Muskox intrusion and other layered intrusions-II 1006:

Liquid”~ boundaries of

original magma

CUMUL’ATE ,SEQUENCE Minerals Rocks

d-e Orlhopyroxene Orthopyrdxenite

c-d Chromite Chromitite

a-b Olivine-(Chromite) Peridotite

Fig. 10. Phase diagram model ilIustrat~g the formation of the lower parts of the Muskox cyclia units containing chromite-rich layers by a oombination of fractional crystallization and contamination. The liquidus relations are the same as in Fig. 6. For simplicity, the contamination is shown as a distinct event from the crystallization. In reality, however, chromite probably crystallized while the siliceous contaminant was being assimilated, in which case the magma would follow an

arcuate path through the chromite field.

A

STiLLWATER GREAT DYKE EUSHVELD

e-f Orthopyroxene q-h Orthopyroxene b-c Orthopyroxene

d-e Olivine _ (Chromite) f-g Olivine-(Chromite) a-b Chromate c-d Chromife e-f Chromite o-b Oiiwne -(Chromife) c-d Olivine. (Chr0mi:ef

b-c ChremltB

Fig. II. Variations on the model in Fig. 10 illustrating the formation of the more common stratigraphic sequences involving chromitite layers in the Stillwater,

Great Dyke and Bushveld complexes.

1006 T. N. IRVME

time to account for the association of anorthosite with some of the Bushveld chromitite, but the mechanism does offer an explanation for the exceptional abundance of o~hop~oxeni~ and reIative dearth of affihated olivine cumulates in this complex.

An attempt to define the main physical processes that appear to be required by the mechanism is shown in Fig. 12. Diagram A depicts ‘normal’ co~~tions during

Fig. 12. Diagrams illustr&ing the physical processes postulated for the formation of ahromitite layers. It is assumed that the intrusion roof comprises a subs~~t~~ proportion of rocks such as pelitic schists snd granite that will yield salic melt at temperatures well below the fiquidus of the basic magma. The tendency of RsU to accumulate at tba top of intrusions would facilitate the melting process. In diagram A, abcde is s possible tom~r~t~re path for the basic magma as it moves in the convection path ABCDE; crystallization should ideally begin at D and continue to E. Branch cf exemplifies the temperature path if the liquid were to descend s cooling wall; branch cg should obtain if the magma cooled to its liquidus while moving along the roof. The paths in diagram B illustrate possible effects of

contamination. For further discussion, see text.

accumulation of the layered series ; B and C portray events relating to episodes of contamination. The temperature and crystallization paths illustrated for the convecting basic magma in A and B are similar to those described in detail by IRVME ~~97~b). &&ice it to note here that the ~on~e~tio~ currents play an integral role in both the e~sta~~zation of the magma and the deposition of the crystals.

The critical feature in Fig. 12A is that the roof contact of the intrusion is continuously exposed to the high temperature of the basic magma owing to the fact that


the minerals formed in the body entirely accumulate on its floor. Thus salic rocks along the contact, such as pelitic schists or granite, would be expected to undergo melting, yielding granitic liquid that would tend to float on top of the basic magma and remain separate because of its low density and high viscosity. Heat transfer models indicate that large amounts of granitic melt can be produced in this way (IRVINE, 1970b), and in the Muskox intrusion and Bushveld Complex, melt of this type appears to be represented by thick zones of granophyre (see section on roof-rock melting below).

In Fig. 12B, the basic magma is contaminated by mixing with a large quantity of the salic melt. Subsequent events could be very complicated, but in general the effect of the contamination should be to lower the actual temperature of the basic liquid (because the salic melt would generally be somewhat cooler) and to reduce ita liquidus temperature (by freezing-point depression)-and, of course, it should cause chromite to crystallize alone. The dissolution of the salic melt might be expected to take place gradually, and in fact there is evidence that some of the Muskox chromite may even have grown in an emulsion-like mixture of t’he two liquids (see section on inclusions in chromite below).

Just what would cause an abrupt episode of magma mixing is a matter of specu- lation, but a strong possibility is that it is touched off by sagging or shifting of the intrusion roof in response to tectonic activity or to major movements of magma in some distant part of the system. Another possibility, illustrated in Fig. 12C, is that the mixing occurs during the influx of new magma, for example, in advance of the formation of a new cyclic unit. If the new magma flowed along the roof as shown (because it was hotter and therefore less dense than the old magma), it would be subject to contamination at an early stage when chromite was most likely to precipitate. But any chromitite layers produced should probably appear at the base of cyclic units,* rather than at an intermediate level as in the Muskox intrusion.

The scheme in Fig. 12 embodies several features that appear important to the explanation of the lateral extent of chromitite layers. Because the melting of the roof rocks is a natural consequence of the fact that the minerals that formed in the intrusion accumulate on its floor, it should be possible for the melt to form (and spread) under large areas of the roof and so be available as a contaminant to large parts of the intrusion. Because the contamination starts at the roof, there is maximum oppor- tunity for its effects to be dispersed through the basic magma before this magma convects down to the floor where the chromite is finally deposited. And if the contamination occurred during the infusion of fresh magma as in Fig. 12C, the lateral

flow would tend to spread its effects. This last possibility would appeas to be of particular interest in the Bushveld Complex and Great Dyke, inasmuch as they contain exceptionally extensive chromitite layers, many of which occur at the base

of cyclic, or otherwise repeated stratigraphic units (e.g. WORST, 1960; CAMERON and DESBOROUQH, 1969).

It might be noted in this regard that the cyclic units themselves, just like chromitite layers, tend to be sharply defined and constant in stratigraphy over long distances.

* An exception might arise if the new magma carried a large proportion of suspended olivine that could settle out before chromite began to crystallize alone.

1008 T. N. IRVINE

The implication is that the changes in magma composition leading to their formation are propagated through an intrusion with considerable efficiency. The fact that the units can be extremely well differentia~d in terms of both phase layering and cryptic layering also implies that they formed from magma that was very well mixed. Indeed, the impression is that the mixing was considerably more rapid than the crystallization, so that even relatively small changes in liquid composition were immediately reflected in the composition of the cumulates.

Another feature of the scheme in Fig. 12, pertinent to the thickness and stratigraphic defi~tion of c~omiti~ layers, is that the oonta~nation process ~en~u s&&o calls for only the blending of liquids, not assimilation of solids, so heat requirements should be minimal. Thus, given the necessary quantity of granitic melt, an intrusion could become intensely contaminated almost instantaneously in terms of its life history.

S~PORTI~~ E~IDENOE AXD D~scussroa

Inchsionns in chromite

Some of the strongest evidence in support of the proposed contamination mechanism comes, remarkably, from features directly visible in the ohromite itself. It has been found that many of the chromite grains in the Muskox chromite-rich layers contain roughly spherical, silicate-rich inclusions, lo-100 pm in diameter, that generally comprise several minerals and so would appear to represent droplets of trapped liquid (Fig. 13). Examination of the inclusions by reflection microscopy (they are rarely large enough to be seen in transmitted light in ordinary thin sections) and by electron microprobe has shown that they variously consist of combinations of orthop~oxene, a chromian titanian phlogopi~ and its sodium analogue, sodic plagioclase, pyrrhotite, chal~opy~~, rutile and probably K-feldspar and quartz. Of more immediate interest, however, are the levels of silica and alkalies. ‘Bulk’ electron microprobe analyses (Fig. 14) show SiO, values up to 86 wt.%, and total alkalies commonly range from 5 to 11 per cent, higher than would ordinarily be expected of basaltic liquid. Several inclusions even gave analyses resembling granite: the one in Fig. 13A, for example, showed 72% SiO,, 19% AlzOs, 5% K,O and 4% N&,0. Thus it would appear that the inclusions probably represent droplets of contaminant granitio melt that were trapped in the chromite while in various stages of mixing with its basaltic parental liquid. If so, then they constitute evidence, as mentioned earlier, that the concentrated chromite grains actually grew in an emulsion-like mixture of the two liquids.

Another type of inclusion in Muskox chromite of interest in this regard is rutile. Some of this mineral, as just mentioned, is found in the silicate inclusions (Fig. 13C), but more commonly it occurs as small, clearly primary, rodlike crystals scattered through the dense population of chromite grains in the chromite-rich layers. The significance of these inclusions relates to the fact that in the system MgO-TiO,-SiO, at low pressures, rutile is not compatible with forsterite but will ~oprecipita~ w&h pyroxene of enstatite composition (MASSAZZA and SIRCRIA, 1958). If these relationships extend to olivine and orthopyroxene such as occur in the peridotite and orthopyroxenite layers adjoining the Muskox chromite-rich layers, then the presence


LLL’-46-w 0 20 100

SIO,, wt %

Fig. 14. Electron microprobe data from spheroidal silicate inclusions in chromite from the Muskox chromite-rich layers. Note that the scale for (Na,O + K,O) is

divided by 2.

of the rutile inclusions would imply that the Muskox magma was considerably enriched in silica just before the concentrated chromite was precipitated, as in Fig. 10.

Inclusions in chromite of the types just described have also been found in the Bushveld and Stillwater chromitite layers. The silicate-rich inclusions in Bushveld

chromite (MCDONALD, 1965) consist principally of orthopyroxene, clinopyroxene, biotite and plagioclase, of which only orthopyroxene is associated as a cumulus phase; those in Stillwater chromite are mainly olivine and biotite (JACKSON, 1961, 1966). In both intrusions the biotite was noted as distinctive, suggesting that concentration of alkalies is exceptional. McDonald postulated that the inclusions represent

droplets of silicate melt trapped in the chromite as it crystallized from an immiscible chromite-rich liquid, but JACKSON (1966) argued that it is unlikely that such a liquid would form in a basic magma. By the present hypothesis, the chromite

would crystallize from the basic magma, and the inclusions would represent contaminant salic melt in various stages of mixing and assimilation, combined in some cases with small amounts of associated crystalline material (e.g. olivine, pyroxene and rutile) .

The occurrence of rutile in the Bushveld Complex as inclusions in chromite has been illustrated by MCDONALD (1965, Fig. 4), and according to E. N. Cameron (personal communication, 1974) it may also be associated as a discrete cumulus phase. Rutile inclusions in chromite in Stillwater chromitite have been noted by Jackson (personal communication, 1969). The fact that rutile is not a usual primary

phase in low-pressure ultramafic and gabbroic rocks suggests the possibility that, like the chromite, it too may have been precipitated in response to contaminat’ion.

1010 T. N. IRVINE

No conclusive data are yet available in this regard, but it would appear significant that the cations in the rutile structture occur entirely on octahedral sites, and that Cr,C, may enter the structure in relatively large concentrations (e.g. Cr,O, contents up to 7 wt.% have been recorded in natural rutiles; MEYER, 1975). The rutile in Muskox chromite shows l-5-2% Cr,O,. Crystal-field stabilization of the Cr3+ ion might therefore have been a critical factor in the formation of the mineral, in which case contamination could have been the controlling mechanism.

Evidence of roof-rock melting and contamination

If the proposed mechanism of origin for chromitite layers is valid, then the intrusions in which the layers occur should show independent evidence of roof-rock melting and contamination. In the Muskox intrusion, the basic magma was emplaced along and just beneath an unconformity between a basement complex of pelitic schists, metamorphosed acidic volcanic rocks, and potassic granite, and a relatively undeformed cover of quartz-rich sandstone. The roof consists partly of hornfelsed schist, partly of granite, and partly of quartzite metamorphosed from the sandstone. The granophyre at the top of the intrusion everywhere includes fragments of quartzite, regardless of the type of roof rock, and many of the fragments have been rounded and embayed by assimilation. In the northeast (Fig. 1) the granophyre is transitional with an extensive unit of ‘contact breccia’ consisting of closely packed blocks and fragments of quartzite up to 5 ft on a side cemented by some lo-20 per cent granophyre. Xenoliths of schist and granite are less common and are restricted to areas in which these rocks form the roof. The schists in the roof are locally permeated by small granitic veinlets, and some of the granite shows coarse, graphic textures that could represent recrystallized melt. Below the granophyre in the layered series and marginal zones, recognizable country rock material is extremely rare, the only occurrences being a few blocks of quartzite in the uppermost pyroxenite and gabbro layers. In view of these relations, it is not difficult to imagine the granophyre as representing magma formed mainly by melting of the schist and granite that floated at the top of the intrusion together with a residue of quartzite fragments, which survived because they were so refractory.

This interpretation appears to be supported by chemical and isotopic relations. The granophyre is extremely pot’assic and so is not the kind of rock that would be expected to form as the end product of crystallization differentiation of normal tholeiitic magma. On the other hand, the granite and all the schist samples that have been analyzed from the roof show very high K,O/Na,O ratios and so, on partial fusion, should yield a highly potassic salic melt (IRVINE and BARAGAR, 1972, pp. 28-32). The Rb-Sr and Sr-isotope relations of the granophyric rocks suggest an age of approximately 1700 m.y., similar to typical K-Ar ages from the basement schists and granites and much older than the 1200-1250 m.y. age indicated for the intrusion itself by K-Ar, Rb-Sr, and paleomagnetic data on the ultramafic and gabbroic rocks (D. H. Loveridge, T. N. Irvine, and R. K. Wanless, unpublished data). Apparent initial s7Sr/86Sr ratios for the ultramafic and gabbroic rocks range from O-705 to 0.709, and a 1700-m.y. isochron fitted to the granophyre data projects to the same range. Such high ratios are indicative of extensive crustal contamination (e.g. FATJRE and HURLEY, 1963).


The Bushveld Complex presents a similar picture but on a grander scale. The complex is enormous, with a layered series about 25,000 ft thick (WILLEMSE, 1969; WAGER and BROWN, 1968). It has highly varied stratigraphy, featuring hundreds of layers ranging from orthopyroxenite, peridotite, anorthosite and chromitite in the lower levels, through norite and two-pyroxene gabbro at intermediate levels, to magnetite, anorthosite and olivine ferrodiorite toward the top. There is no upper border zone of mafic rocks that could have protected the roof; and the remaining roof rocks, which are mainly felsite (i.e. a salic volcanic rock) and highly metamorphosed pelitic sediments, show strong evidence of partial melting (WAGER and BROWN, 1968; WILLEMSE, 1969; VON GRUENEWALDT, 1972). Thegranophyreinthecomplex, together with associated granite, occurs both at the roof contact and as sills in the felsite and is locally as thick as 7000 ft. IRVINE (1970b) showed by a simple thermal model that the basic intrusion could have melted some 3000-3500 ft of liquid of granitic composition, and VON GRUENEWALDT (1972) has supported the model and argued for conditions that would lead to even more melting. In view of the size of the complex, one can readily visualize countless events in which the melt might have mixed with the basic magma and modified its crystallization order, thereby causing many stratigraphic complications.

As in the Muskox intrusion, this interpretation appears to be supported by Rb-Sr and Sr-isotope relations. Data obtained by DAVIES et al. (1970) on the granitic and dioritic rocks yielded an isochron indicating an age of 1954 m.y. and an initial %Sr/%Sr ratio of 0.716. Data on the gabbroic and ultramafic rocks gave initial ratios of 0.7069-0.7090, based on the same age. (The latter rocks gave only very low a’Rb/YSr ratios, and so no reliable independent age could be established for them.) The high initial ratios are strongly indicative of contamination, and DAVIES et al. (1970, p. 589) remarked that, to explain the data, it was necessary to postulate extensive differentiation after contamination, which is essentially the nature of the contamination mechanism proposed here.

The steeply dipping layered rocks of the Stillwater Complex have been unroofed through a combination of erosion and deformation, and there is no mention of xenoliths, assimilation, or contamination in the available descriptions of the body (e.g. HESS, 1960; JONES et aE., 1960; JACKSON, 1961). BONINI et ab. (1969) reported a large gravity anomaly adjoining the stratigraphically upper side of the complex, which they ascribed to an extension at depth, and they suggested that the exposed area of the body might represent as little as 10 per cent of its total extent. If such is the case, then some roof rock might still be available at depth to sampling by drilling. Data so far obtained from the complex on Rb-Sr and Sr-isotope relations (STEUBER and MURTHY, 1966; KISTLER et al., 1969; FENTON and FAURE, 1969) have not yielded a definitive age, and none of the suggested possibilities is compatible with a U-Pb age of 2750 m.y., reported by NUNES and TILTON (1971). For an age of 2750 m.y., four of the investigated samples show nominal initial *Sr/*6Sr ratios of about 0.701, a value that could be mantle derived. Most samples, however, give values of about O-703, and two would have much higher ratios. FENTON and FAURE (1969) suggested that the two high values were due to serpentinization, a reasonable interpretation, but it is apparent that the problem warrants further study with a view to possible effects of contamination.

1012 T. N. IRVINE

The Great Dyke also has been unroofed by erosion, and there seems no possibility that it has any extensions at depth. It does, however, include various country rock xenoliths, principally of greenstone, hornblende gneiss, banded ironstone (quartz- magnetite rock), quartzite, graywacke and granite, plus a few blocks of exotic ultramafic rocks and chromitite representative of older ultramafic bodies in the region (WORST, 1960). The xenoliths are not abundant but apparently are wide- spread in the gabbroic cumulates that form the upper part of the complex. Interest- ingly, however, the one mentioned occurrence in the ultramafic cumulates is a block of granite, about 300 ft long and 80 ft wide, situated between the upper two pyroxenite layers, which is a part of the section that also contains two chromitite layers. Worst noted rocks similar to the xenoliths among the wall rocks of the intrusion, but he considered the xenoliths to have come from the roof, implying that the body did not have a complete upper border zone to protect the roof, at least in the later stages of layered series accumulation. Worst also remarked that among the sedi- mentary xenoliths, quartz& appeared to survive the best, and indeed most of the xenoliths are types that would largely withstand melting in a basic or moderately ultrabasic magma. The main exception is the granite, and some of it is described as being cut by ‘acid veins’ and showing granophyric or graphic intergrowths of quartz and feldspar, features that might be related to melting. Worst’s summary of the regional geology indicates that granite and metasedimentary schists are the predom- inant rocks adjoining the dyke, so if the roof had the same constitution, one could infer from the preserved features of the dyke that large amounts of these rocks had been melted, leaving only the xenolithic residue of more refractory units listed above.

DAVIES et al. (1970) obtained a Rb-Sr isochron for the Great Dyke giving an age of 2532 m.y. and an initial ratio of 0.7025. The latter value indicates that there has not been much contamination, but so too does the overall lithology of the intrusion, which is dominated by olivine cumulates. In fact, the Great Dyke is so like the lower part of the Muskox intrusion as to suggest that it too may have been open repeatedly on a large scale to loss as well as addition of magma. In such a case contamination might not be cumulative in the intrusion and so might leave only a limited stratigraphic record that would be difficult to detect.

Role of oxidation

One of the first questions that is raised by the proposed origin for chromitite layers is, How much contamination is required to form a given layer? It is evident that a large amount of magma must be affected. For example, if half the chromium in the liquid could be precipitated, then to form 1 ft of chromitite containing 40 per cent Cr,O, would require 1000 ft of liquid like the Muskox chilled margin containing O-08 per cent Cr,O,. But whether this much precipitation would require 5 per cent contamination or 50 per cent is unknown. This aspect of the problem must be tackled by experiments on natural materials.

The answer, however, also depends on the role of oxygen. The presentation above has emphasized the effects of silica and alkalies, but chromite crystallization can be critically dependent on the oxidation state of the magma, and the contamination mechanism does offer a convenient means for introducing oxygen (e.g. as H,O in the granitic melt). An evaluation of this factor requires information on the oxidation state of the concentrated chromite, particularly as compared with that ofdisseminated

Crystallization sequenoes in the Muskox intrusion and other layered intrusions-II 1013

chromite that may have coprecipitated with silicate minerals at an immediately

preceding stage. In an attempt to obtain this information the author has made electron

microprobe analyses of chromites through the Muskox layered series leading up to and including the chromite-rich layers. From the data, values for the oxidation

ratio Fea+/(Fe 2+ + Fe3+) have been calculated on the basis of the spine1 R,O, stoichiometry, and estimates of the oxygen fugacities at which the mineral may have equilibrated have been made following a method outlined by IRVINE (1965). The fo,

results are numerically reasonable in terms of experimental data obtained by HILL and ROEDER (1974) on chromite crystallization in tholeiitic magmas, but they are based on several assumptions and so are regarded a,s significant only in the respect that they show that the fo, values derived in this way correlate closely with the iron oxidation ratio (Fig. 15). The data show some systematic variations with

tOOFe3?(Fc?fcF~+) Log(fo,)

,100

1200

,300

,400

f

.z ; T-i g ,500

1600

1700

iQO0

:

: 56~. Feldspathic

,’ peridotite . B

mu4 Olivlne clln!xwEsmui% .’ . : m. .” . . ‘: ;4: Feldspathic . .

.’ : perldotite -

..,. e

Dunite

16

. ‘20.. Lii ., Per’dotite I

. B

. .

= .”

Y

. .

--

JAi-L-L %%?I+ Chromite-rich layers

Fig. 15. Plot of the oxidation ratio of iron in Muskox chromite in the North drill- hole section leading up to and including the two chromite-rich layers, computed from electron microprobe analyses on the basis of spine1 R,O, stoichiometry. Also shown are log for values estimated from the analyses on the assumption that the chromite equilibrated with olivine and orthopyroxene at 1300’K (1027%), by a

method outlined by IFNINE (1965).

1014 T. N. IRVINE

stratigraphic height, but there is no clear indication that the concentrated chromite is more oxidized than the disseminated grains.

A principal problem with this type of approach is that the chromite may have changed in composition after accumulation by reaction with intercumulus liquid and associated silicate phases. Reaction with the liquid might explain, for example, the slightly higher oxidation ratio of the chromite in the peridotite and feldspathic peridotite as compared with that in the dunite (Fig. 15) inasmuch as they are essentially distinguished from the dunite by larger proportions of intercumulus materials. A further complication is that there is evidence in the Muskox intrusion that intercumulus liquid filtered upward through the layered series for hundreds of feet, reacting continuously en route (Irvine, unpublished data). Thus it must be concluded that, if increases info, did contribute to the precipitation of the Muskox chromite-rich layers, either they were too small to be clearly reflected in the composition of the chromite or their record has been obliterated by later events. Other intrusions may be more definitive in this regard.

APPLICATION OF THE CONTAMINATION MECHANISM TO OTHER TYPES OF MAQMATIC ORE DEPOSITS

If the contamination mechanism is viable, then it should also pertain to magmatic ore deposits other than just chromite, especially layered deposits of magnetite and sulphides.

Mugnetite

The magnetite layers of the Bushveld Complex are obvious candidates for consideration inasmuch as they are very similar in appearance and lateral extent to chromitite layers (e.g. WILLEMSE, 1969; MOLYNEUX, 1970). They occur in association with anorthosite layers at a relatively high stratigraphic level in the complex, just where olivine reappears as a cumulus phase after being absent through about 13,000 ft of section. At this level, magnetite has been a subordinate (disseminated) cumulus phase for about 600 ft, and the mafic silicates have become moderately rich in iron, their Mg/(Mg + Fe) ratios averaging about 0.5. The general mineral association is closely represented in the systems MgO-FeO-Fe,O,-SiO, (MUAN and OSBORN, 1956), CaMgSi,O,-Mg,SiO,-FeO-E‘e,O,-SiO, (PRESNALL, 1966) and CaAl,Si,O,-Mg,Si04- FeO-Fe,O,-SiO, (ROEDER and OSBORN, 1966), in all of which oxidation is a factor of prime concern ; thus we are led naturally to the possibility that the magnetite layers were precipitated because the magma was suddenly oxidized. The process is entirely feasible in terms of the physical chemistry, and given that the contamination mechanism provides a means for introducing oxygen, it is perhaps unnecessary to look further for a mechanism of origin, only for substantiating evidence. On the other hand, the association of the magnetite with anorthosite is not obviously ac- counted for by oxidation, and it might be that there are other factors relating to contamination that are more critical to the formation of both rock types. The problem warrants much more investigation.

Sulphide minerals occur in small but conspicuous concentrations in the Muskox chromite-rich layers (SMITH, 1962 ; CHAMBERLAIX, 1967) and in some of the chromitite layers of the Stillwater Complex (PAGE and JACKSON, 1967). In the Bushveld

Crystallization sequences in the Mu&ox intrusion and other layered intrusions---II 1015

complex, sulphides and chromite are distinctive phases in the important platiniferous Merensky Reef (e.g. LIEBENBERQ, 1970). It is generally believed that the sulphides were precipita~d as an immiscible sulphid~oxide liquid, and the association with the chromitite strongly suggest’s that the liquid was concentrated by the same processes as the chromite.

Information on the solubility of sulphide in magmas and on the determining factors is still very limited. The best available data for basaltic liquids come from a study by SKINNER and PECK (1969), which indicated that a Kilauea tholeiite became saturated with sulphide liquid at 0.038 wt.% S and 1065°C. NALDRETT (1973, p. 9) has quoted a value of 0.16 per cent S for saturation of a picritic liquid at 1450°C. It has long been known from studies of slags that sulphide solubility in silicate melts increases with the Fe0 content and temperature of the melt and decreases with its SiO, content and oxygen fugacity (e.g. VOGT, 1919; RICHARDSON and FINCHAM, 1954). From these relations, RICHARDSON and FINCHAM (1954) argued that the volubility is essentially determi~~ed by the bonding of sulphur with the Fez+ in the melt, and more recent investigators have generally supported t’his interpretation (e.g. MACLEAX, 1969; HAUCHTON and ROEDER, 1971; HAUGHTON and SKINNER, 1972). All the above-mentioned factors are basically represented in the system FeS-FeO- Fe,O,--SiO,, and the join FeS-FeO-SiO, is of parhicular interest in the present context

In this join, as seen in Fig. 16A, certain liquid compositions in the fayalite liquidus field can be shifted by the addition of silica into a two-liquid field where they will exsolve or precipitate a sulphide-rich liquid. Translating this effect into a model diagram for tholeiitic magmas (Fig. 16B), one sees that it could yield sequences of cumulates with concentrations of sulphides at the same stratigraphic levels as were described earlier for chromitite layers (Figs. 10 and 11). It appears possible that the effect in the experimental system is due, at least in part, to relative differences in the crystal-field stabilization energies of Fe%+ in oxide, silicate and sulphide structures (cf. BURNS, 1970, Tables 6.2, 6.5 and 7.1), in which case the effect for nickel and copper sulphides should probably be more pronounced, Ni2+ and Cu2+ showing much greater stabilization. This possibility may be explained by means of the following equilibria :

(Ma+ 1 octahedral Silicate liquid - - (Mnfeetabedral)Snlphide liquid

wn+ 1 tetrahedral Silicate liquid - octabearal Mafia silicate mineral )

where pi:fn+ = Xi”*+, CWf, Pea+, Mg2f.

Thus, as the frequency of octahedral sites in a cooling basic silicate liquid just saturated in olivine or pyroxene and sulphide liquid is reduced through contamination by salic material, these equilibria should shift strongly to the right for NW, Cu2+ and Fez+, causing them to be preferentially expelled into the other phases. But if the sulphur in the silicate melt has dissolved largely by bonding to these ions (as apparently it does for at least Fe2+), then it too should preferentia~y exsolve ; hence the precipitation of sulphide liquid to the exclusion of the silicate minerals is expected.

It would appear that this mechanism for precipitating sulphide liquid from basic magmas may be particularly important in the case of the Sudbury nickel ores.

1016 T. S. IRVINE

Fe0 wt % SiO,

FeS

CUMULATE SEQUENCE

d-e Orthopyroxene-(Sulphide liquid) c-d Sulphide liquid a-b Olivine - (Sulphide liquid)

ulphides Oxides)

Olivine Not to scale Sulphide

(Oxides)

Fig. 16. (A) Liquidus relations for the join FeS-FeO-SiO,, after MACLEAN (I969), showing how certain liquids in the fayalite primary phase field can be forced to precipitate a sulphide-rich liquid through the addition of silica. (B) Sohematic representation of similar relations for tholeiitic magmas, showing how a combination of fractionation and oon~min&tion might produce & concentration of sulphides euch as occurs in the Muskox chromita-rich layers, Compare with Figs.

IO and II.

Over the years, the concentration of these ores around the generally inward-dipping footwall contact of the oval-shaped Sudbury Irruptive hss repeatedly been attributed to gravitational accumulation of immiscible sulphide liquid; and recent studies (e.g. DIETZ, 1964; FRENCH, 1968) have show~l that the Irruptive was probably emplaced in response to the impact of a Iarge meteorite, having been intruded beneath an extensive mass of shattered and partly melted material that was evidently formed by impact from thick units of quartz&! and argillite and an underlying complex of granitic gneiss, an ideal situation for salic contamination. Another important recent discovery (SOUGH, PODOLSKY et cd., 1969; NALDRETT and KULLERUD, 1967) is that the ores are Iargely contained in a ‘sub-layer’ of small noritic, gabbroic, and dioritic intrusions that are also rich in xenoliths, some of these consisting of country rock schists, and many of distinctive, cumulate-type ultramafic and gabbroic rocks ranging from dunite through peridotite, orthopyroxenite and websterite, to olivine gabbro and norite. The main part of the Irruptive, by contrast, consists of thick,

Crystallization sequences in the Muskox intrusion and other layered intrusions-11 1017

rudely stratiform layers of noritic and gabbroic cumulates overlain by a similarly thick unit of micropegmatite (granophyre). No clear age distinction has been established between the mafic cumulates and the micropegmatite, but there is some opinion that the sub-layer intrusions are younger than the norite (NALDRETT et al.,

1973, p. 211). Working from the base summarized above, NALDRETT et al. (1973) proposed a

tentative sequence of events leading to formation of the sulphide ores. The following is a modified sequence, based on the hypothesis that the sulphides were precipitated by the contamination mechanism.

As the basic magma of the Irruptive was emplaced, olivine was a principal mineral to crystallize, and accordingly there developed a complex zone of olivine- bearing cumulates in the lowest parts of the funnel-shaped floor. But with the convection accompanying emplacement and crystallization, the magma was quickly contaminated with large amounts of partly melted salic material from its roof, and as a result the ensuing period was marked by wholesale precipitation of sulphide liquid and by the erratic but progressive appearance of orthopyroxene in place of olivine in the crystallization order. The sulphide liquid presumably collected mainly in pools on top of the olivine-bearing cumulates, but as accumulation of silicates resumed, it would tend to seep to some extent into interstices between the settled crystals. Eventually conditions stabilized, and the main units of norite and gnbbro were formed. During this period, orthopyroxene was joined in the crystallization order, first by plagioclase and later by augite, magnetite and apatite. However, because of the high viscosity acquired by the magma through contamination, the fractionation of the crystals was very imperfect, and the cumulates that formed were poorly layered and retained large amounts of pore liquid as are now evidenced by abundant interstitial granophyre in the norite and gabbro (cf. NALDRETT et al., 1970). In the meantime, the roof of the intrusion was further melted by the heat from the basic magma and underwent some gravitative differentiation in itself, distinguished particularly by the segregation of refractory quartzite fragments, which floated upward to form a quartzite breccia with granophyre matrix at the present roof contact of the Irruptive (cf. STEVENSON, 1963) .* The siliceous melt eventually resolidified as the micropegmatite.

At some time toward the end stages of solidification, perhaps in response to a major episode of subsidence in the center of the Irruptive (an event that appears to be indicated by the lopolithic form of the body), the sulphide liquid together with substantial amounts of basic silicate magma (possibly representing intercumulus liquid that had been preserved in the hot, deep part of the structure) were squeezed upward along the footwall contact to form the ore-bearing sub-layer intrusions. Carried in these intrusions were pieces of the deep ultramafic and gabbroic cumulates, plus fragments of country rock schists picked up en route.

The key features of this sequence of events in the present context are that (1) it relates both the sulphide ores and the exceptional abundance of orthopyroxene in the Irruptive to the meteorite impact, and (2) it allows for the development of Ni-rich sulphide in that the sulphide liquid is precipitated at an early stage before the basic magma is depleted of Ni through extensive fractionation of olivine and pyroxene.

* It appears that this quartzite breccia, is almost identical with the contact breccia at the top of the Muskox intrusion.

1018 T. N. IRVINE

If it should prove that the sub-layer intrusions are older than the norite, the events would have to be rearranged, but these features could be retained.

A question that is raised that points to a possible test of the mechanism is whether any chromium was precipitated with the sulphides. GASPAERMI and~ALDRETT (1972) have reported Cr,O, in concentrations of S-13 wt.% in magnetite in the norite, enough to suggest that the magma should have been capable of crystallizing chromite at an earlier stage. It is unlikely that chromite would form once orthopyroxene began to crystallize, but earlier precipitation might be expected. To the author’s ~owledge, no one has recorded chromite in the ores as other than an accessory phase in some of the ultramafic xenoliths, but some of the ores are rich in what appears to be primary magnetite in which chromium has been detected (HAWLEY, 1962), and perhaps an intensive search would reveal substantial concentrations. On the other hand, it should be borne in mind that the extensive precipitation of sulphide might tend to suppress the crystallization of chromite, for exampIe, by depleting the magma in Fez+.

CoNoLnsIoNs

From this account, two inferences of considerable general significance may be drawn. One is that the differences between the chemical structures of basic and salic silicate melts would appear to be extremely important in determining the precipitation of magmatio ores, especially the ores of transition metals. The other inference is that the compositional characteristics of the cumulates that collect at the bottom of a layered intrusion can be strongly dependent on the events that occurred at its top. The proposed contamination mechanism of origin for chromitite layers and similar ore deposits combines these two factors, and an attempt has been made to illustrate its capab~ities and potential, and some possible implications. An important attribute of the mechanism that might be noted is that it should frequently be amenable to testing in a variety of ways by field, chemical, mineralogical and isotopic methods.

Acknowledgements-Much of the information presented on the Muskox intrusion stems from work done at the Geological Survey of Canada, and the writer is very grateful for the support provided. Survey colleagues to whom particular thanks are due are C. II. S~H, D. C. FINL)LAY, R. K. WANLESS and W. D. LOVERIDGE. At the Geophysical Laboratory, the electron microprobe work was facilitated through instruction by C. G. HADIDIACOS and L. W. FINGER, and the experimental work, through help from I. KUS~IRO, R. J. ARCULUS, R. H. MC~AE;J;ISTER and E. F. OSBORN. The manuscript has been considerably improved as a result of comments and suggestions by R. J. ARCULUS, P. M. BELL, E. F. OSBORN, D. RUMBLE, D. MSHAW and H.S.YODER, Jr.

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1020 T. N. IRVINE

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